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Measurement Methods for Evaluation of

Thermal Integrity of Building Envelopes

by

Richard A. Grot Douglas M. Burch

Samuel Silberstein Lawrence S. Galowin NOVEMBER 1982

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ational Bureau of Standards

epartment of Commerce

Washington, D.C. 20234

Office of Design and Construction

Public Building Services

General Services Administration

Washington, D.C. 20405
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KATIONAI. BUREAU

OF STANDARDS

UBRARY

NBSIR 82-2605

MEASUREMENT METHODS FOR EVALUATION OF

MAR 14 1983

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(2 • THERMAL INTEGRITY OF BUILDING ENVELOPES

Richard A. Grot

Douglas M. Burch

Samuel Silberstein

Lawrence S. Galowin

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Center for Building Technology

Washington, DC 20234

November 1 982

Prepared for Public Buildings Service

General Services Administration

Washington, DC 20405

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director
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ABSTRACT

This report presents various measurement and inspection techniques for

assessing the thermal performance of the exterior envelopes of buildings. The

inspection techniques include the use of ground-based infrared thermographic

surveys, aerial infrared surveys, tracer gas air infiltration measurements,

pressurization tests for measuring the tightness of the building envelope, and

spot radiometer surveys for detecting gross defects. Heat flow meters, a portable calorimeter, and a microprocessor-driven envelope testing unit are also

considered. For each technique, recommended procedures are provided; they

include equipment requirements, conditions under which the techniques can be

carried out, calibration, accuracy, and limitations.

An Executive Summary provides an overview of the Building Diagnostic Program of

which Phase 1 is covered in this report. Field test evaluations will be carried

out in Phase 2.

Key Words: Air infiltration rates; envelope thermal performance; infrared

imaging; radiometers; thermographic surveys; tracer gas techniques.

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EXECUTIVE SUMMARY

BACKGROUND

The Federal government presently owns and leases thousands of buildings

throughout the United States. Also, the General Services Administration (GSA)

and other federal agencies construct new buildings each year. Many of the new

and existing buildings have thermal defects in their envelopes as a result of

poor workmanship, a misunderstanding of the construction specifications and

aging of materials. Many of the older buildings were also compromised in their

thermal performance through their design emphasis on first cost, disregarding

energy efficient components or systems because fuel and power were plentiful

and inexpensive. Consequently, buildings frequently use excessive amounts of

energy due to envelope thermal defects.

Several diagnostic techniques have evolved which can be applied to locate and

identify defects in building envelope in new and existing buildings. Through

exacting problem identification remedial or retrofit actions can be carried out

which result in cost effective energy savings.

The availability of these new techniques prompted the General Service

Administration to work through the National Bureau of Standards (NBS) to

develop a multi-phased research effort to implement a diagnostics program for

the federal government. The first phase was devoted to the development of

procedures that GSA could use to evaluate the thermal integrity of the building

envelopes. To improve the capability of this project to meet the needs of other

agencies, the Phase I effort was developed under the guidance of a peer review

group consisting of representatives from the General Services Administration,

the Department of Defense, the Department of Housing and Urban Development, the

Department of Energy and the Public Works of Canada.

Phase I ; Development of Recommended Procedures for Evaluating Thermal Integrity

of Building Envelopes

This Phase I report addresses the measurement techniques, inspection procedures

and data analysis procedures required to assess various aspects of the integrity of the envelope of buildings. The eight generic methods of building diagnostics that are described include; (1) ground Infrared thermographic surveys

for locating sources of air leakage, missing Insulation, defective or damaged

insulation, thermal bridges and bypasses, and moisture damage; (2) aerial infrared thermographic surveys for assessing roof damage; (3) air infiltration meassurement of the building under natural conditions; (4) induced pressurization

techniques of measuring the tightness of the building envelope and major components of the building envelope; (5) spot radiometers, both temperature and

energy flow types, for estimating the heat loss from major building components

during an energy audit; (6) heat flow meters for measuring the thermal conductance and heat loss from small sections of the building envelope; (7) portable

calorimeter boxes for measuring the thermal conductance of large areas of the

building envelope; (0) envelope thermal testing units for the determination,

in situ, of the heat transmission of building elements.

iv

i

For each of these methods there is a description of; the technique; the

equipment used; the theory underlining the measurement; the interpretation

methods and assessment of the anticipated accuracy; the availability of equipment and of firms offering the service; the training requirements for users of

the equipment; and the potential restrictions to the application of the method.

Whereas building equipment interactions with the envelope are usually important

to building energy use, they are not covered in this report.

Phase 11 ; Application and Evaluation of Recommended Procedures

In Phase II, the methods described in the Phase I Report are being applied and

evaluated in a sample of 4 large federal buildings (over 100,000 square feet)

and 4 small federal buildings (less than 100,000 square feet). Each building

category represents a significant fraction of the gross area of federal buildings. Also the size of 100,000 square feet is about where different applications of the proposed inspection procedures are expected to be required (due to

economic factors of application and scale). The collection and interpretation

of data will indicate whether modifications to the diagnostic procedures will

be required for broad practical implementation in a cost-effective manner.

Techniques being evaluated by NBS include;

a. Measurement of air infiltration using tracer gas technique. Up to ten

readings will be taken for each building over a period of time to

reflect varying weather conditions and with the building in a normal

operating mode. An additional set of readings will be taken with the

inlet and outlet dampers closed (following coordination with GSA

operating personnel).

b. Measurement of building tightness using fan pressurization tests. For

small buildings, an external fan will be used. For large buildings,

the building system fans and dampers will be used (following proper

coordination with GSA operating personnel). During tests, observations will be made of typical building elements such as doors, windows

and joints to determine if air leakage sites can be identified and

evaluated

.

c. Measurement of thermal conductance of major building components using

heat flow meters, calorimeters, or envelope thermal testing units

(whichever are appropriate) in areas representative of the total

building envelope.

d. Interior and exterior thermographic surveys under suitable winter

conditions.

e. Aerial infrared roof inspections of the four large buildings.

f. Energy audits of the four small buildings using spot radiometers to

cover the same surfaces as surveyed by thermographic techniques.

V
Considerations used in the selection of the test sites included building size,

climate, travel time and distance, building design and operating characteristics. The buildings under evaluation include:

When the evaluations are completed NBS will prepare case study reports for each

building describing: the results of the tests; the interpretation of the

results with use of the data for developing building retrofit options; estimates of the costs of the recommended retrofits; cost estimates for performing

the test procedures; projected estimates of the energy savings from retrofits.

In addition procedures will be developed for the economic evaluation of the

benefits of the diagnostic techniques as applied to each category of building.

Impacts of this Research Effort

The diagnostics program will provide GSA/PBS with methods to improve the energy

performance of federal buildings. In both existing and new buildings several

methods can be used to locate and identify defects in the building envelope.

In existing buildings, deterioration will often produce thermal anomolies long

before serious damage has occurred. The detection of the anomolies can lead to

corrective action before more costly repairs are required. In new construction,

in situ nondestructive methods can be used to verify the thermal integrity of

the envelope systems as the construction proceeds.

Significant and wide ranging contributions for improving building perfomance

and for new building design can be derived from the results obtained in the

field site demonstrations. The economic evaluation criteria will establish a basis for cost effective decisions on the use of diagnostics and for determining

efficieat retrofit packages for allocation of limited PBS resources.

In addition, costs determined by performing the test procedures will be directly

applicable for PBS planning purposes. The development of comprehensive technical requirements for inclusion in PBS procurement specifications will assure

that both new buildings and retrofits realize their anticipated savings and

attain desired level of quality in federal buildings, designed, constructed or rehabilitated by PBS.

Large (>100,000 ft^) Small (<100,000 ft^)

Anchorage, AL

Columbia, SC

Norfolk, VA

Springfield, MA

Ann Arbor , MI

Fayetteville, AR

Huron, SD

Pittsfield, MA
ACKNOWLEDGMENTS

The authors of this report appreciate the assistance and guidance given by

David Dibner, Erma Striner, David Eakin and John Holton of Public Buildings

Service of the General Services Administration. It was at their Initiative

that this work was started. Assistance was also given by Preston E, McNall,

Noel Raufaste, Mary Reppert, Robert R. Jones, and Yui-May Chang of the National

Bureau of Standards during various stages of preparation of the report.

Special recognition is given to the peer group review panel: Millard Carr,

Department of Defense; Richard Munis, U.S, Army Cold Region Research and Engineering Laboratory; George Courville, Oakridge National Laboratory; Dan

Kluckhuhn, Department of Housing and Urban Development; and Peter Mill of

Public Works Canada and to his staff of the Architectural Sciences Division,

Audrey Kaplan, Gregory McIntosh, and Robert Hawley, for the exchange of information and sharing Canadian experiences on the diagnostic program. The discussions on developments in measurement methods with John Shaw and Bill Brown,

National Research Council of Canada, with David Harrje, Princeton University,

and with Robert Sonderegger, Lawrence Berkeley Laboratory are also acknowledged.

Special requests for accelerated support services of Ulesia Gray and Brenda

Thompson of the CBT Word Processing Center and the Graphics Department of NEL

are acklowledged.

vil
TABLE OF CONTENTS

Page

ABSTRACT iii

EXECUTIVE SUMMARY iv

ACKNOWLEDGMENTS vii

1. INTRODUCTION I 1.1 BACKGROUND 1 1.2 POTENTIAL DIAGNOSTIC PROCEDURES APPLICATIONS 2 1.2.1 Application to New Construction 3

1.2.2 Application to Existing Buildings 5

2. GROUND INFRARED THERMOGRAPHIC SURVEYS 9

2.1 SUMMARY 9

2.2 DESCRIPTION OF INFRARED IMAGING SYSTEMS 9

2.2.1 Thermographic Equipment Specifications and

Evaluation 15

2.2.2 Conditions Under which Thermographic Surveys can be

Performed 20

2.3 CONDUCTING INTERIORS SURVEYS 21

2.3.1 Measurement Technique 21

2.3.2 Guidelines for Conducting Surveys 21

2.4 CONDUCTING EXTERIOR SURVEYS 22

2.4.1 Measurement Techniques 22

2.4.2 Guidelines for Conducting Surveys 22

2.5 INTERPRETATION OF THERMOGRAPHIC DATA 23

2.6 ACCUPJVCY OF THERMOGRAPHIC SURVEYS 27

2.7 AVAILABILITY OF THERMOGRAPHIC EQUIPMENT AND SERVICES 27

2.8 TRAINING OF THERMOGRAPHIC OPERATORS 27

2.9 PREVIOUS USES OF THERMOGRAPHY 28

2.10 PROGRAM RESTRICTIONS TO THE APPLICATION OF THERMOGRAPHIC

SURVEYS 29

REFERENCES 30

3. AERIAL INFRARED SURVEYS 34

3.1 SUMMARY 34

3.2 CONCEPT OF AERIAL INFRARED THERMOGRAPHY 34

3.3 ACCURACY OF TECHNIQUE 39

3.3.1 Pitched Ventilated Roofs 39

3.3.2 Low-Slope Built-Up Roofs 39

3.4 GUIDELINES FOR CARRYING OUT SURVEYS 41

3.5 EQUIPMENT SPECIFICATIONS 43

3.6 INTERPRETATION OF AERIAL THERMOGRAPHHIC DATA 45

3.7 PREVIOUS USE, AVAILABILITY AND COSTS 45

3.8 PROGRAM APPLICATION 45

3.9 TRAINING REQUIREMENTS 45

REFERENCES 46

4. AIR INFILTRATION MEASUREMENT METHODS 47

viii
TABLE OF CONTENTS (Continued)

Page

4.1 SUMMARY 47

^,.2 DESCRIPTION OF EQUIPMENT-ALL TRACER GAS METHODS 48

4.2.1 Tracer Gases 48

4.2.2 Sulfur Hexafluoride Measurements 51

4.3 THEORY 51

4.3.1 All Tracer Gas Methods 51

4.3.2 The Tracer Decay Method 55

4.3.3 "Constant" Concentration Technique 65

4.3.4 Constant Flow Technique 68

4.4 PROGRAM RECOMMENDATIONS 69

4.5 RESTRICTIONS TO IMPLEMENTATION 69

REFERENCES 70

5. FAN PRESSURIZATION TECHNIQUES 74

5.1 SUMMARY 75

5.2 DESCRIPTION OF EQUIPMENT 76

5.3 THEORY 78

5.4 MEASUREMENT GUIDELINES 78

5.5 COMPONENT TESTING BY FAN PRESSURIZATION 82

5.6 USE OF BUILDING'S AIR HANDLING SYSTEM 84

REFERENCES 86

6. TEMPERATURE MEASURING SPOT RADIOMETERS 89

6.1 SUMMARY 89

6.2 DESCRIPTION, SPECIFICATION AND COST OF EQUIPMENT 89

6.3 THEORY 91

6.4 GUIDELINES FOR CONDUCTING MEASUREMENTS 92

6.5 INTERPRETATION OF RESULTS 92

6.6 ACCURACY OF THE TECHNIQUE 93

6.7 AVAILABILITY OF EQUIPMENT AND ENERGY AUDITS WITH SPOT

RADIOMETERS 94

6.8 PREVIOUS USE 94

6.9 TRAINING REQUIREMENTS FOR PERSONNEL USING SPOT RADIOMETERS ... 94

6.10 PROGRAM APPLICATION RESTRICTIONS 94

REFERENCES 95

7. RADIOSITY MEASURING SPOT RADIOMETERS 96

7.1 SUMMARY 96

7.2 DESCRIPTION AND COST OF EQUIPMENT 96

7.3 THEORY 96

7.4 GUIDELINES FOR CARRYING OUT MEASUREMENTS 99

7.5 INTERPRETATION OF DATA 99

7.6 ACCURACY OF THE TECHNIQUE 99

7.7 AVAILABILITY OF EQUIPMENT AND PREVIOUS USE 99

7.8 TRAINING REQUIREMENTS 99

7.9 PROGRAM APPLICATION RESTRICTIONS 100

REFERENCES 101

ix
TABLE OF CONTENTS (Continued)

Page

8. HEAT FLOW METERS 102

8.1 SUMMARY 102

8.2 DESCRIPTION AND COST OF EQUIPMENT 102

8.3 CALIBRATION OF SENSORS lOA

8.4 GUIDELINES FOR CARRYING OUT HEAT FLOW METER MEASUREMENTS 105

8.5 ACCURACY OF TECHNIQUE 107

8.6 INTERPRETATION OF DATA 107

8.7 PREVIOUS USE 107

8.8 TRAINING REQUIREMENTS 107

8.9 PROGRAM APPLICATION RESTRICTIONS 108

REFERENCES 109

9. PORTABLE CALORIMETER BOXES 110

9.1 SUMMARY 110

9.2 DESCRIPTION OF EQUIPMENT 110

9.3 MEASUREMENT TECHNIQUE 112

9.4 GUIDELINES FOR CONDUCTING MEASUREMENTS 113

9.5 SPECIFICATIONS FOR THE CALORIMETER BOX 113

9.6 ACCURACY 114

9.7 INTERPRETATION OF DATA 114

9.8 PREVIOUS USE, AVAILABILITY OF SERVICE AND TRAINING REQUIRED .. 114

9.9 PROGRAM APPLICATION RESTRICTIONS 114

REFERENCES 115

10. ENVELOPE THERMAL TESTING UNIT 116

10.1 SUMMARY 116

10.2 DESCRIPTION OF EQUIPMENT 116

10.3 MEASUREMENT TECHNIQUE 116

10.4 ACCURACY OF THE METHOD 118

10.5 PREVIOUS USE, COST, AVAILABILITY OF EQUIPMENT AND TRAINING

REQUIRED 118

10.6 PROGRAM APPLICATION RESTRICTIONS 118

REFERENCES 119

11. GLOSSARY 120

X
LIST OF FIGURES

Page

Figure 2.1 Block diagram of thermographic system 10

Figure 2.2 Schematic of basic spot radiometer 11

Figure 2.3 Schematic of portable line-scanner 12

Figure 2.4 Schematic of prism-scanning imaging system 12

Figure 2.5 Schematic of rotating mirror scanning infrared imaging

system 13

Figure 2.6 Schematic of low-cost multiple-raster Infrared imaging

system 13

Figure 2.7 Schematic of pyroelectric vidicon 14

Figure 2.8 Schematic of MRTD test for infrared imaging systems 16

Figure 2.9 Relation between MRTD and building defects 16

Figure 2.10 Instrument sensitivity requirement R5/R15 for recognition

@ 8-14 microns-exterior survey 18

Figure 2.11 Instrument sensitivity requirements R5/R15 for recognition

@ 3-5 microns 18

Figure 2.12 Instrument sensitivity requirement R2/R10 for recognition

@ 8-14 microns 19

Figure 2.13 Instrument sensitivity requirement R2/R10 for recognition

@ 3-5 microns - exterior survey 19

Figure 2.14 Example of thermogram vids in a cavity wall retrofitted

with foam insulation 25

Figure 2.15 Example of thermogram of voids in a cavity wall retrofitted

with cellulose insulation 25

Figure 2.16 Example of thermogram showing two levels of insulation in a cavity wall initially Insulated with 3-1/3" glass fiber

batts over which foam is installed as a retrofit ......... 25

Figure 2.17 Thermogram showing air infiltration around an improperly

weatherstripped door 25

Figure 2.18 Thermogram showing air penetration under ceiling

insulation 26

Figure 2.19 Example of an exterior thermogram of a retrofitted

dwelling Notice the missing insulation in a bay cavity

and the abnormally warm roof 26

Figure 2.20 Example of thermal reflection recorded by an 8-14 micron

system. Image in the side of the house is the

neighboring house 26

Figure 2.21 Example of thermal reflection recorded by an 8-14 micron

system. Light area along the chimney in the siding 26

Figure 3.1 Schematic illustration of the use of a line scanner to

perform aerial infrared thermography 35

Figure 3.2 An aerial infrared photograph of a large number of

residential roofs (provided by Texas Instruments) 36

Figure 3.3 Transmittance of the whole atmosphere as a function of

wavelength 36

Figure 3.4 Average atmosphere transmittance as affected by incident

angle and outdoor relative humidity 38

Figure 3.5 Aerial thermogram of three test houses 40

xi
LIST OF FIGURES (Continued)

Page

Figure 3.6 Depression in surface temperature on a built-up roof

having 4 inches of insulation as a function of wind

speed 42

Figure 4.1 Sample of tracer gas decay method data 56

Figure 4.2 Comparison of concentration versus time relationships four

four simple models ranging from perfect mixing to complete

nonmixing 56

Figure 4.3 Schematic of air sampling bag procedure 57

Figure 4.4 Schematic of 26-story Park Plaza building in Newark, N.J. .. 60

Figure 4.5 Schematic of sampling network for tower of Park Plaza

building 60

Figure 4.6 Detail of sampling network for tower of Park Plaza building. 61

Figure 4.7 Typical sequence of test data from tower of Park Plaza

building 61

Figure 4.8 Daily average air exhange rates for Park Plaza building .... 62

Figure 5.1 Installation of large fan in a tall building 74

Figure 5.2 Schematic of blower door assembly used by Princeton

University 76

Figure 5.3 Schematic of errors in fan pressurization measurements 81

Figure 5.4 Direct method for testing a wall 84

Figure 6.1 Spot radiometer 89

Figure 6.2 Exterior wall schematic 90

Figure 7,1 Use of reference pad for measuring incident radiation 98

Figure 8.1 Schematic drawing of heat flow meter 103

Figure 8.2 Example of commercially available heat flow meter 103

Figure 9.1 Construction details of calorimeter 110

Figure 9.2 Illustration of measurement technique 110

Figure 10.1 Schematic of envelope thermal testing unit 116

Figure 10.2 Outside blanket 116

Figure 10.3 Inside blanket 116

xii
LIST OF TABLES

Page

Table 1.1.1 Phasing of Test Procedures for New Construction 4

Table .2.2 Phasing of Test Procedures for Existing Buildings 6

Table 2.1 Required Instrument Sensitivities for Interior

Surveys (MRTD) 17

Table 3.1 Required MRTD to Permit an Uninsulated Built-Up Roof to be

Distinguished from One Having 2 Inches of Perlite

Insulation 44

Table 3.2 Required MRTD to Permit an Uninsulated Pitched Ventilated

Roof to be Distinguished from One Having R-11 Ceiling

Insulation 44

Table 4.1 Gases and Techniques for Tracer Dilution Method 50

Table 4.2 Atmospheric Constituents 50

Table 5.1 Approximate Fan Capacities Required for Variouus Spaces 77

Table 6.1 Summary of Uncertainties in Measuring the Thermal Resistance

of Insulated (Uninsulated) Walls 93

Table 7.1 Summary of Uncertanity in Measuring Thermal Resistance of

Insulated (Uninsulated) Wall 100

Table 8.1 Typical Phase Lag for Various Wall and Roof Constructions ... 106

Table 9.1 Required Measurement Periods for Various Building Component

(based on information in ref's. [2,3]) 113

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1
1 INTRODUCTION

1 . 1 BACKGROUND

The Federal government presently owns and leases thousands of buildings

througlout the United States. The General Services Administration (GSA) con- structs new buildings each year some of which have thermal defects in the

building envelopes due to either poor workmanship or a misunderstanding of the

construction specification. A strong need exists for viable, in situ and

nondestructive survey techniques to verify the thermal integrity of the

envelope systems of office buldings under construction. In this way, thermal

defects can be identified and located, and the contractor can carry out remedial action prior to departure from the construction site. All existing

buildings degrade with age and require periodic maintenance. The potential for

serious deterioration in the building will often produce thermal anomalies long

before serious damage has occurred. The detection of these anomalies can lead

to corrective action before more costly repairs are required. To assure the

quality of remedial actions taken to improve existing federal buildings,

inspection techniques are also required.

A majority of federal buildings are not energy efficient because they were

designed at a time when energy was plentiful and inexpensive. Economic circumstances dictated the first costs as more important than future energy costs.

As a result, these buildings often use excessive energy. In the case of both

new and existing buildings, several diagnostic techniques can be applied to

locate and identify defects in building envelopes, thereby permitting selection

of cost effective remedial or retrofit actions to be carried out in order to

achieve energy savings.

This report presents reviews of the technical methods for inspections with

measurement methods to provide the data, procedures to conduct analyses and

apply interpretation techniques for assessing the integrity of the exterior

envelopes of federal buildings. The inspection techniques considered are:

ground-based infrared thermographic surveys, aerial infrared surveys; tracer

gas air infiltration measurement; pressurization tests for measuring the tightness of the building envelope; and spot radiometer measurements (both the temperature measuring type and radiosity-measuring type) for detecting gross

defects. Heat flow meters, a portable calorimeter and an envelope thermal

testing unit developed by Lawrence Berkeley Laboratories are also considered

for determining the thermal characteristics of the envelope. Applications of

the instrumentation and test methods are based upon previous usage experience*

and indications are provided concerning availability of equipment and services,

user training requirements, and restrictions.

The techniques described in this report can be used to find, locate and assess

the heat loss resulting from the following deficiencies:

Information from cited references, specifications, brochures and illustrative

examples, have been retained in the units presented by the authors or manufacturers

.

1
1) uninsulated walls

2) uninsulated ceilings/roofs

3) partially insulated areas

4) air leakage

5) heat loss at joints

6) thermal by-passes

7) below-grade heat loss

8) thermal bridges

9) wet insulation

10) shrinkage in foam insulation

11) settlement of loose-fill insulation

12) air movement around vapor barriers

In addition to locating deficiencies, the techniques can be used to quantify

heat losses, to determine the extent of deterioration, and to suggest remedial

action. They are applicable to both new and existing buildings, although the

manner in which they are applied will differ (see 1,2 below).

A common presentation format was adopted to describe the required method,

equipment and instrumentation; where ever applicable the information was organized as follows:

Summary

Description of Equipment

Theory

Guidelines for Conducting Measurements

Interpretation of Results

Accuracy of Techniques

Availability of Equipment and Services

Previous Usage

Personnel Training Requirements

Program Application Restrictions

Cited references for each test procedure are listed at the end of each chapter,

1.2 POTENTIAL DIAGNOSTIC PROCEDURES APPLICATIONS

The diagnostic procedures described in this report are broadly applicable to

development of cost effective reductions in the energy usage of buildings. The

appropriate methods are summarized below for use in different stages of new

construction and for existing building improvement retrofits. For new buildings the process of construction is divided into six steps; design, prototype

evaluation, construction, pre-occupancy acceptance, first year occupancy and

post occupancy evaluation. For existing buildings, the process of retrofitting

the building is divided into the following stages: diagnosis of deficiencies,

quantification of thermal deficiencies, prototype retrofit evaluation, retrofit

quality control, post retrofit evaluation, and end of warranty evaluation,

1.2.1 Application to New Construction

The application of the diagnostic procedures to the construction of new

buildings would occur in 6 stages of construction: 1) the design stage during

2
which planning of the tests are included in the traditional design methodology

of buildings; 2) a prototype evaluation of the proposed construction practices

and materials; 3) during-construction quality control; 4) preoccupancy testing;

5) during-the-f irst-year-of-occupancy evaluation of the building; and 6) post

occupancy evaluation. The tests applicable to each of these stages are summarized in table 1.2.1.

Deslg 1 of the Building

During the design of the building, plans should be made for incorporating the

test procedures into the construction process and to assure that the building

specifications are written in such a way that the results of the test can be

used to verify that the specifications are met. Building specifications must

be determined for: thermal conductances of major building sections; acceptable

air infiltration rates; the extent of tightness of building components, joints

and the total envelope.

The construction methods to be used should be analyzed in order to identify

potential areas where deficiencies in workmanship may occur and potential areas

for the existance of thermal bridges and by-passes. A time schedule for applying the tests should be developed and Interfaced with the construction schedule

for the building.

Building Prototype Evaluation

If innovative construction procedures or materials are to be used in the

construction of the building, an evaluation of prototypes of the section or components of the building should be undertaken.

The prototypes should be tested for air leakage using fan pressurization tests

for components. If possible, thermography should be used to determine abnormal

temperature patterns in the prototypes. The thermal characteristics of the

prototype should be determined using the ETTU, calorimeter or heat flow meters.

In order to ensure that the building will meet the specifications and to allow

for timely correction of poor quality workmanship which will decrease the energy

performance of the building, tests would be performed during construction to

assess the quality of the workmanship. The test procedures appropriate for

quality control are: thermographic inspection of typical finished walls and

joints, fan pressurization testing of finished components and sections and

quantitative determination of the thermal characteristics of finished building

sections using heat flow meters, the ETTU or calorimeters. The testing should

be the responsibility of the building contractor and should be performed by

his personnel or by a private inspector, under contract to him.

Preoccupancy Testing

In order to determine that the specifications for the building were met,

preoccupancy testing should be carried out. The tests for this stage are those

which could not be performed during the quality control Inspection stage; those

include air infiltration testing using tracer gas methods, total building fan

3
Table 1,2.1 Phasing of Test Procedures for New Construction

Stage

I, Design of the building 1)

2 )

3)

4)

II. Building prototype 1)

evaluation (if appropriate) 2)

3)

4)

III, During-construction 1)

quality control

2 )

3)

IV. Pre-Occupancy Testing 1)

2 ) 3)

4)

V, First Year of Occupancy 1)

VI. Post Occupancy Evaluation 1)

(or after 1st year) 2)

3)

Activity or Test Procedures Appropriate

develop building specifications which

tests must verify

a) thermal conductance

b) air infiltration rates

c) tightness of building components,

joints and total envelope.

identify potential areas of deficiencies

in workmanship

identify potential areas of thermal

bridges and by-passes

develop time schedule for applying tests

fan pressurization testing of components

thermographic inspection of prototype

section

quantitative U-value of prototype

section with ETTU or calorimeter

quantification of effect of thermal

bridges with heat flow meters

fan pressurization of finished components

and sections

thermographic inspection of typical

finished walls and joints

quantitative U-value of typical finished

wall section (ETTU or calorimeter)

complete tests of stage II which could

not be finished

air infiltration by tracer gas

total building pressurization test

complete thermographic inspection

air infiltration measurements by tracer

gas

thermographic inspection

total building fan pressurization

fan pressurization of components if total

building fan pressurization is different.

4
pressurization testing and a complete thermographic inspection. Data from

these tests should be represented in such a way that the observed deficiencies

can be corrected before occupancy of the building.

First Year Occupancy Testing (Warranty Period)

During the first year of occupancy the air infiltration rate of the building

under normal usage conditions should be measured using the air sample bag

method. The results of this test should be compared with the anticipated air

infiltration rates for the building.

Post Occupancy Evaluation

At the end of the shakedown period for the building, tests should be performed

to determine whether deterioration in the building envelope has occurred as a means of recognizing latent (hidden) defects and to note degradation rate of

materials and performance. Such information will enable criteria to be evaluated and improved. This effort can be associated with a formal Post Occupancy

Evaluation program. The tests appropriate for the evaluation are: thermographic inspection of the building in order to detect abnormal thermal patterns

in the envelope, total building fan pressurization testing and if necessary an evaluation of the air leakage of defective components using fan pressurization

techniques for air leakage deterioration or quantitative thermal tests by heat

flow meters, ETTU or calorimeters for thermal conductance defects.

1.2.2 Applications to Existing Buildings

The application of the diagnostic procedures to existing buildings would occur

in six stages: 1) a diagnostic stage in which the deficiencies in the building

are identified; 2) a quantification stage in which the energy losses and the

potential for physical building deterioration are determined where there is some doubt as to the cost/benefit of correcting the observed deficiency; 3)

the testing of prototype retrofit measures if there exists some uncertainty in

the effectiveness of proposed corrections to the observed deficiencies; 4)

during-retrof it , quality control of the retrofit work; 5) post retrofit evaluation of the workmanship; and 6) testing of the building during post occupancy

evaluation. Table 1.2.2 presents a summary of the applicable test procedures

at each of these stages. It is not proposed that all the tests listed in table

1.2.2 be applied to every retrofitted building. Stages 1, 4, and 5 should be

applied to most retrofitted buildings. Stages 2, 3, and 6 should be used only

as required or as means to provide data for future evaluation in which the

stages are not utilized.

Diagnostic Stage

The purpose of the diagnostic stage inspection of an existing building is to

identify the existing thermal deficiencies in the building envelope. For large

buildings the appropriate test procedures for diagnosing the building thermal

deficiencies are: 1) air infiltration tests using the air sample bag technique

of the tracer cleaning method; 2) ground thermographic inspection of the building's envelope from both the exterior and interior; and 3) aerial infrared

5
Table 1.2.2 Phasing of Test Procedures for Existing Buildings

Stage

I. Diagnosis Stage

II. Quantification of

Deficiencies

III. Prototype Retrofit

Testing

IV. During Retrofit

Quality Control

V. Post Retrofit

Evaluation

VI. End of Retrofit

Warranty Evaluation

(or after one year)

Activity or Test Procedure Appropriate

1) Air infiltration by tracer gas

2) Complete exterior and interior

thermographic inspection (or for

small buildings, energy audit with

spot radiometer)

3) Aerial roof survey (large building)

1) Fan pressurization tests

a) whole building

b) components

2) Heat loss from thermal bridges

with heat flow meters

3) U-values of major components if

uncertainty exists

1) Fan presurization of prototype

solutions to leakage of components

2) Thermographic inspection of

prototype

3) Quantification of U-value if necessary

1) Fan pressurization of components

for quality control

2) Thermographic inspection of quality

control

1) Whole building fan pressurization

2) Air infiltration by tracer gas

3) Complete thermographic inspection

4) Quantification of U-value if necessary

1) Fan pressurization of whole building

and/or tracer gas tests

2) Component testing if necessary

3) Thermographic inspection

6
inspection of the roof. For small buildings the tests applied would be:

1) air infiltration tests using the air sample bag techniques; and 2) either a ground thermographic inspection of the building's envelope or an energy audit

of the building using spot radiometer, if the testing proves this technique

viable. The air infiltration tests will determine if the buildings are experiencing high air exchange rates. This test should be done between 5 to 10

times under typical weather conditions with the inlet and exhaust dampers both

open and closed. The thermographic inspection (or the energy audit using spot

radiometer) should be of such nature that the major thermal deficiencies are identified in the thermal envelope. In other words, identification should be

made of areas of missing insulation, areas of improving installed insulation,

existence of thermal bridges, existence of heat bypasses, location of major

sources of air leakage, and areas of moisture damage. Inspections should be

made in such a way that the tests required in the quantification stage can be

undertaken and specific information can be given to the retrofit contractors.

The data gathered in this stage should be analyzed in such a way that the

extent of the thermal deficiencies can be determined, the cause identified,

and corrective action proposed.

Quantification of Thermal Deficiencies

After the diagnosis of the thermal deficiencies in the building, a judgment

should be made to determine whether quantitative data are required to evaluate

the effectiveness and benefits of the retrofit measure proposed to eliminate

the thermal deficiencies. The appropriate diagnostic test for this quantification are; fan pressurization tests of the whole building, using either the HVAC

system fans for large buildings or an external pressurization fan for small

buildings and the pressurization tests of typical joints and components of the

building, the measurement of the heat loss from thermal bridges with heat flow

meters; the measurement of the thermal characteristics (U-values) of major

components, if they cannot be accurately determined from building material and

construction data. In addition to determining the energy losses due to the

thermal defect, the data gathered by these tests should be used to assess the

potential for further physical building deterioration which could result from

the thermal deficiency if left uncorrected.

The tracer gas measurements of the air exchange rate of the building in the

diagnosis stage provides for an estimate of the total energy loss due to air

leakage in the building. The thermographic Inspection identifies the sources

of the leakage. The fan pressurization testing of this stage will allow estimation of the energy loss due to these leakages and allow the assessment of

specific retrofit measures. Similarly, the measurement of heat flow on typical

thermal bridges and the measurement of the thermal characteristics (U-value

and dynamic response if thermal mass effects are to be assessed) will provide

data for estimating the energy losses due to the defects observed in the

diagnosis stage.

Prototype Retrofit Testing

In large buildings, or in complexes of similarly constructed buildings, the same retrofit action may be repeated many times. In such a situation, it is prudent

7
to evaluate proposed retrofit measures In a small number of applications before

applying them to the complete building or complex. The test procedures appropriate for the evaluation of prototype retrofit measures are: fan pressurization testing of prototype solution to the air leakage of components; thermographic inspection of prototype installations; and quantification of the heat

losses using heat flow meters, the ETTU, or calorimeters.

Quality Control During Retrofit

The quality of the retrofit installation can be assessed by performing fan

pressurization tests of the leakage of retrofitted components and by thermographic inspections. The results of these tests can be compared to the results

of the prototype stage tests to determine if deviations are occurring due to

poor quality workmanship. In this type of testing, thermographic inspections

with low-resolution imaging systems should be adequate and provide the most

cost effective procedure. This testing should be the responsibility of the

installation contractor and could be performed by his personnel or by private

Inspectors under contract to him.

Post Retrofit Evaluation

The post retrofit evaluation will determine whether the applied retrofit

measures achieved the expected results in improving the energy performance of

the buiding and preventing continued deterioration of the building envelope.

The test procedures which can be used at this stage are: a complete thermographic inspection (or for small buildings an energy audit using spot radiometers); air infiltration measurements using the air sample bag technique; whole

building pressurization tests; and quantification of the heat losses from

retrofitted sections of the building using heat flow meters, the ETTU, or calorimeter.

End-of-Retrof it Warranty Evaluation

There is the potential that the applied retrofits will deteriorate with time

and lose their effectiveness. This is especially true of many solutions to air

leakage problems. At the end of the retrofit period or during post occupancy

evaluation (or after one year) tests may be applied to the building to determine whether deterioration in the retrofit measures has occured. The appropriate tests for this evaluation are thermographic inspection of the building and

air infiltration tests using either the tracer gas technique or fan pressurization of the building. If these tests show signs of deterioration in the retrofit measures, component testing using fan pressurization or quantification heat

loss measurements using heat flow meters, the ETTU, or calorimeter may be

required to assess the magnitude of the deterioration.

8


2. GROUND INFRARED THERMOGRAPHIC SURVEYS

2 . 1 SUMMARY

Infrared thermography is a qualitative technique for locating and identifying

thermal defects in a building envelope [1-6]’*. For a thermographic survey an infrared imaging system (see fig. 2.1) is used to provide an image of a surface

in which the variations in intensity of each point (usually referred to as the

gray tone) correspond to variations of the apparent radiance temperature along

the surface. Variations in the gray tone of a thermal image produced by a thermographic imaging system correspond to variations in the heat flow along

the surface and to variations in the thermal resistance along the building

envelope. A copy of the thermal image produced by a thermographic system is

called a thermogram. The most common means of producing thermograms are to

photograph the display of the thermographic system or to directly record the

video output of the system for subsequent reproduction. The thermal picture is

a hard copy which documents the type of defect to a trained investigator (i.e.,

missing insulation, air infiltration, thermal bridges, etc.) and the extent of

the defect (i.e., the percent of the building component which is uninsulated,

the length of the crack producing the air infiltration, etc.). When the thermogram is accompanied with information specifying the range in apparent radiance

temperatures included in the display and the inside-to-outside temperature difference across the envelope, the thermal pattern can be interpreted by trained

personnel for use in energy audits, as a regulatory tool for the enforcement of

codes, for the certification of a building's thermal integrity, for the quality

control of weatherization retrofits, for the detection of moisture damage to

roofing systems, and for the detection of deficiencies in mechanical heating

systems and solar systems.

2.2 DESCRIPTION OF INFRARED IMAGING SYSTEMS

There are several types of infrared devices used in the United States to

inspect buildings. These devices include: [6-9]

a) infrared thermometers (spot radiometers)

b) thermal line scanners

c) single-element raster scanners

d) multiple-element raster scanners

e) pyroelectric vidicon instruments

f) forward looking infrared (FLIR) instruments

g) Schotsky infrared diode arrays (future).

Infrared thermometers (see fig. 2.2) have been in use for building inspections

and also for the measurement of thermal conductivities of building elements.

Tests at the National Bureau of Standards and the U.S. Army Cold Regions

Laboratory [10-12] have shown that these devices do not give accurate thermal

resistance values and that operators of these devices would have difficulty

assessing the extent of most defects. However, these devices do have a useful

References listed at end of each chapter.

9
Figure 2.1 Block, diagram of thermographic system [13]

10
CAL SLIDE

AMP AMP

Figure 2.2 Schematic of basic spot radiometer [6]

function in thermographic inspections of buildings for measurement of the

absolute temperature at a location within a thermogram. The application of

spot radiometers for the determination of thermal conductivities is discussed

in section C, chapters 6 and 7, of this document.

Two types of thermal line scanners have been used for building inspections [6].

Each of these systems produces a single straight-line scan of a target along

which the emitted and reflected radiant energy from the target along this line

is displayed. The hand-held version of this system (fig. 2.3) superimposes the

thermal trace over a visual picture of the target. The other class of line

scanners used is of the aerial-type mapper where the instrument is mounted in a moving vehicle, either a van or an aircraft. Single-element raster scanners

used in the United States operate in both the 3-5 micron and 8-14 micron range,

and use both refractive (fig. 2,4) and reflective (fig. 2.5) scanning systems

to produce a two-dimensional thermal image of the target. These systems have

a temperature resolution of about 0.2 degrees Celsius and produce between 128

to 25b lines and 128 to 256 pixels per image at scan rates up to 30 frames per

second. Low-cost multiple-element raster scanners (fig. 2.6) have been developed which use multi-detector arrays [6,14] in order to reduce the constraint

imposed by single-element scanners due to the trade-off between the speed of

response and the signal-to-noise characteristics of the detector. Scanning

improvements allow a faster frame rate with no reduction in signal-to-noise

ratio, or improved signal-to-noise performance without decrease in frame rate.

However, multiple-array detectors do have uniformity limitations due to the

difficulty in matching the detectors in the array. Pyroelectric vidicon systems (fig. 2.7) are television camera tubes in which the target is made up of

a thin layer of pyroelectric material which is sensitive to infrared radiation

11
Figure 2.3 Schematic of portable line-scanner [6]

CAMERA LENS TRANSFER DETECTOROPTICAL-MECHANICAL SCANNER OPTICS HOUSING

ROTARY CHOPPER

Figure 2.4 Schematic of prism-scanning imaging system [6]

12
HORIZONTAL

MIRROR

Figure 2.5 Schematic of rotating mirror scanning infrared imaging system [6]

PRISM ASSEMBLY

6 SIGNAL AMPLIFIERS

6 INFRARED DETECTORS IN ARGON COOLED

CRYOSTAT

EYE CUP

ROTATING DOUBLE-SIDED

MIRROR WITH 10 ANGLES

OF TILT TO OBTAIN 60

SCAN LINES

INCOMING

INFRARED

RADIATION

LED ARRAY

6 LIGHT EMITTING DIODES

SILICON WINDOW

Figure 2.6 Schematic of low-cost multiple-raster infrared imaging system [6]

13
BACKING ELECTRODE FOCUSING AND SCANNING COILS

Figure 2.7 Schematic of pyroelectric vidicon

[6,15]. The target is scanned by a controlled electron beam and the image is

reconstructed electronically. These systems have been used in the past mainly

for surveillance and general purpose thermal viewing. The major limitations

for their acceptance as a practical tool for Inspecting buildings have been the

requirement for panning or chopping (the pyroelectric target responds only to

temperature changes) and the lack of uniformity in the pyroelectric layer which

leads to a non-uniform spatial response of the equipment. However, progress is

being made in the development of this type of equipment, especially with the

addition of digital electronic image processing, which could alleviate both of

these limitations in the near future.

Forward-Looking Infrared Instruments (FLIR) are high-performance real-time

scanning instruments which have excellent spatial and thermal resolution. They

have been used primarily for military and surveillance purposes. They are capable of producing thermal Images with a temperature resolution better than

0.5 degrees Celsius, with images containing 525 lines with 525 pixels per line

at 60 frames a second. These units are fully television-compatible. The

Schotsky infrared diode arrays (probably to become available in the future)

consist of two-dimensional infrared diode arrays which were developed by sev- eral manufacturers for military applications. Commercial systems of this type are not yet available for building inspections. These arrays do offer the

potential for the development of a low-cost thermal viewer. However, the

problem of matching detectors in the array may lead to spatial uniformity

deficiencies which could restrict their applications.

The cost of infrared equipment varies depending on the type: infrared spot

radiometers cost from $300 to $1500; hand-held line scanners are approximately

14
$8000; low-resolution imaging systems cost $1400; high-resolution imaging

systems range from $27,000 to $70,000; and pyroelectric vidicon systems cost

approximately $20,000. The services of infrared inspectors using high resolution equipment may be contracted for between $600 to $1000 per day plus travel

expenses

.

2.2.1 Thermographic Equipment Specifications and Evaluation

From the wide range of equipment available in the United States it is difficult

to determine which equipment characteristics are primarily important for building inspections. In general, thermographic hardware is characterized in manufacturer brochures with a variety of descriptions and non-standard specifications [10,17]o For example, thermal sensitivity is described in many cases as minimum detectable temperature difference (MDTD) with no indication given of the

spatial extent of the target. In other cases, thermal sensitivity is described

as noise-equivalent temperature difference (NEAT), minimum resolvable temperature difference (MRTD) at some unknown spatial frequency, or simply as "thermal

sensitivity." ASHRAE Standard 101 [18] has emphasized the need for more standardized specifications of the sensors used for various applications. The

parameters which are meaningful for equipment specification fall into five

categories

:

1. spatial resolution

2. noise characteristics

3. signal transfer properties

4. geometric properties

5. summary measures.

The first four of the above categories are objective laboratory measurements.

The fifth category is the most critical for qualitative use of infrared imaging

equipment, since it involves imagery interpretation by human observers at the

sensor display and incorporates the other four parameters. The performance

parameter, summary measures, is based on the observer's ability to see a standardized bar pattern when the temperature difference between the pattern and

its background is varied (see fig. 2.8). When the type of the anomaly to be

detected is specified in terms of its spatial and temperature characteristics

under given environmental conditions, it is then possible to specify the MRTD

requirement of any sensor, which might be used to conduct a particular type of

inspection, in order to detect or recognize the defect. American test standards

specify the spatial frequency for the MRTD at 0.5 cycles/cm and 0.13 cycles/cm.

The MRTD requirement is usually then specified as the temperature difference

produced by insulation levels of R-5 to R-15 (void to fully insulated wall area)

or R-10 to R-15 (partially insulated to fully insulated wall area) for a given

inside-outside temperature difference across the building. The methodology for

relating the MRTD to the ability of the infrared imaging system to detect a defect is shown in figure 2.9.

The required instrument sensitivities for interior and exterior surveys are presented below:

15
Propoftsd MRTD Test for Building Diagnostic

Sensors

Oistanca,

D

IR

Figure 2.8 Schematic of MRTD test for infrared imaging systems [16]

Relationship Between MRTD

and Recognition of Structural Faults

Crltleal Fault Dimension.h

Figure 2.9 Relation between MRTD and building defects [16]

16
Interior Surveys

Required instrument sensitivities for interior surveys are given in table 2.1.

Table 2.1 Required Instrument Sensitivities for Interior

Surveys (MRTD)

Tj-Tq MRTD °F

”F R5/R15 R10/R15

10 0.5 0.125

20 1.0 0.250

30 1.5 0.375

40 2.0 0.500

50 2.5 0.625

The first column of the minimum resolvable temperature

differences are those required to distinguish an R5 region

from an R15 region; the second column are the differences

required to distinguish an RIO region from an R15 region.

Tj - inside air temperature, Tq - outside air temperature.

Consider a situation in which it is desired to determine the location of a partial void which has a thermal resistance of R-10 compared to the wall around

it whose thermal resistance is R-15. In this case, it is essential that the

infrared device have sufficient sensitivity to enable distinguishing between

R-10 and R-15. For an outdoor air temperature 20“F lower than the Inside air

temperature, use of table 2.1 indicates that a system having an MRTD of 0.25°F

or less is required. Similarly, for a greater indoor/outdoor temperature

difference, an Infrared device having a larger MRTD is permissible (i.e., for

a temperature difference of 40°F, table 2.1 indicates an MRTD of 0.5°F).

Exterior Surveys

When the purpose of an exterior infrared survey is to distinguish an uninsulated region of a building envelope (R-5) from an insulated region

(R-15), the instrument sensitivity, expressed in terms of MRTD at 30°F, must

be less than the value given in figure 2.10 for an infrared scanning system

operating in the 8-14 micron wavelength band and less than the values given in

figure 2.11 for an infrared imaging scanning system operating in the 3-5 micron

wavelength band. Here the wind speed is the local wind speed moving past the

exterior surface being examined. A considerably lower MRTD is required for an infrared scanning device operating in the 3-5 micron wavelength band than one operating in the 8-14 micron wavelength band, due to the much greater effect

of object temperature on instrument sensitivity for the 3-5 micron wavelength

band. Consider the following example:

17
REQUIRED

INSTRUMENT

SENSITIVITY

(°Fj

WIND SPEED (mph)

Figure 2.10 Instrument sensitivity requirement R5/R15 for

recognition @ 8-lA microns-exterior survey [18]

Figure 2.11 Instrument sensitivity requirements R5/R15 for

recognition @ 3-5 microns [18]

18
2.5

r T

WIND SPEED, MPH

Figure 2.12 Instrument sensitivity requirement R2/R10 for

recognition @ 8-14 microns [18]

WIND SPEED, MPH

Figure 2.13 Instrument sensitivity requirement R2/R10 for

recognition @ 3-5 microns - exterior survey [18]

19
A thermographic Imaging system is used to locate insulation voids of an exterior wall of a building. At the time of the survey, the local wind speed

moving past the surface is 7.6 mph and the indoor-outdoor temperature difference is 50°F. The required MRTD referenced to 86°F is 0.54°F for an infrared

imaging device operating in the 8-14 micron wavelength band (see fig. 2.10)

and 0.22°F for an infrared imaging system operating in the 3-5 micron wavelength band (see fig. 2.11).

2.2.2 Conditions Under Which Thermographic Surveys Can Be Performed

The conditions under which thermographic data can be obtained depend upon both

the types of defects which are to be detected and the type of infrared imaging

systems employed [20-21] . For high resolution imaging systems (systems with a minimum resolvable temperature difference less than 0.54°F with a spatial frequency of 0.52 cycles/cm), it suffices that a) the minimum temperature difference across the building envelope is greater than 18°F for a period of at least

8 hours prior to inspection, b) the transient conditions are such that there is no more than a 30-percent variation in the temperature across the surface for a period of 24 hours prior to the inspection, and c) that there is no direct solar

radiation on the inspected surface for a period of 3 hours for light construction and 8 hours for masonry and masonry veneer construction. During exterior

inspections the wind should be less than 15 mph and the surfaces should be dry.

It is recognized that both exterior and interior thermographic surveys are technically possible. If it is possible to observe a class of defects from the

interior, it is generally preferable to obtain data from the Inside; greater

temperature variations exist on interior surfaces than on exterior surfaces and

the sensitivity of infrared equipment is greater at interior ambient conditions

than it is under normally encountered exterior ambient temperatures. However,

certain practical considerations , such as the ease of performing the survey and

the accessibility, often dictate the type of survey. Also, certain classes of

defects such as heat bypasses into attics, excessive basement losses, and roof

inspections for moisture damage, are best observed from the exterior.

The above conditions guarantee that the building is sufficiently close to steady

state so that the anticipated thermal patterns of the defects are those produced

in steady state. Although it is preferred that steady state conditions exist

at the time of inspection thermographic inspections can be undertaken under

other conditions if sufficient knowledge is used in taking and interpreting the

thermographic data. Under different conditions, other classes of thermal patterns will exist. For example, a wall exposed to direct solar radiation will,

after a sufficient time, experience a temperature reversal — studs and voids

will appear warm and the insulated section cold on interior inspection. An

exterior inspection about an hour after sunset will show the studs warmer than

the Insulated section on both interior and exterior surveys due to the differences in the thermal capacities of the two sections. On many veneer surfaces,

interior surveys are technically possible an hour or two after the sun has been

shining on them. In southern climates, roofs suspected of moisture damage can

be inspected in the evenings after being exposed to solar radiation. In such

cases, the interpretation of the thermograms is not based on differences in the

thermal conductance between the damaged and undamaged sections but on the

differences in their heat storage.

20
In using thermographic systems operating in the 8-12 micron range, care should

be taken to ascertain that none of the apparent image is caused by the reflection of thermal radiation from extraneous heat sources. An extraneous heat

source can be an adjacent hot object such as a radiator, a chimney, the operator

himself or the night sky.

2.3 CONDUCTING INTERIOR SURVEYS

2.3.1 Measurement Technique

For an interior survey, a thermographic imaging system is used inside a building. A series of thermal images is obtained for all portions of the

interior envelope of the building. For each measurement, the infrared camera

is placed sufficiently far from the subject surface that the field of view of

the infrared camera will subtend as much of the interior surface as is possible

without loss of thermal and spatial resolution.

The sensitivity setting of the equipment is adjusted at the highest possible

setting (lowest minimum resolvable temperature difference) which will prevent

all portions of the subject surface from being displayed at a saturated level

(either white or black). If prominent thermal defects are displayed in the

thermal picture, the oscilloscope screens of the television monitor are photographed, using a conventional camera. To assist in the interpretation of

results it is desirable to obtain a visible-spectrum photograph of the same scene displayed in the infrared picture.

A 2.3.2 Guidelines for Conducting Surveys

Preferably, thermographic surveys are performed within a building rather than

from the exterior in order to avoid the problem of wind "scrubbing away" surface temperature differences depicting thermal anomalies in the building

envelope and degrading instrument sensitivity due to reduced object temperature.

It is recommended that at least an inside-to-outside temperature difference of

20 °F exist at the time of a survey, so that sufficiently large temperature

differences will occur to permit thermal anomalies in the building envelope to

be readily distinguished. Surveys may be carried out during either the day or night. For day surveys it is important that solar loading not negate insideto-outside temperature differences, thereby causing thermal anomalies to become

indistinguishable. Night surveys are preferred because more stable heat transfer conditions will usually prevail; meaningful survey results will be obtained

by the elimination of interference from solar loading and reflections.

Preparation for the survey may include elimination, to the extent practical, of

thermal artifacts such as furniture located near exterior walls. During the

survey, a thermogram shall be taken of each surface of the building where

anomalies may exist and which are regarded as significant (i.e. major air

infiltration paths, large uninsulated regions of a wall or ceiling, etc.).

* Mostly from ref. [18].

21
Data should be recorded with each thermogram to proivde reference to a calibrated gray scale for the purpose of indicating the relationship between

contrast and temperature difference. Often, the reference requirement can be

accomplished by simply recording the instrument sensitivity setting, which is

the range of radiance temperature displayed in the thermal picture. That

information, along with the inside-to-outside temperature difference, will permit a thermal anomaly to be interpreted correctly by a third party. During the

survey, the following data should be recorded:

1. date of survey

2. time of survey

3. indoor temperature

4. outdoor temperature

5. general wind condition (i.e., calm, moderate wind, etc.)

6. unusual surface finish (i.e., low emittance surface)

7. building construction

8. data for calibrating gray-tone scale

2.4 CONDUCTING EXTERIOR SURVEYS

2.4.1 Measurement Technique

For an exterior survey, a thermographic imaging system is used to obtain a series of thermal images of the exterior envelope of a building. For each

measurement, the infrared camera is placed sufficiently far from the subject

surface so as to provide the maximum field of view for the infrared camera without loss of thermal and spatial resolutions. The sensitivity setting of

the equipment is adjusted at the highest possible setting (lowest minimum

resolvable temperature differences) which will prevent all portions of the

subject surface from being displayed at a saturated level (either white or

black.). If conspicuous thermal defects are displayed in the thermal picture,

then the oscilloscope screens of the television monitor are photographed,

using a conventional camera. It is desirable to obtain a visible-spectrum

photograph of the same scene displayed in the infrared picture to assist in

the interpretation of results.

204.2 Guidelines for Conducting Surveys

In 2.3.2 it was noted that thermographic surveys are preferably performed from

within a building rather than from the exterior. However, exterior surveys are frequently more readily accomplished.

The infrared imaging system used for the survey may be portable, cart-mounted,

ground-vehicle-mounted, or mounted on an aerial platform. The field of view

of the infrared imaging system should be at least 18 degrees. In carrying out

the survey, an attempt should be made to include as much of the exterior sur- face, as possible, of the building without causing its horizontal field of view

to subtend more than 10 meters and its vertical field of view to exceed 6

meters. Surveys should not be carried out with larger fields of view unless

it can be shown that the MRTD of the instrument meets the requirement of fig~

ures 2.10, 2.11, 2.12 and 2.13 at the particular field of view. Surveys should

22
1

be carried out at least 3 hours after sunset and before sunrise to avoid

surface temperature differences from variations in solar absorptance which

could be incorrectly interpreted as thermal anomalies. The minimum inside-tooutside temperature difference at the time of survey, and for a minimum of 4

hours prior to the survey, should not be less than 18°F. Exterior surveys

should not be carried out during periods when the wind speed exceeds 15 mph, in

order to avoid the problem of the wind completely scrubbing away temperature

differences depicting thermal anomalies.

Exterior surveys should proceed to the extent practical only when the view of

building surfaces is substantially clear of water, snow, and/or ice, and when

no fog is present. Data should be recorded with each thermogram to allow

reference to a calibrated grey scale, or two reference temperatures, for the

purposes of indicating the relationship between contrast and temperature

difference. Often this may be accomplished by simply recording the instrument

sensitivity setting which is the range in radiance temperature displayed in the

thermal picture. This information, along with the inside-to-outside temperature

difference, will permit a thermal anomaly to be interpreted correctly by other

investigators. For the purposes of carrying out exterior surveys of buildings,

infrared cameras operating on the 8-14 micron region are preferred over those

operating in the 3-5 micron region, in order to obtain much greater instrument

sensitivity. Consider the following example;

Both 8-14 micron and 3-5 micron infrared detectors have an MRTD of 0.36°F at

86°F. When these detectors view an object at 68°F, the MRTD for the 8-14

micron detector is degraded to 0.5°F whereas the MRTD for the 3-5 micron detector is degraded to 1.2°F.

During the survey, the following data should be recorded;

1 . date of survey

2. time of survey

3. indoor temperature

4. outdoor temperature

5. data for calibrating gray-tone scale

6. wind speed at each surface measured

7. unusual surface finish (i.e., low-emittance surface)

8. building construction

As a final note, when conventional photographic equipment is used outdoors,

the required exposure times for films may be substantially altered at temperatures below normal (room) temperature. Guidelines for compensating for this

effect can be obtained from photographic film manufacturers.

2.5 INTERPRETATION OF THERMOGRAPHIC DATA

The interpretation of thermographic data [20-21] is a process which consists of

comparing the thermal patterns of a building with previously observed and cataloged thermal patterns for various types of thermal defects. The major steps

in this process are; 1) determination of anticipated thermal patterns, given

physical building data and the environmental conditions to which the building

23
was subjected, 2) acquisition of actual thermal patterns under environmental

conditions which will produce thermal irregularities identifiable by the infrared system employed and for the defects which one desires to observe, 3) deduction of the temperature distribution on the inspected surface, correcting for

emissivity changes, and eliminating apparent temperature patterns produced by

the reflection of radiation from extraneous sources, 4) comparison of the

observed temperature patterns with the anticipated thermal patterns, 5) deduction of the types of defects in the structure which could produce the observed

irregularities in the thermal patterns, and 6) verification of the conclusions,

using ancillary measurements, if reasonable uncertainty remains, as to the cause

of the thermal irregularities.

The inspection of buildings using infrared scanning systems can produce

information which is useful for locating and assessing heat losses from buildings, such as: 1) areas of the exterior wall which were not insulated; 2) areas

of the roof not properly insulated; 3) shrinkage or fissures in fully insulated

wall areas; 4) air leakages around doors and windows; 5) excessive heat loss

around door and window frames; 6) excessive heat loss in the joints between

walls, ceilings, and floors; 7) excessive heat loss in unconditioned spaces,

8) air penetration into interior cavities; and 9) moisture damage to insulation.

Each of the above defects produces thermal patterns which have distinct pattern

characteristics. In the evaluation of thermographic data one should assess:

1) the lack of uniformity of the observed radiation intensity in the image;

2) the coincidence of changes in radiation intensity with structural features;

3) the sharpness and nature of the contours separating areas with different

thermal characteristics; and 4) the changes in thermal patterns caused by

equipment positioning and adjustment.

Some typical defects observed in single-family dwellings [20] are shown in

figures 2.14 to 2.19. Missing insulation in two cavity-filled walls is detected

in the thermal images taken from the interior of the dwellings in figures 2.14

and 2.15. Missing insulation usually produces regular and well-defined shapes

not associated with structural features; the contour of the boundary between

the void and the insulated sections is usually sharp. In frame construction,

voids usually appear colder than the framing members. Two levels are observed

in the thermal pattern in figure 2.16. That both of these levels are insulated

can be ascertained by noting that the studs are colder than the area between

them in both regions. Figures 2.17 and 2.18 show examples of air leakage. Mr

leakage usually appears at joints and junctions in the building envelope, and

often produces thermal patterns with irregular variations. In the interpretation of thermograms for the purpose of locating voids or missing insulation,

some care should be exercised to ensure that the thermal patterns observed are not due to air leakage around improperly installed insulation. Information on the condition of the wall before the application of the insulation, the type of

insulation installed, and the location of possible sources of air penetration,

helps in correctly interpreting thermal patterns.

The thermogram in figure 2.19 shows the type of information which can be

observed during exterior surveys. Figures 2.20 and 2.21 show some examples of

thermal reflections observed during surveys using 8-14 micron systems.

24
Figure 2.15 Example of thermogram

of voids in a cavity

wall retrofitted with

cellulose insulation.

Figure 2.16 Example of thermogram

showing two levels of

insulation in a cavity

wall initially insulated

with 3-1/2" glass fiber

batts over which foam is

installed as a retrofit.

Figure 2.17 Thermogram showing air

infiltration around an improperly weatherstripped door.

Figure 2.14 Example of thermogram

voids in a cavity wall

retrofitted with foam

insulation.

25
Figure 2.18 Thermogram showing air

penetration under

ceiling insulation.

Figure 2.20 Example of thermal

reflection recorded

by an 8-14 micron

system. Image in the

side of the house is

the neighboring house.

Figure 2.19 Example of an exterior

thermogram of a retrofitted dwelling. Notice

the missing insulation

in a bay cavity and

the abnormally warm roof

Figure 2.21 Example of thermal

reflection recorded by

an 8-14 micron system.

Light area along the

chimney is a reflection

of the chimney in the

siding.

26
Reflection can be detected by viewing the surface from different angles. If

the patterns move, they probably are due to reflections from extraneous sources.

2.6

ACCURACY OF THERMOGRAPHIC SURVEYS

Thermographic surveys are qualitative at the present state of development. If

the guidelines for carrying out the thermographic surveys are fulfilled, then

properly trained theriaographers can detect the classes of thermal deficiencies

in over 90 percent of the cases [2]. However, untrained operators do not

usually arrive at consistent results [28, 45, 46]

.

The difficulties seem to arise from: 1) performing inspections under unsuitable

conditions; 2) not understanding fully the principles of heat transfer in buildings; 3) not understanding the characteristics of the infrared equipment; 4) not

performing simple additional measurements to verify the conclusions drawn from

the infrared inspections; and 5) attempting to perform infrared inspections with

a minimum of effort.

If there are doubts as to the results of a thermographic survey, ancillary

measurements should be made.

2.7

AVAILABILITY OF THERMOGRAPHIC EQUIPMENT AND SERVICES

There are approximately six manufacturers of spot radiometers in the United

States, two manufacturers of hand-held line scanners, two manufacturers of

low-resolution imaging equipment, four manufacturers of pyroelectric vidicon

and three manufacturers of high-resolution imaging systems. There are over 50

firms in the United States offering thermographic services for energy conservation assessments. A detailed description of most of the manufacturers and ser- vices can be found in ref. [9] , as of December 1978. High resolution infrared

cameras cost in the range of $25,000 to $500,000, low-resolution systems cost

$8,000 to $14,000; thermographic survey services without analysis ranges from

$500 to $700 per day.

2.8

TRAINING OF THERMOGRAPHIC OPERATORS

One of the major difficulties in successfully applying thermography to

buildings has been lack of trained operators and interpreters of thermographic

data. This shortcoming has been clearly recognized by Public Works Canada [20-

22] who have developed a training program for the various levels of technical

expertise required for performing, analyzing and interpreting thermographic

surveys of buildings.

Public Works Canada has defined three levels of investigation in building

thermography, each of which requires a different degree of professional

expertise. The levels of investigation consist of the following.

Level I (Thermographic) : Locating thermal anomalies and interpreting causal

mechanisms. Both activities produce qualitative results and are performed by a thermographer (i.e., a trained IR-system operator with para-professional

knowledge of building science).

27
Level II (Thermographic): Interpreting the significance of the problem in the

construction and recommending appropriate remedial action. Both activities

produce qualitative results and are performed by a thermologist (i.e., a trained IR-system operator with professional knowledge of building science).

Level III (Building Science): Interpreting the cause and effect of building

problems, and detailing design recommendations to correct existing problems

and prevent future ones. All activities produce qualitative results and are performed by a building scientist (i.e., a trained interpreter of IR thermal

images with specialized knowledge of building science).

For each of these levels, a person must obtain an appropriate level of

proficiency in;

• the fundamentals of building physics,

• infrared theory and equipment,

• building enclosure and heat transfer principles.

In implementing a diagnostic program for federal buildings, the Canadian

training program provides a useful model. It will be necessary to have federal

employees exposed to the basic principles put forth in the Canadian training

program in order to be able to communicate effectively with the professionals

who perform and interpret the results of the thermographic surveys. The professionals who undertake the surveys should be trained to the level specified

in the Canadian training program,

2.9 PREVIOUS USES OF THERMOGRAPHY

Thermography has been widely used as a research tool in the United States. A

summary of published work on thermography can be found in reference [23].

Applications where thermography have been shown to be effective are:

a. Quality Control of Building Retrofits [24-27]. In this application, the use of thermography seems economically viable. The retrofit of buildings

is difficult under the best of circumstances, and it has been demonstrated

that (although the building owner has invested a sizable amount of money to

fix the building) much of the work is poorly done and that there are often

serious defects remaining in the building.

b. Moisture Damage to Roofs [2B-31]. In this application, thermography can be

used to identify those areas which have been damaged, and also the sources

of the moisture. There are good economic reasons for repairing only the

damaged areas, not the whole roof.

c. The Inspection of Commercial and Institutional Buildings [32-42]. The

pioneering work in the area is now being performed at Public Works Canada.

They have undertaken a program to inspect 100 federal buildings in the

various provinces of Canada.

d. The House-Doctor Approach to Weatherization. Princeton University has

developed a method of using infrared imaging systems, along with fan

28
depressurization of the building, to discover air leakage paths. These

leakage paths are repaired at the time of inspection with the infrared system. (This may be the only viable means of being certain that a dwelling

is air tightened properly.)

2.10 PROGRAM RESTRICTIONS TO THE APPLICATION OF THERMOGRAPHIC SURVEYS

There should be no serious program restrictions to the application of

thermographic surveys to the diagnosis of federal buildings. However, some care should be taken in its implementation. The inspection of the buildings

in warm climates, though theoretically possible, is a problem area where little

practical experience exists. Thus, earlier stages of the program should be

structured to emphasize colder climates, progressively moving to warmer climates as experience is gained. The number of buildings inspected should be

consistent with staff capabilities to utilize the results, i.e., thermographic

surveys produce large amounts of data and there is a tendency to collect these

data and then not properly or completely analyze them. Emphasis should be

placed on data analysis techniques and methods for presenting the data in a form useful to the building operators.

29
REFERENCES

[1] Paljak, L. , and Pettersson, B. Thermography of Buildings, Swedish Council

of Building Research, Svensk Byggtjinst, Stockholm, Sweden, 1972.

[2] Pettersson, B., and Axen, B» Thermography - Testing of the Thermal

Insulation and Untightness of Buildings, Swedish Council of Building

Research, Svensk Byggtjinst, Stockholm, Sweden, 1980.

[3] Mill, P. A. D. The Principles of Building Science and Thermography

Needed to Diagnose the Performance of Building Enclosures, Proceedings

of ASHRAE/DoE-ORNL Conference on Thermal Performance of the Exterior

Envelopes of Buildings, ASHRAE SP 28, New York, 1981.

[4] Mill, P. A, D. Development of PWC Capabilities in Thermographic

Diagnosis of Building Envelope Deficiencies, Public Works Canada, Report

Series No. 27, Ottawa, Canada, 1979.

[5] Kaplan, H. ABC's of Infrared Radiation as Applied to Energy Loss in

Structures, Proceedings of Thermosense II, American Society of

Photogrammetry , Falls Church, VA. : 15-36; 1980.

[6] Kaplan, H. Infrared Equipment Available for Energy Surveys, Proceedings

of Therraosense II, American Society of Photogrammetry, Falls Church, VA.

87-102; 1980.

[7] Shumaker, D. L. , Snow, F. J., Barthle, R. C. , Culp, T. L. , and Wood, J. T.

Status of Thermal Imagery Technology as Applied to Energy Conservation,

DoE Report, HCPIM 4228-01, 1979.

[8] Headley, R. B., Larsen, R. J., Goldberg, G. G. , and Boyd, R. J. Infrared

Thermography Requirements Study for Energy Conservation, report prepared

for the Energy Research and Development Administration, Contract

No, EX-76-C-01-2109, 1977,

[9] Mill, P. A. D. Developments of PWC Capability in Thermographic Diagnosis

of Building Envelope Deficiencies, Public Works Canada, Report No 27,

Ottawa, 1979.

[10] Grot, R. A., Munis, R. H. , Marshall, S. J., and Greatorex, A. A

Comparative Test of Applicability of Various Thermal Scanning Systems

for Detecting Heat Losses in Buildings, Proceedings of the Fourth Biennial Infrared Information Exchange, AGA Corp., Secaucus, NJ., 1979.

[11] Munis, R. H, , and Grot, R. A, Current Status of ASHRAE Standard for

Conducting Tlsermographic Energy Audits of Buildings, Proceedings of

Thermosense I, American Society of Photogrammetry, Falls Church, VA.

,

1979.

30
[12] Treado, S. J. , and Burch, D. M. Evaluation of Hand-Held Infrared

Thermometers for Wall Thermal Resistance Determination, Proceedings of

the Fourth Biennial Infrared Information Exchange, AGA Corp., Secaucus,

N.J., 1979.

[13] Hurley, C. W. , and Krleder, K. G. Applications of Thermography In

Industry, Proceedings of Infrared Information Exchange 76, AGA Corp,

Secaucus, N.J.

[14] Dirk, J. TE-Cooled Sensors for Thermographic Instruments, Proceedings

of Thermosense IV, Society of Photo-Optical Instrumentation Engineers,

1982.

[15] Levitt, R. S., Rao, N.V., and Fisher, R. Pyroelectric Vldlcon Camera

Systems: Status and Capabilities, Proceedings of Thermosense IV, Society

of Photo-Optical Instrumentation Engineers, 1982.

[16] Wood, J. E., Collins, J. J., and Snow, F. J. Application of Image

Evaluation Technology to Heat Loss Diagnostic Sensors, SPIE 23rd Annual

Technical Symposium, August, 1979, San Diego, CA.

[17] McIntosh, G. B. and Flllppene, A. F. Minimum Resolvable Temperature

Difference (MRTD) Testing: Equipment Specification for Building Performance Diagnostics, Proceedings of Thermosense IV. Society of PhotoOptical Instrumentation Engineers, 1982.

[18] Application of Infrared Sensing Devices to the Assessment of Building

Heat Loss Characteristics, ASHRAE Standard 101, Atlanta, 1981.

[19] Thermal Insulation - An Infrared Method for Qualitative Detection of

Thermal Irregularities In Buildings, International Standards

Organization, draft proposal ISO/TC 163/SC1/WG4.

[20] Grot, R. A. Interpretation of Thermographic Data for the Identification

of Building Heat Loss, Proceedings of the Society of Photo-Optical

Instrumentation Engineers, Vol. 254: 24-29; 1981.

[21] Mill, P. A. D. Thermographic Interpretation of Building Enclosure

Deficiencies In Canadian Public Buildings, Proceedings of the Society of

Photo-Optical Instrumentation Engineers, Vol. 254: 121-137; 1981.

[22] Mill, P. A. D. Canadian Experience In Certification and Training of

Thermography Personnel, Proceedings of Thermosense III, Society of PhotoOptical Engineers, Bellingham, WA. , Vol. 254: 169-184; 1981.

[23] Mill, P. A. D. , Kaplan, A. G. , and McIntosh, G. B, Thermographic Training

Manual, Level I, International Building Thermographic Training Course,

Public Works Canada, Ottawa, Department of Energy, USA, and General

Services Administration, USA, March, 1981.

31
[24] Marshall, S. J. Infrared Thermography of Buildings - An Annotated

Bibliography, USACRREL SR 77-9, March 1977.

[25] Grot, R. A., Harrje, D. T., Johnston, L. C. Application of Thermography

for Evaluating Effectiveness of Retrofit Measures, Proceedings of

Infrared Information Exchange 1976, AGA Corp., Secaucus, NJ.

[26] Grot, R. A. An Assessment of the Application of Thermography for the

Quality Control of Weatherization Retrofits, Proceedings of Thermosense

II, American Society of Photogrammetry , Falls Church, VA. , 1981.

[27] Harrje, D. T., and Grot, R. A. Energy Conservation in Housing:

Retrofitting and Documentation, Proceedings of the First Canadian

Symposium on Thermography and Energy Conservation, 1976.

[28] Axen, B., and Holmsten, D. Swedish Experience in the Application of

Thermography for Establishing Porosities and Quality Control in Residential Dwelling Retrofits, Proceedings of the Society of Photo-Optical

Instrumentation Engineers, Vol. 254: 77-90; 1981.

[29] Toblasson, W. and Korhonen, C. Detecting Moisture in Roof Insulation with

Hand-Held Infrared Camera, Second International Symposium in Cold Region

Engineering, University of Alaska, August 1976.

[30] Sopko, V. Detection of Moisture in Commercial and Industrial Built-Up

Roofing Systems, Proceedings of Thermosense IV, Society of Photo-Optical

Instrumentation Engineers, 1982.

[31] Duffy, P. R. Low-Re solution Infrared Building Thermography in a Subtropical Environment, Proceedings of the Society of Photo-Optical

Instrumentation Engineers, Vol. 254: 139-144; 1981.

[32] Marshall, S. J. Detecting Moisture in Buildings Using Thermography,

Proceedings of the Society of Photo-Optical Instrumentation Engineers,

Vol. 254: 111-117; 1981.

[33] Malone, R. A« , and Orlove, G, L. Benefits of Commercial and Industrial

Building Envelope Thermography, Proceedings of Thermosense II, American

Society of Photogrammetry, Falls Church, VA. : 161-170; 1980.

[34] Hawley, R. Public Works Canada - Regional Thermographic Project,

Proceedings of Thermosense IV, Society of Photo-Optical Instrument

Engineers, 1982.

[35] Munis, R. H. , R. H. Berger, Marshall, S. J. and Bash, M. A. Detecting

Structural Heat Losses with Mobile Infrared Thermography, Part I, Part

II, Part III and Part IV, USACRREL RR 326, 348: 76-33; 1974 through 1976.

[36] Munis, R. H., and Marshall, S. J. Infrared Thermography of Buildings:

Qualitative Analysis of Five Buildings at Rickenbacker Air Force Base,

Columbus, Ohio, USACRREL, SR 77-26, Sept. 1977.

32
[37] Munis, R. H. and Marshall, S. J. Qualitative Assessment of Window Heat

Loss Using Infrared Thermography, Proceedings of the First Canadian

Symposium on Thermography and Energy Conservation, 1977.

[38] lunis, R. H., and Marshall, S. J. Qualitative Assessment of Window

Infiltration Loss, Federal Office Building, Burlington, Vermont,

USACRREL SR 77-29, Sept. 1977.

[39] Munis, R. H. , and Vogel, P. E. J. Excess Heat Loss Study of the

Harold F. Scott School, Warwick, Rhode Island, USACRREL, March 1977.

[40] Munis, R. H. , Marshall, S. J., and Vogel, P. E. J. Pinpointing Locations

of Excessive Heat Loss in Ten Selected Office Buildings, Concord, New

Hampshire, USACRREL, 1978.

[41] Mill, P. A. D. Thermographic Diagnosis of Building Deficiencies,

Public Works Canada, Report No. 28, Ottawa, 1979.

[42] Mill, P. A. D. Thermography - New Science Tool, Public Works Canada

Report No. 29, Ottawa, 1979.

[43] Harrje, D. T., Dutt, G. S., and Beyea, J. Locating and Eliminating

Obscure but Major Energy Losses in Residential Housing, ASHRAE Trans.

Vol. 85, Part II: 521-534; 1979.

[44] Harrje, D. T., Dutt, G. S., and Gadsby, K. J. Isolating the Building

Envelope, Proceedings of Society of Photo-Optical Engineers, Vol. 254:

50-56; 1981.

[45] Regan, R. Comparative Analysis of the Performance of IR Contractors for

Quality Control of Retrofit Insulation of Residential Dwellings,

Proceedings of Thermosense IV, Society of Photo-Optical Instrmentation

Engineers, 1982.

[46] Chang, Y-M. , and Grot, R. A. Quality of Inspection by Infrared

Thermographic Systems for Weatherization Retrofit Installations, NBSIR

82-2510, 1982.

33
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3. AERIAL INFRARED SURVEYS

3 . 1 SUMMARY

Aerial infrared thermography is an imagery process utilizing an infrared line

scc.i nar generally mounted underneath an aircraft, which produces an apparent

radiance temperature in a partial map of the terrain below the aircraft.

In surveying low-slope built-up roofs of commercial and industrial buildings,

this technique is usually effective in identifying those roofs that are either

uninsulated or have defective roof insulation. This technique has been shown

to be particularly effective in locating regions on individual built-up roofs

that have defective, missing, or wet insulation thereby permitting local

repairs to be carried out.

With regard to surveying pitched ventilated roofs of residential buildings,

variations in ceiling resistance cause differences in apparent radiance temperatures among the roofs displayed in an aerial infrared image which are smaller,

by approximately a factor of three, than those for low-slope built-up roofs.

Field surveys have shown that under optimum climatic conditions (i.e., clear

sky and low wind), a highly sensitive infrared imaging system viewing a small

cluster of residential roofs having the same roof emittance and exposed to the same microclimate will produce a thermal picture, ranking the buildings cor- rectly according to their heat loss characteristics. Under non-optimum conditions the variations in roof emittance, local wind speed and local outdoor

temperature (throughout the macroclimate) may often cause differences in apparent temperature which mask out those differences in radiance due to variations

in ceiling resistance. When viewing individual pitched ventilated roofs, it is

not possible to distinguish uninsulated portions of the ceiling.

3.2 CONCEPT OF AERIAL INFRARED THERMOGRAPHY

In conducing an aerial infrared survey, an infrared line-scanning system

mounted in an aircraft scans the terrain, building up a thermal image as the

aircraft progresses along a flight line, as shown in figure 3.1. The scanner

is basically an optical telescope, with its narrow field of view continuously

redirected by a spinning flat mirror. The mirror causes the system to scan in

a plane perpendicular to the direction of flight of the aircraft. Such systems

have one or more cryogenically cooled thermal radiation detectors in the focal

plane of the telescope which convert the focused thermal energy received by the

detector into an electrical signal. This electrical signal can be processed

into a visual image on a cathode-ray tube (CRT) which can be photographically

recorded or digitized and recorded on a magnetic tape recorder for future

processing .

An aerial infrared photograph of a large number of residential roofs is shown

in figure 3.2. Variations in gray tone from light to dark correspond directly

to variations in apparent radiance temperature. Parts of the thermal picture

Much of this description was taken from ref. [1]

34

Fiqure 3.1 Schematic illustration of the use of a line scanner to perform

aerial infrared thermography [1]

35
Figure 3.2 An aerial infrared photograph of a large number

of residential roofs (provided by Texas Instruments)

Figure 3.3 Transmittance of the whole atmosphere as a function

of wavelength [2]

36
appearing in lighter gray tones have a higher apparent radiance temperature

than other parts of the thermal picture. Infrared line-scanning systems usu- ally have a sweep angle (0), as depicted in figure 3.1, which does not exceed

110 degrees, in order to restrict distortion at the outer edges of the thermal

picture [3J. Flights are most often conducted at altitudes between 1000 and

2000 ft. The instantaneous field of view of the infrared line scanner varies

with the particular system but will generally range between 1 and 2.5 milliradians on a side. Aerial infrared thermography is usually performed in the 8

to 14 micron wavelength band because of the high atmospheric transmission over

that part of the infrared spectrum (see figure 3.3). Atmospheric transmission

data presented in figure 3.4 shows that the average atmospheric transmittance

at a distance of 1640 ft above ground level are 0.93 and 0.97 at outdoor relative humidities of 95 and 20 percent, respectively. In addition, the same data

shows that an increase in path length due to an increase in the view angle, has

a very small effect on the atmospheric transmittance over this particular

wavelength band.

The theoretical consideration for thermal radiation sensed from surfaces, such

as roofs, by an infrared line scanner includes both self-emitted radiation and

reflected radiation from the sky. The apparent radiance temperature (T^) is

defined as the temperature of a perfectly black surface which would radiate the same amount of thermal radiation as a real surface at a temperature (Tg) and

having a surface emittance (e). An equation relating the apparent radiance

temperature of a surface to its actual temperature has been derived by Goldstein

[5j. For the convenience of the reader, a derivation for the equation is given

below.

For the 8-14 micron wavelength band, the self-emitted energy from a perfectly

black surface at the apparent radiance temperature (T^) is equal to the selfemitted and reflected sky radiation from the actual surface at a temperature

(Tg), or

Ta’^ = e-Tg"^ + ( 1-e )

• (3.1)

Here the radiant energy from the sky is based on the spectral sky temperature

(Tgky) instead of the calorimetric sky temperature (Tgjj^y) , since radiant

energy is being considered only over the 8-14 micron portion of the infrared

spectrum. The spectral sky temperature is lower than the calorimetric sky

temperature because the atmosphere is relatively transparent over the 8-14

micron wavelength band. The exponent n is 5 instead of 4 because radiation is

only being considered between 8-14 microns instead of the whole infrared spectrum.

Solving eqn. (3.1) for (T^) gives:

- T„ •[£ + (1-e)«3^]

1/5

(3.2)

Here 3 is the ratio of the spectral sky temperature (Tg^^y) to the surface

temperature (Tg). The accuracy of equation (3.2) was investigated by numerically integrating the Planck distribution function for the self-emitted and

37
ATMOSPHERIC

TRANSMITTANCE

Figure 3,4 Average atmosphere transmittance as affected by

incident angle and outdoor relative humidity [4]

38
reflected components of the roof radiance. Equation (3.2) was found to be

accurate to within a fraction of a degree Fahrenheit over a representative

range of surface temperatures and sky temperatures. It should be pointed out

that equation (3.2) does not account for thermal reflections from nearby

buildings

.

3.3 ACCURACY OF TECHNIQUE

3.3.1 Pitched Ventilated Roofs

Several winters ago, the National Bureau of Standards, in cooperation with the

Texas Instrument Company, carried out an aerial infrared survey of a three

house cluster located in Missouri. The test houses had similar floor plans

and identical roof emittances. The celling thermal resistance levels were as follows

:

Building

Insulation

Level

12

3

R-30

R-11

R~0

During the survey, the indoor temperatures within the three test houses vjere

maintained at 70°F. An aerial thermogram obtained during this survey is given

in figure 3.5.

When that thermogram was taken, the outdoor temperature was 20°F, the sky was

clear, and the wind speed varied between 5 to 7 mph. Note that the uninsulated

roof of building 3 appears lighter (depicting higher heat loss) than the other

two buildings. Also, note that it is difficult to distinguish the different

insulation levels in buildings 1 and 2. These results indicate that, when

aerial Infrared surveys are performed under optimum (clear sky and low wind)

conditions, uninsulated roofs can be distinguished from insulated roofs, provided that the roofs are exposed to the same microclimate and have the same emittance. When aerial infrared surveys are carried out for a large group of

houses, typical variations in roof emittances and variations in local wind

speed and local outdoor temperatures throughout the macroclimate cause differences in the apparent radiance temperature of roofs, which may frequently mask

out those differences due to variations in ceiling thermal resistance.

3.3.2 Low-Slope Built-Up Roofs

In the case of low-slope built-up roofs, variations in roof resistance cause

differences in apparent radiance temperature among the roofs displayed in an aerial infrared photograph, which are approximately a factor of three greater

than those for pitched ventilated roofs. Variations in roof emittance and variations in local wind speed and local outdoor temperature throughout the macroclimate produce differences in radiance temperature which are larger than those

between built-up roofs having 2 and 4 inches of Insulation. Therefore, there

39
Figure 3.5 Aerial thermogram of three test houses

40
is a good chance that built-up roofs which have little or no roof insulation

will be displayed in a lighter gray tone than more insulated built-up roofs

included in an aerial infrared thermogram. However, if all the built-up roofs

displayed in an aerial infrared thermogram are well insulated, then variations

in other parameters such as roof emittance, local wind speed, and local outdoor

temperature may also cause particular roofs to appear warmer than other roofs,

which may be incorrectly interpreted as the absence of roof insulation without

knowledge from ground inspections. The field measurements of Goldstein [5J

have shown such effects.

The aerial infrared techique has been shown to be very effective in locating

regions of individual low-slope built-up roofs that have defective roof insulation [6, 7J. These regions will show up as "hot spots" in the radiance temperature map. A common problem with built-up roofs is that the exterior membrane

becomes ruptured, permitting water to penetrate the roof system and wet the

insulation. Regions having wet insulation will conduct more heat and will

appear warmer tnan regions having dry insulation. The merit of such a survey

technique is that defective regions can be located, permitting local repairs to

be carried out instead of replacing the whole roof system.

3.4 GUIDELINES FOR CARRYING OUT SURVEYS*

Airborne thermographic surveys should be conducted when the buildings are heated. Building roof surfaces should be clear of water, dew, snow or ice.

The outside ambient air temperature at ground level should be at least 40°

F

below the building interior air temperature, unless the survey is conducted

under clear-sky conditions. In this case, a temperature difference of 30°F can

be used. In all cases, however, it is preferable that the outside ambient air

temperature not exceed 45°F.

The survey should be conducted at least 4 hours after sunset. The sky

conditions should be homogeneous, either clear or solid overcast, to permit

uniform radiation exchange to occur. The wind speed should be less than 15 mph

measured at the nearest weather station. It is desirable that a conventional

aerial photograph be taken during daylight hours of the same scene displayed in

the thermal image. The formation of dew or frost on a roof may change its emittance, and the phase change which occurs releases latent heat to the roof which

increases its surface temperature. Both of these effects may substantially change

the apparent radiance temperature of a roof. Thus it is recommended that aerial

infrared surveys be carried out under conditions without dew or frost formation.

The mathematical model for aerial infrared surveys was used to investigate the

depression in surface temperature for a roof due to radiation exchange with a clear night sky (see figure 3.6). For the analysis, a pitched ventilated roof

having R-30 ceiling insulation was used, since the depression in surface temperature would be greater for this roof than for roofs having less insulation.

Dew or frost would form on this roof before it would form on roofs having less

thermal insulation. A criterion for precluding the formation of dew or frost

Most of this information was obtained from ref. [8].

41
WIND SPEED,MPH

Figure 3.6 Depression in surface temperature on a built-up roof having 4

inches of insulation as a function of wind speed

42
would be that the dew-point spread (difference between the dry-bulb and

dew-point temperatures) of the outdoor air should be larger than the surface

temperature depression from that of the outdoor air.

To illustrate the use of figure 3.6, consider the following example: An aerial

infrared survey is carried out at an outdoor temperature of 20°F and a relative

humidity of 75 percent. The dewpoint temperature corresponding to this psychrometric condition is 14°F, giving a dew-point spread of 6°F. From figure 3.6,

the depression in the surface temperature for wind speeds less than 7 mph is

greater than 6“F. Therefore, if this aerial infrared survey is to be performed

without dew or frost formation on a built-up roof, then the local outdoor wind

speed should be greater than 7 mph.

The following data should be recorded during an aerial infrared survey:

1. Date of survey

2. Time of survey

3. Outdoor ground-level temperature

A. Wind speed at ground level

5. Sky conditions (i.e., clear, solid overcast, etc.)

6. Flight line location and orientation

7. Approximate ground speed and altitude of aircraft

8. Site conditions (i.e., foliage, roofs having low emittance,

etc.

)

3.5 EQUIPMENT SPECIFICATIONS

The spectral response of the detectors used in airborne infrared imaging

scanners should be within the spectral region 2.0 to 14.0 microns. However,

for most applications the 8.0 to 14.0 micron subregion is preferred since it is

optimal as an atmospheric transmission window for most measurement conditions.

The spatial resolution of the airborne infrared scanner is expressed as the

object plane resolution, which is a function of the instantaneous field of view

and the altitude above the roof surfaces being surveyed; it is also referred to

as the ground resolution (expressed in terms of the side of a square scanning

spot). The object plane resolution at the flight path nadir should not exceed

a 3-foot scanning spot.

The usable f ield-of-view for airborne infrared imaging scanners should be no larger than 90°. Devices with wider fields of view or scan angles may be used,

but the actual usable data for thermographic assessment shall be no greater than

+ 45° with respect to the flight path nadir.

It is desirable that the line scanner incorporate two internal blackbody

reference sources, so that the range in radiance temperature displayed in the

thermal image is known.

Automatic-gain systems are not recommended. Manual gain adjustments for signal

amplification are necessary to preclude changes in background component temperature conditions from masking building radiance characteristics. Scanners should

43
employ a method of displaying the scanner video signal or thermal map to the

operator during data collection to facilitate proper settings of the instrument

controls. Data may be recorded directly on hard copy film or on magnetic tape,

disc, or on other media in analog or digital form for subsequent processing into

hard copy thermograms.

Scanners should be operated within an aircraft speed and altitude envelope to

provide contiguous line scanning or overscanning of the building roof surfaces.

The thermal image should not be significantly degraded as a result of aircraft

vibration, roll, or other unwanted motions.

Required instrument sensitivities for an aerial line scanner system to

distinguish an uninsulated built-up roof from one having 2 inches of perlite

insulation are given in table 3.1. Note that as the ground-level temperature

increases and as the wind speed increases, the instrument sensitivity must be

increased (minimum resolvable temperature difference must be reduced) to permit

these insulation levels to be distinguished.

Table 3.1 Required MRTD to Permit an Uninsulated Built-Up Roof

to be Distinguished from One Having 2 Inches of

Perlite Insulation.

Exterior Wind Speed

Temperature 0 mph 7 . 5 mph 15 mph

0°F 7.5°F 2.6“F 1.6°F

20°F 6.6°F 2.2°F 1.3°F

40 °F 5.0'’F 1.6°F 0.9°F

Required instrumentation sensitivities for pitched ventilated roofs are summarized in table 3.2. Here the minimum resolvable temperature differences

will permit an uninsulated pitched ventilated roof to be distinguished from one having R-11 ceiling insulation. Note that the minimum resolvable temperature

differences are less than 0.5°F at wind speeds of 15 mph.

Table 3.2 Required MRTD to Permit an Uninsulated Pitched

Ventilated Roof to be Distinguished from ( Having R-11 Ceiling Insulation

Exterior Wind Speed

Temperature 0 mph 7.5 mph 15 mph

0°F 2.9°F 0.8°F 0.5°F

20°F 2.4°F 0.7°F 0.4°F

40°F 1.7°F 0.5°F 0.3°F

44
3.6

INTERPRETATION OF AERIAL THERMOGRAPHIC DATA

The general principles used for the interpretation of ground-based thermographic

data always apply to interpretation of aerial thermographic data. However, more

care should be taken before conclusions from air inspections are drawn [9-10]

from aerial thermographic data since the aerial survey thermograms record

emitted and reflected radiation and tonal differences are a function of object

temperature, emissivity and the surroundings. The interpreter must have knowledge about the particular structure and know the overflight conditions, primarily aircraft altitude, time-of-night of data collection and the environmental conditions prevailing at the time of data collection and the roof conditions, (snowcovered , dry, wet). The tonal differences in an aerial thermogram

are relative, and therefore only comparisons of buildings in close proximity

should be made. Analyses should be restricted to only the roof of the building.

3.7

PREVIOUS USE, AVAILABILITY AND COSTS

Aerial infrared thermography has been widely used by utility companies,

municipalities, or institutions which have large numbers of buildings in close

proximity, which they want to inspect. There are approximately eight firms

offering this service in the United States [llj. Aerial infrared equipment is

manuf acturered by about five companies and costs from $100,000 to $250,000.

The cost of aerial surveys start at about $5,000 (and range upward) depending

on the number of buildings to be examined.

3.8

PROGRAM APPLICATION

Aerial infrared surveys are most suitable for the inspection of large built-up

roof areas. They can readily detect areas of moisture damage on areas of

insufficient insulation, and aid in the performance of ground-based inspections.

In large facilities they are useful for inspecting underground distribution

systems. For the evaluation of the building envelope, aerial infrared surveys

are restricted to inspection of flat roofs.

3.9

TRAINING REQUIREMENTS

Aerial infrared thermography should only be undertaken by specially trained

technical personnel. The staff training requirements are similar to those of

level 2 in the Canadian training program. The interpretation of aerial

thermograms requires a training equivalent to level 3 (see Section 2.8).

45
REFERENCES

[1] Tanls, F. Jo, Sampson, R. E., and Wagner, T. W. Thermal Imagery for

Evaluation of Construction and Insulation Conditions of Small Buildings,

report prepared for the Federal Energy Administration, Contract No. CO-04

50233-00, July 1976.

[2] Hazard, W. R., and Underwood, S. A. Modeling Heat Loss with Aerial

Thermography, report prepared for the Department of Energy, Contract

No. EM-78-C-01-A1A8, June 1973.

[3] Aerial Infrared Thermograms and Residential Heat Loss, Ecological

Services of Texas Instruments, Incorporated, February 1978.

[4] Headley, R. B., Larsen, R. J., Goldberg, G. G. , and Boyd, R. J. Infrared

Thermography Requirements Study for Energy Conservation, report prepared

for the Energy Research and Development Administration, Contract

No. EX-76-C-01-2109, April 1977.

[5] Goldstein, R. J. Application of Aerial Infrared Thermography, ASHRAE

Transactions, Vol. 84, Part 1: 207-226; 1978.

[6] Tobiasson, W. , and Korhonen, C. Detecting Moisture in Roof Insulation

with a Hand-Held Infrared Camera, Second International Symposium in Cold

Regions Engineering, University of Alaska, August 1976.

[7] Link, Jr., R. E. Air-Borne Thermal Infrared and Nuclear Meter Systems

for Detecting Roof Moisture, Paper No. 30, Proceedings of the Symposium

on Roofing Technology, National Bureau of Standards, September 21-23,

1977.

[8] Application of Infrared Sensing Devices to the Assessment of Building

Heat Loss Characteristics, Proposed ASHRAE Standard 101 P, June 1981.

[9] Evans, J. B. Interpretation of Aerial Thermograms, Proceedings of the

Society of Photo-Optical Instrumentation -Engineers , Vol. 254: 30-34;

1981.

[10] Lawrence, G. , Branner, A., MacKay, H. Interpretation of Aerial

Thermographic Data, Proceedings of Thermosense IV, Society of PhotoOptical Instrumentation Engineers, 1982.

[11] Shumaker, D. L. , Snow, F. J., Barthle, R. C. , Culp, T. L., and Wood, J. T

Status of Thermal Imagery Technology as Applied to Energy Conservation,

DoE Report HCP/M 4228-01, 1979.

46


4. AIR INFILTRATION MEASUREMENT METHODS

4 . 1 SUMMARY

Excessive air leakage in a building is indicative of, and may serve as a primary diagnostic tool for determining construction flaws. Measurements of

the air leakage characteristics of a building provide data to determine

the four primary factors of: (a) the natural air leakage rates occurring in

the structure under various climatic conditions and use patterns; (b) tightness

of the building compared with other buildings and with itself after corrective

measures have been completed; (c) location of the leakage paths in the structure; and (d) the magnitude of each air leakage path [1]. Air leakage is the

uncontrolled entry of air from outdoors. Direct measurements under natural

conditions can be accomplished by "labeling” indoor air with a gas not normally

found in either outdoor or indoor air, such as sulfur hexafluoride (SF^) [2,3],

or by labeling it, in great excess, with a normal component such as carbon

dioxide (CO2

) [4-6] , or even by measuring the excess concentration indoors over

outdoors of a naturally produced component (CO2 , CO or radon). Such substances

are called tracer gases. Results obtained by this method are not meaningful,

nor useful for comparison with other buildings or with the same building at

different times, unless they consist of long-term average results which encompass many different weather conditions, or the indoor and outdoor temperatures,

wind speed (and perhaps less important for many buildings the wind direction)

are specified.

Another purpose of air leakage tests is to determine whether overall building

ventilation is adequate. There are numerous complaints in office buildings

of symptoms related to an insufficiently clean air supply [7,8], for example,

headaches, nausea, fatigue, respiratory symptoms, and feelings of stuffiness.

These complaints may be due to excessive concentrations of CO, smoke, formaldehyde or other contaminants, or to excessive relative humidity, heat or insufficient air movement. Building ventilation rates are often specified in order to

reduce the potential for indoor contamination at 5 i/sec per person or greater

[9,10]. Failure to achieve the specified rates indicates a potential for

employee complaints of symptoms and of contamination by such pollutants as radon "daughters" (decay products) which cause no complaints. Naturally, overall leakage rates are not the whole story; ventilation might be more adequate

in some parts of a building than in others and insufficient local air movement

might occur even when total ventilation is adequate [11,12]. However, an excessively low air exchange rate is considered to be prima facie evidence of

poor air quality. Tracer gas methods are the preferred means of determining

whether ventilation is adequate because it is very difficult to relate envelope

permeability measurements to naturally induced pressure differences.

Various air infiltration measurement techniques using tracer gases are classified as: tracer decay, constant concentration, and constant flow methods.

The first method requires varying degrees of automation, the latter two methods

are primarily automated. Sulfur hexafluoride (SF^) concentration measurements

will be considered in detail because it is the most commonly used tracer gas

and the theory and data analysis is common to all.

47
4.2 DESCRIPTION OF EQUIPMENT—ALL TRACER GAS METHODS

4.2.1 Tracer Gases

Many gases have been used for tracer gas measurements; table 4.1 lists some of

them and their important properties [13]. According to Honma [14], tracer

gases should have the following attributes:

1. The content of the gas in ordinary air must be relatively small (see

table 4.2), and there must be no source of the same gas in the building

concerned.

2. It must be possible for the low concentration to be accurately assessed

and detected.

3. The density of the gas must be as near as possible to that of air.

4. The gas must not react with the constituents of air.

5. The gas must not react with, or be adsorbed onto, the surfaces of walls,

furniture, clothes, etc.

6 . The gas must not be harmful to the human body.

7. The gas must not be flammable.

8 . The gas must be easy to handle, easily available and inexpensive.

In addition. Hunt suggests that the analytical method for data analysis should

be readily available and lend itself to automation [ 2 ].

Sulfur hexafluoride has recently become the most widely used tracer gas because

it satisfies all of Honma' s and Hunt's criteria except for its high density

compared to air. It is the only gas listed in table 4.2 whose concentration

can be measured in the ppb range; the others must be present at concentrations

in at least the ppm range or even higher. Therefore a small tank of SF 5 will

suffice to measure a large building, whereas several large tanks of other gases

may be required. Table 4.3 shows that although SF^ is relatively expensive per

unit volume, the total cost per building is among the lowest of all tracer gases

because of the extremely small volume required [3]. A point that requires care,

however, is that a small leak from an SF^ tank could affect the measured infiltration rate; this is unlikely to occur when other gases are used. One might

expect that because of its high density, SFg would settle out of air towards

the lower portion of a room, where the detector is located, and give lower

apparent infiltration rates than a lighter gas. Similarly, a very ligVit gas

like helium (He) might be expected to rise towards the ceiling, away from the

detector, and yield spuriously high infiltration rates. In practice, these

problems have has not been found to occur. Where gas-dependent differences in

infiltration rates were found, a physical cause, unrelated to molecular weight,

could be determined or the difference in rate was small. Howard [15] reported

higher apparent infiltration rates with hydrogen (H2

) than with nitrous oxide

(N2 O) in buildings with bare gypsum walls, but obtained similar rates with the

two gases in buildings with less porous walls, such as concrete or painted gypsum. Hunt and Burch [16] double-labeled air inside a townhouse having gypsum

board surfaces with SF^ and He, two inert gases with a molecular weight ratio

of 36/1. Helium did not disappear faster than SF^; the opposite occurred, but

the difference in infiltration was only 17 percent. Similarly, Grlmsrud et al.,

[17] obtained infiltration rates with SF^ which were 10 percent greater than

those obtained with N2O and methane (CH4

).

48
Table 4.1 Gases and Techniques for Tracer Dilution Method [17J

Trdcer

Measuring

Apparatus

Maximum

Allowable Concentration, in Air

( vol/vol)

Minimum

Detectable Concentration, ppm Toxicology [43] Chemical Inertness Comments [42]Density

Compared to dry air

Hydrogen Katharometer [43] 4% (lower

explosive

limit)

200 nontoxic extremely

reactive In presence of oxygen and

heat or flame

flammable or explosive in presence of oxygen and heat

or flame

0.07

Helium Katharometer 144,45] 300 nontoxic nonreactive nonreactive 0.14

Carbon

monoxide

inf rared

absorption; heat

of absorption measurement 145), gas chromatograph

followed by

reduction to

mettiane and measurement

witli flame

ionization

detector l4oJ

50 ppm

0.4

5 combines with

hemoglobin

to produce

asphyxia

can be dangerous

when exposed

to open flame

can also react with oxygen in air in sufficient

concentration;

may explode when

when exposed to open flame

1.0

Carbon

dioxide

inf rared

absorption 147); gas chromatograph

with thermal

conductivity

detector

5000 ppm

1

70

nontoxic very soluble in water

1.5

Sulfur

hexai luo- ride

electron

capture t34,34,4B)

gas chromatograph

1000 ppm 0.000002 nontoxic chemically inert

when pure When heated to decomposition

(550°C), toxic

byproducts may

be formed

4.9

Nitrous

oxide

infrared [49]

absorption

25 ppm [50] 1 nontoxic very soluble in water

can form explosive

mixtures in air

1.5

tthane flame ionization

[51] detector 172] gas chromatograph with

flame ionization

detector

3X (lower

explosive

limit)

5 nontoxic will burn when

exposed to flame

may explode in presence of oxy- gen and heat or flame

1 .u

Methane infrared

absorption [52] 5% (lower

explosive

limit)

5 nontoxic will burn when

exposed to flame

may explode in presence of oxy- gen and heat or flame

0.53

49
Table 4.2 Atmospheric Constituents [3]

Average Typical

Tropospheric Indoor and

Background Urban Ambient

Concentrations

ConcentrationsAnthropogenic

Compound ppm ppm Sources

«2 0.5 0.5 • • • He 5.2 5.2 • • • CO 0.1 5 to 50 combustion

CO2 320 30 to 5000 combustion

N2O 0.3 0.3 to several ppm combustion

CH3 CH3 1.5 X 10"^ 0.1 incomplete

combustion

CH4 1.5 2 to 5 incomplete

SFb 10 "^

< 10“^ combustion

telephone

switching

stations

Table 4.3 Relative Gas Cost for Tracer Dilution Study Taking

Account of Detectability [3]^

Ghs

Gas Volume

Per Dollar (m^)

Maximum Volume Measurable

Per Dollar Spent on Tracer Gas (m^)

He 0.08 300

COn 1.5 1.5 X 10°

CO 0.16 1.5 X 10^

CH3 CH 3 0.37 30,000

NnO 0.34 3 X 10^

A 0.06 3 X 10^°

3 Gas Costs figured on June 1977 prices on West Coast of United States.

50
Bassett et al., [18] compared SF 5 , CO2 and direct flow (induced controlled

air exchange) measurements in 2 rooms, 2 houses and a school building in a careful study that conclusively demonstrated that for practical purposes the

three methods give interchangeable results. Air was exhausted from the test

spaces by fans. Agreement between gases and flow measurements, or between the

two gases, was nearly always at the 95 percent confidence level; any disagreement was only a little beyond the expected experimental error. The only significant difference noted was that in one sheltered room used to study the

effects of low flow rates, both tracer gases yielded an air exchange rate of

0.13 h“^ . The authors attributed this effect to pressure differences induced

by the air handling circulation fan.

4,2.2 Sulfur Hexafluoride Measurements

Some details of performing measurements using SF^ are described. Before

starting any measurements it is necessary that the detector be brought to a stable condition. Also, it is important to calibrate the apparatus to ensure

that operation is in the linear range. A linear response together with a sufficiently high standing current (corresponding to a potential of 70 V or higher) indicate proper detector operation. The detector should frequently

be cleaned with ethanol or acetone, and the column should occasionally be

cleaned by passing air heated to 100°C through it.

Sufficient SF 5 is injected to give a final concentration of about 20 to 50 ppb,

depending on the detector. Sampling is done at various times after mixing is

completed. Sulfur hexafluoride concentration is determined by a gas

chromatography-electron capture-detector unit. The unit pumps air through an aluminum oxide column that separates SF^ from oxygen (O 2 ), which also captures

electrons; O 2 is eluted first. The gases then pass through a detector containing a radioactive source of electrons, either tritium (H3

) or nickel (Ni^ 3 ).

The decrease in the standing current established by the source is measured and

converted into arbitrary concentration units. The column is flushed with argon

(Ar) or nitrogen (N 2

) between samples, depending on the type of detector.

The chromatography-detector unit is compact and simple to operate. It costs

approximately $6000 to $8000. It is portable, but tanks of SF^ and Ar or N 2 must also be transported along with it. An attachment can be made available

so that up to ten sites may be automatically sampled almost simultaneously.

This sampling procedure is in contrast to other types of detectors, such as infrared absorption spectrometers, which may be operated in a truly continuous

mode. For such detectors, no elaborate automation schemes are necessary for

continuous operation, unlike that for SF^ detectors,

4.3 THEORY

4.3.1 All Tracer Gas Methods

There are several ways of using tracer gas to measure infiltration rates [9];

( 1 ) a single injection followed by tracer gas concentration decay; ( 2 ) continuous injection at a constant rate; and (3) frequent injections to try to maintain constant concentration. All of these methods have shortcomings but the

51
first method is the most widely used and is undoubtedly the simplest. All

three approaches are based on mass balance; that is, all the tracer gas

released into the building must be accounted for, so whatever is not in the

building must have leaked out. It is assumed that tracer gas mixes thoroughly

and instantaneously with air.

The parameters related to the infiltration rate are the indoor and outdoor

temperature difference and the wind speed. The simplest equation, with

those parameters as independent variables, can be expressed as

I = a + b»AT + c»w (4.1)

where

I = air exchange rate, h”^

AT = indoor-outdoor temperature difference, °K

w = wind speed, m/s.

While the relationship assumed may not be physically correct [19], it has been

found to be as accurate as more complicated models for houses [20]. More com- plicated equations, for example, taking into account both shielding by nearby

buildings and wind direction, are probably required for tall buildings. The

best known equations of Tamura and Shaw equations [21,22] require knowledge of

air leakage characteristics of building components, which are determined by

fan pressurization tests discussed in Chapter 5. Indoor-outdoor temperature

differences and winds are the sources of the indoor-outdoor pressure differences that cause air infiltration. There is little air leakage during mild

weather even in poorly constructed buildings. Conversely, there may be a great deal of air leakage during heavy wind conditions in even well constructed

buildings. Not all air leakage paths are equally detectable by air leakage

measurements; under low wind conditions, where temperature difference induced

leakage predominates (stack or chimney effect), most of the leakage may be

concentrated in openings relatively far from the neutral pressure height

(that height at which net indoor-outdoor pressure difference vanishes). The

stack effect arises because the density gradient of air is steeper at higher

than at lower temperatures, in winter it is therefore greater indoors. The

indoor-outdoor pressure difference increases with height, i.e., becomes more

marked the taller the building. Under wind induced infiltration, of course,

most of the leakage occurs in windward and leeward faces of the building.

Therefore, any defective construction at approximately midheight on the

walls not facing into prevailing winds can easily be overlooked by tracer

gas tests.

The mass balance equation is:

V«C(t) + q*C(t) = F(t) (4.2)

where

O V = volume (m ) O C = tracer gas concentration (g/m^)

52
o q = air infiltration rate (m'^/h)

F = tracer gas flow rate (g/h)

t = time (h).

A useful quantity, the air exchange rate, is related to the infiltration

rate by:

I = q/V (4.3)

where

:

I = air exchange rate (h“^)

The term "infiltration rate" is also sometimes used for I.

The solution of equation 4.2 is given by:

C(t) = Cq e + e J^e F(u)/V du (4.4)

o where

:

3

Cq = concentration at zero time (g/m )

Usually F is constant; in this case:

C(t) = Cq e + (1 - e "^^) (F/q) (4.5)

The "volume" used above is, strictly speaking, the "effective" volume [23,24]

and not necessarily the physical volume of the space being measured, which may

be either greater or smaller. There are several reasons for the difference:

furnishings may occupy space, the air with which the tracer gas is well mixed

may not fill the entire space (in contradiction to the assumption underlying

the entire treatment), or the tracer gas may enter an adjoining space (storage

areas, basement or attic). Lawrence Berkeley Laboratory (LBL) researchers

[23,24] claim that the effective volume may differ from the physical volume by

up to 50 percent; however, when the divergence is so large the reasons for the

discrepancy can probably be readily determined. Usually, the difference is

probably less than 25 percent, in which case it is of the same order of magnitude as the scatter in the infiltration rate data caused by changing wind

speeds, temperature differences, occupant effects and poor mixing.

Buildings —especially large ones —have uniform infiltration rates only in

special cases. To treat tracer gas measurements in the greatest generality,

multisector buildings must be assumed. The immediately following subsection

deals with such buildings. The theoretical discussion presented above will be

expanded below, in the descriptions of tracer decay, constant concentration and

constant flow techniques.

53
Multisector Infiltration Measurements

If a building is divided into distinct sectors which exchange air incompletely

among one another, a multisector method for measuring infiltration might be

required. For some purposes (e.g., energy conservation and fresh air supply)

only the outdoor contribution to building air exchange rates is important,

while for other applications (e.g., migration of smoke and contaminants from

one place to another) sector-to-sector air exchange rates are also important.

A sector may be part of a room [11,12], especially when air supply ducts are poorly situated; it may be a room itself [14] (as in a building with no mechanical air circulation systems); it may be a collection of rooms (e.g., in

a building with air supply units that serve different floors or sections of the

building [25]; or a single-family house with an attic or basement [26,27]); or

it may be the entire building. In the latter case, and in the case of a building having sectors with little communication among each other, multisector

techniques are unnecessary.

Multisector techniques may involve either tracer decay, constant concentration

or constant flow techniques. The tracer method is the simplest technically but

the constant concentration method is by far the simplest to analyze, provided

only external air exchange rates are of interest. There appears to be no really

adequate method for measuring sector-to-sector air flows unless the number of

sectors being considered at any one time is small. Otherwise too much equipment

is required. Air-bag sampling provides a technique to determine concentrations

across several sectors; the sampling can be performed simultaneously or successively to obtain time average results. Sampling of different sectors at different times provides measurements that may not be consistent since transient

disturbances can cause changes in concentrations.

Another method for measuring air flows in multisector buildings is to

simultaneously use two or more different tracer gases, one in each sector, and

to measure all of the gases in each sector. Again, this approach is only practical for a small number of sectors at a time. Etheridge [26] reported on the use of 2 tracer gases, CO 2 and N2O, simultaneously in a constant concentration

mode to measure the infiltration rate of an entire house and its loft.

The equation for the multisector case (actually, a set of simultaneous

equations) is similar to the single-sector equation 4.2, with the addition of

terms for exchanges among sectors. In the constant-concentration case, those

terms disappear and the treatment is reduced to the single-chamber case. In

both the tracer decay and constant flow methods, conditions can be designed to

reduce, but not eliminate, the size of these terras relative to indoor-outdoor

air exchange rates.

The equations are given by [14,28]:

N

= Z q^. (C.-Ci) - q^o (4.6)

j = l

where

54
subscript "i" refers to the 1 th sector, i = subscript "o" refers to outdoors

= air flow rate from sector j to sector i.

The volume and infiltration rate expressions are

V =

I = 2qio/v (4.7)

As usual, Cq is taken to be much smaller than Cj^; for many tracer gases, such

as SF^, = 0 and this case is shown in equation 4.6. In many multisector

cases, most of the q^^j = 0 since many sectors are not adjacent. Now, if the Cj^ are nearly constant in equation 4.6, the first term on the right is small and

equation 4.6 reduces to the single-chamber case. For example, if one ensures

that the concentration of gas is more or less constant throughout the building

at the start of the experiment by seeding gas appropriately, then the concentrations will remain approximately equal as long as too much time is not allowed

to elapse (approximately 1 hour). This procedure is the basis for treating a multifloor house as a single sector in the tracer-decay air-bag method. Of

course, by measuring concentrations in the different sectors, one can determine

whether or not concentrations are sufficiently close to a constant value

throughout the building. In the constant concentration method;

and I from (4.7) can readily be determined by summing all of the flows in the

building.

4.3.2 The Tracer Decay Method

The tracer-gas decay method [13] is very versatile and is the simplest of the

tracer-gas measurement systems. It can be used for short- and long-term measurements. The measuring equipment may be located on site or the samples may

be collected into air bags and analyzed offsite [Ij. Injection of gas and

measurements of the concentrations can be made manually or automatically to

study the dependence of air exchange rates as weather changes over time.

A single injection of tracer gas suffices for each measurement and no complicated apparatus is required to control the concentration or automatically

Inject gas, although several automated techniques have been developed and are described below. As much time as necessary may be allowed for mixing the tracer

gas with air. A fan may be used to thoroughly mix the tracer gas with the air

and then be turned off, or the fan may be run throughout the entire measurement

procedure. Bassett et al., [18] report that a fan might Induce an artificial

air flow, but only at very low air exchange rates; 0.13 h“^ in their study. It

is convenient to inject tracer gas into the building’s air handling system, if

it has one. In that case, air is sampled at the return air ducts. If the

C(t) = 0 = C for all i except 0 so that;

^i “ ^lo ^o (4.8)

Summary

55
Figure 4.1 Sample of tracer gas decay method data

Figure 4.2 Comparison of concentration versus time

relationships for four simple models ranging

from perfect mixing to complete nonmixing.

56
STEP 1 STEP 2 INJECTION miiNc V V.

- Vj-l N'

* J

STEP 3

FILL sample

lAG

• • Mill »

IN Ml M »• 1*1 STEP 5

FILL 2od -«o3 STEP 6

SNIP RAGS TO LABORATORY

»• it ft u. ft ft It 1 M

ft Ml » ('

Hi iI' sample iag RN M

lit «' • u

Mftt tWM

FIELD EQUIPMENT

STEP 7 DETERMINE TRACER CONCENTRATIONS

IN LABORATORT © -a

SFc DETECTOR

STRIN6ES OF TRACER CAS SF|

(MlZDNANO SAMPLE PUMP AIR SAMPLE RAGS

|2 Ret floor]

Figure 4.3 Schematic of air sample bag

procedure

57
building is large and/or divided into many rooms (and there is no mechanical

air handling system) it may be difficult to obtain a uniform tracer gas concentration throughout the building. Even if a uniform distribution of tracer-gas

is accomplished initially, concentrations may decay at different rates in different parts of the building. The measurements will reveal the extent of the

non-uniformities; if the divergence in room concentrations is large a multisector method should be used.

The basic tracer decay technique, followed by air-bag and automated variants

of air infiltration measurement methods are described below.

Description of Equipment—Basic Tracer Decay Technique

1, Tracer gas monitor. The monitoring instrument should either have been

calibrated by the manufacturer or should be calibrated on site. Standard

mixtures of at least two different concentrations used in the range of the

test should be used,

2, Sampling network. The network consists of tubing, tubing junctions, a pump, and possibly an aspirator. This network is used to draw samples from

remote locations within a structure, blend them, and bring the blended sample

to a convenient place for analysis. In general, it is best to avoid plasticized tubing such as vinyl, and use copper, stainless steel, or possibly

polypropylene or nylon. The experimenter should be aware that surface

absorption within the sampling network can be a major source of confusion

in any concentration decay measurement.

3, Syringes for Injecting tracer gas.

4, Circulating fans. Fans are used to circulate air within a structure and

should be capable of circulating air in any direction. The building's air

handling system can also serve this purpose.

5, Meteorology stations, A portable meteorology station that records wind

speed and direction and outside temperature is required to provide local

external conditions,

6, Indoor temperature monitor.

7, Tracer gas. A cylinder or container of gas selected from among those listed

in table 4.2 is necessary as a source of tracer gas for the test. The con- centrations listed in table 4,2 should never be exceeded. Good experimental

practice is to ensure that the maximum allowable concentration of the particular tracer gas is below the maximum by at least a factor of four. The initial tracer gas concentration should never exceed the OSHA time-weighted

average for substances included in the latest OSHA-controlled gases list.

Description of Automated Tracer Decay Equipment

Automated air infiltration measuring equipment using the tracer decay method,

developed by researchers at the National Bureau of Standards (NBS) and Princeton University, has been used in the United States since 1974. These systems

58
use an electron capture gas chromatograph which measures SF^ in the ppb range,

as previously described [29-36]. The latest version of this equipment consists

of an S-100 bus microcomputer with two 5 1/4 inch dual-sided floppy disc drives,

a real-time clock, a CRT terminal, an electron-capture detector gas chromatograph, a 10-port sampling manifold, 5 injection units, and interfaces for both

analogue and digital data [33]. This system costs approximately $20,000 and

must be specially assembled. This system has been used in air infiltration

studies in large buildings.

O Figure 4.4 shows a schematic of a 26-story, 450,000-m'^ office building (from

another on-going project in which air infiltration measurements are currently

being taken) where two systems are being used. One system measures the air

infiltration rates in the tower (floors 3 to 26) portion of the building and is

located in the mechanical equipment room of the 26th floor. The sampling network for this system is shown in figure 4.5 and 4.6. This system includes a local weather station on the roof, and thermistors to measure interior temperatures on each of the sampled floors. The second system is located in the lower

mechanical equipment room and monitors the four lower zones of the building and

an adjacent four-story Plaza Building. A typical trace of data from the tower

is shown in figure 4.7. Injection occurs 10 minutes before each hour. Note

that the tracer gas is fairly well mixed after 20 minutes. Figure 4.8 shows

the average daily air infiltration rates for April and May, 1981. Data are usually not collected on weekends or holidays, when the building HVAC systems

are not operating. Such air infiltration measuring systems have operated 18

days without attention and for 30 days without data loss.

Hartmann and Muhlebach [34] of the Swiss Federal Laboratories for Material

Testing and Research designed a tracer-decay system with a controller that can handle up to six rooms. Data are analyzed off line on a central computer. The

system also measures air temperature, humidity, wind velocity and direction,

and pressure. An infrared analyser (MIRAN) is used to measure the concentration

of the tracer gas, N2 O, in the 10 to 20 ppm range.

Theory—Tracer Decay Technique

In the tracer decay method, there is no source of tracer gas once the

measurement has begun, equation 4.5 then becomes:

C(t) = Cq e (4.9)

and then solving for the infiltration rate

I = (l/t) In (Co/C(t)) (4.10)

Measurement Guidelines [13] —Basic Tracer Decay

The basic tracer test is carried out as follows:

Step 1: An amount of tracer gas sufficient to produce a readily detectable

response in the gas-measuring instrument is released at one or more points in

each sector of the test structure, or into the air handling system. The

59
SCHEMATIC OF PARK PLAZA BUILDING

Newark, N.J.

Figure 4.4 Schematic of 26-story Park Plaza Building

in Newark, N.J.

TRACER SAMPLING NETWORK

South return^l y 1 sample J]|||jpj

/ 1 pumps =111 1

— North return

zi

1 22nd floor /C^— 20th floor

1 Z|

1

1—16th floor A

i

1 -y-\

/ j

1

1 ^

- 11th floor

1

1

1 ^1

1

1^

- 7th floor A

1

1>- - 3rd floor ^

Figure 4.5 Schematic of sampling network for tower of Park

Plaza Building

60
1

add Nl N0UnUN33N03 )03V)U

(V3

O

l-i o

>4-1

u

o

s

4-1 I)c

&0c

•H

T3 aoc •r^ •H P

CQ o.B

ta

reN (0

re rH

>4-1 0. o

fH

•r^(0

re a.

4-) OJ o of

>atuM

3

130 H

61

Figure

4.7

Typical

sequence

of

test

data

from

tower

of

Park

Plaza

Building
Figure 4.8 Dally average air exchange rates for Park Plaza

Building

release site is governed by the location of the air handling system(s) (or

mixing fan(s) in a structure with no air-handling system). Tracer gas release

can be from a disposable syringe.

In a building with central-heating and air-conditioning system(s), the main

fan(s) is (are) operated continuously. Here, tracer gas is Introduced into the

main supply or return duct(s), preferably in the vicinity of the main fan(s).

Fans are used to circulate the air and mix the gas in a building without

central-heating and air-conditioning system(s). Tracer gas is released at one

or more points within the structure. Care must be taken not to affect the

pressure distribution within the structure.

Step 2: Allow at least 30 minutes for mixing of the tracer gas.

Step 3: Homogeneity of tracer concentrations is tested by sampling at a number

of building locations. Monitoring the decay of tracer concentration is begun

when concentrations differ by less than 5 percent of the average concentration

(measured within the building). In multi-story structures, 2 widely separated

samples are required on each floor.

Tracer samples may be measured at a single central location by drawing samples

from a number of locations through a common network (multipoint sampling).

Alternatively, individual samples (grab samples) at a number of distinct locations should be obtainable.

62
When multipoint sampling is used, the sensors are to be located at strategic

points within the test structure and conducted to the central location. With a single measurement device the sampling network conveys blended air samples to

the analyzer; if the dilution rates are different for different rooms or floors

the samples drawn by this method yield air leakage rates slightly lower than

the true average rate. For example, if one of the rooms (or floors) leaks air

at twice the rate of the other room, the result of analysis of the blended

samples of the two rooms leads to an air leakage rate estimate about 4 percent

lower than the true average rate (see figure 4.2).

The number of sampling intervals and their duration depend on several

considerations, generally, an interval should not exceed 2 hours because the

air exchange rate may vary too much over longer intervals (unless measurement

of the long-term air exchange rate is the objective). Enough tracer gas must

remain at the end of the measurement period for detection purposes. The number

of sample times should be chosen so that each successive calculated value of I does not change from the previous one by more than 10 percent [35]. Alternatively, one can calculate the standard error of the slope of the regression

curve of In C vs. t (or its correlation coefficient) and continue taking more

measurements until these values stabilize sufficiently [32]. For large-scale

air-bag sampling, where great precision is not required, a single time interval

of about 1 to 2 hours is usually used.

In addition to scatter, another reason several measurements are recommended is

to be sure one is operating in the linear response range of the instrument. If

there is a consistent trend away from linearity on a log-linear plot of concentration vs. time, either excessive tracer gas may have been injected, or inadequate mixing may be the cause. Hunt and Burch [16] derived relationships as shown in figure 4.2 relating various deviations from linearity to particular

mixing problems. The converse is not true; a linear response does not guarantee

thorough mixing.

Interpretation of Results—Basic Tracer Decay

Calculate the air infiltration rate as described above or by a finite difference

method

;

I = (ti+j - t.)"^ iln(C./C.^.p (4.11)

where subscript "i" denotes the i th Interval.

Accuracy of Technique—Basic Tracer Decay Technique

With this method a mean and standard deviation may be computed as follows:

I = E li/N

Si = [(Eii^ - i2/n)/(n-i)]^/2

where

:

(4.12)

63
N = number of sampling intervals

Sj = standard deviation (h

The errors Involved in tracer gas concentration measurement are approximately

2.5 percent [9]. This error is negligible in comparison to the usual "scatter"

caused by wind gusts and to a lesser extent, changing temperatures and furnace

cycling.

Air Bag-Sampling Measuring Guidelines [1]

There are several advantages to air-bag sampling. A single detector suffices

for any number of measurements. This procedure is useful for a multisector

building where a large number of detectors would be prohibitively unwieldy and

expensive. The test is performed as described in figure 4.3. Steps 1 and 2,

injection and mixing of tracer gas (SF^), are as described above for the tracer

decay method.

Step 3: Fill initial sample bags. After adequate mixing of tracer gas, one fills an Initial air sample bag in each sector of the building (while walking

around) using a small manually- or battery-powered pump. A convenient air

bag size is 5 or 10 liters. The Important point is that the air bag must be

filled slowly, ensuring that an integrated sample is collected. A hand pump

requires about 50 strokes to fill a bag and the electric pump takes about

5 minutes to perform this same function.

Step 4; Decay of tracer gas. Tracer gas concentration requires 1 to 2 hours

to decay. During this period the mechanical air-handling systems are operated

in the desired mode.

Step 5: Fill final sample bags. The procedure in Step 3 is repeated, obtaining

a second sample bag of air for each sector of the building.

Step 6: Ship sample bags to laboratory.

Step 7: The tracer gas concentration is determined as described above for the

tracer decay method.

Accuracy of Technique—Air Bag Sampling

In an infiltration test of a mobile home under controlled temperature conditions

in the absence of wind, agreement between infiltration rates obtained by air bag

and basic tracer gas methods was within 15 percent [35].

Previous Use—Air-Bag Sampling

Infiltration rates of large numbers of buildings can be conveniently measured;

Grot and Clark [36] evaluated energy conservation retrofit measures in hundreds

of homes using this method. The bags were sent by mail to the laboratory for

contents analysis and then reused.

64
Training Requirements-~Air-Bag Sampling

Very little training is required to sample in this way; no highly skilled

technicians are needed.

4.3.3 "Constant" Concentration Technique

Summary

In practice, the "constant" concentration method is a special case of the tracer

decay method, but with short intervals between automated tracer gas injections.

The advantages of this method are that: 1) measurements may be taken over long

periods of time; ( 2 ) nearly instantaneous responses to changes in weather can

be studied [33]; and 3) data analysis is straightforward since gas injection

rate is simply proportional to infiltration rate. The disadvantages are as follows. Since mixing is never instantaneous, the system is always responding

to a previous, not the current concentration, and the equipment is more complicated than that used for other methods. It is difficult to consider a constant

concentration system which is not automated.

Description of Equipment

Several groups have designed automated equipment which frequently monitors

tracer gas concentrations and injects an amount required to maintain constant

concentration. A variety of gases are used by the groups. These groups

include the British Gas Corporation (N2O and CO2

) [26], the Division of

Building Research of the National Research Council of Canada (NRCC) (SF 5 ) [37],

the Institute of Technology in Taastrup, Denmark (N2O) [38], and the Swedish

National Testing Institute (N2O) [39].

British Gas Corporation Method

The British Gas Corporation automated air infiltration unit [26], called

Autovent, is based upon a microprocessor and rapid sample analysis. Gas is

released so as to maintain constant concentration in each room of a house.

Rooms are read in sequence for 6 seconds each, an injection valve is opened and

the duration of injection is recorded. Results are summarized for 30-minute

periods. Sampling is accomplished through tubes of equal length. The two most

recent concentrations are used to determine the amount of N

2O injected to maintain a constant level of gas prior to the next sampling. The nominal concentration is maintained at 50+2 ppm. Injection takes place for as long as half

of the period (up to a maximum of 30 seconds). Each of the injection lines is

calibrated prior to the experiment so that the injection rata is suitable for

each room. A 4- to 5-fold ratio of maximum to minimum air infiltration rates

for each room can be accommodated.

The British Gas Corporation method has been used over a period of almost 2 years

with up to twelve rooms measured simultaneously. Six houses have been carefully

analyzed with an overall reported accuracy of 10 percent. Etheridge et al.,

[26] also used this system to measure two different tracer gases simultaneously

in order to obtain infiltration rates for an entire house and the loft.

65
National Research Council of Canada (NRCC) Method

In the NRCC method, concentration is measured every 2 or 2 1/2 minutes. Fixed

amounts of SFg can then be injected up to 90 times over the next interval, the

intervals spaced as closely as 0.9 seconds apart. The same electron capturegas chromatography unit is used to measure tracer gas concentration as described

above for the tracer decay method. Since a level of 15 ppb SF^ is generally

maintained, the amount of tracer gas used during an experiment is very small.

At the start of an experiment, it usually takes about 1 hour to reach the

desired setpoint level of SF^.

The major weakness of the apparatus is the operation of the detector. Under

heavy usage, the column and the electron capture detector requires considerable

maintenance, cleaning and calibration. Zero drift of the detector is a con- tinuing problem. In addition, the switching valve, which uses a spool valve

arrangement with 0-rings, requires maintanence. Small leaks from the pressurized SF^ supply, that is normally kept in the house being measured, can yield

incorrect air change rates. Comparing NRCC experience with that of other

investigators, the maintanence problems cited here suggest that Infrared would

be preferable to electron capture-gas chromatography equipment if the higher

concentrations (50 ppb range) were tolerable.

Kumar et al., [37], in a report on two houses, claim that agreement between

constant concentration and tracer decay methods was better than 2 percent. In

order to achieve good mixing in these houses, the furnace fan was operated

continuously. The reason for the successful use of 2-minute intervals in

these houses is that the air exchange rate was stable for almost 2 hours at

0.6 h”l.

The NRCC apparatus has been functioning for three years and is now available as

a commercially packaged unit [40]. The unit has demonstrated its ability to

hold SF^ concentrations in a house constant to within 4 percent over 15-minute

intervals and 2 percent over a one-hour interval. The unit has run 72 hours

unattended. To date more than twenty houses have been tested using this

equipment

The Danish Institute of Technology Method

The Danish Institute of Technology automated air infiltration system [38]

is also microcomputer-based. There are many similarities between the British

Gas Corporation system and the Danish system. Injection of N2O is essentially

continuous. Small (12-cm diameter), fans are located near the N2O injection

port in each room to promote rapid mixing of tracer gas. Ten solenoid valves

are used to control injection, and 10 other valves control sampling to the

URAS-7N infrared detector. The experimental design also provides a tank of N 2O

reference gas at 50 ppm to check periodically the design value of 50+2 ppm

N2 O concentrations within the home. Temperature and humidity sensors are added

to further increase reading accuracy. The tubing that runs throughout the house

has been kept as unobtrusive as possible (only 3 to 4 mm in diameter) since the

study emphasizes occupancy effects. Metal tubes are used near door hinges to

provide tubing paths, so that door operation is unaffected. The N2O Injection

orifices for each room were carefully designed to provide choked flow and were

66
subsequently calibrated. The system can operate for up to six days unattended.

Records are maintained on floppy disc, with a viewing screen provided to check

on site operation. Also a statistical package can be used on site to provide

further system checks.

The Danish Institute of Technology research effort is focused on the effect of

such occupant activities as opening and closing of windows and doors on air

exchange rates in each of ten rooms in a house south of Copenhagen.

Greater than 10-fold variations in air change rates can be accommodated (from

a few tenths to greater than 5 h“l) with an accuracy from 5 to 10 percent.

Swedish National Testing Institute Method

The Swedish National Testing Institute automatic air infiltration measurement

method [40] is similar to the Danish one. The short response time of the N2O

analyzer allows collection of a large number of samples per unit time. In the case of field measurements on a variety of buildings, the problem of a 2-hour

equipment warmup time for the URAS-7N analyser has been solved by a 12 V/220 V

transformer in the instrumentation van. Warmup is accomplished in transit.

An alternate MIRAN lOlA analyzer requires 10 minutes or less to warm up. The

arrangement of the ten tubes to the analyzer allows nine air samples and one fresh air purge. The fresh air must be raised to room temperature to avoid

analysis problems. The pumping system transports the samples to the analyzer

through 10-m long, 6-mm diameter plastic tubes within the house. When operated

from the van, special 25-mm tubing is used to thermally insulate the enclosed

nine plastic tubes. Measurements are made at set intervals and used by the

microprocessor to calculate air exchange rates for each room.

Theory—"Constant" Concentration Method

The equation that describes the "constant" concentration method is derived

from equation 4.2 by setting C(t) = 0, then

C(t) = F/q (4.13)

In practice, since the mixing of tracer gas takes about 20 minutes, V«C(t) is

unequal to zero and the "effective" volume term cannot be eliminated by this

technique. This is the reason for placing quotation marks around the term

constant

.

Because of the time lag the system is always responding to the concentration

prevailing minutes before. If the concentration is rapidly changing, either

an excessive (or an insufficient) quanitity of tracer gas will be injected.

This system response can cause the method to break down as the concentration

drifts hopelessly away from the constant value it is set to maintain, according

to Condon et al., [30], who suggest the use of a mixing factor to compensate

for the time lag. They calculate the mixing factor only for the period immediately after tracer gas is first injected as C(t)»V/FQ. It is unclear how this

technique can be used in practice.

67
One of the major advantages of the constant concentration technique over other

methods is its effectiveness in dealing with air infiltration in buildings with

sectors having separate air supply systems that only partially communicate with

each other. (If there were no communication at all, each sector could be

separately measured by any available technique.) In practice, the number of

sectors must not be excessive since a separate instrumentation system is

required for each one, unless air-bag sampling is used.

4.3.4 Constant Flow Technique

Summary

The constant flow technique, like the constant concentration technique, permits

continuous measurement of the ventilation rate. Sherman et al., [41] call this

system "continuous flow." The instrumentation is simpler, however, as no feedback

loops are needed to maintain constant concentration.

The disadvantages are that the Infiltration rate may drop considerably from one interval to another (due to changing weather conditions) causing the concentration to rise beyond the detection limit of the detector. A microprocessor can

be used to adjust the rate of tracer gas Injection in order to avoid the offscale problem. Measurements can only begin when equilibrium is reached.

In common with the constant concentration method, constant flow methods are nearly always automated.

Description of Equipment—Constant (Continuous) Flow Technique

The continuous flow method was developed at LBL to permit automated

measurement of infiltration in a test space at half-hour intervals. This interval was chosen to ensure stability in the the controller feedback loop. This

unit originally used N2O with an infrared analyser but later, due to U.S. exposure limits (see table 4.2), was modified to use SF^, with an infrared analyser.

The system was designed to permit researchers to carefully examine the mechanisms that drive infiltration: weather and mechanical systems. The system is

designed around a microcomputer that: (1) controls the injection of tracer gas

into the test space; (2) selects the sampling port used during an interval;

(3) processes and records weather and system operation data; (4) calculates and

records half-hour average infiltration values; and (5) computes a new injection

flow rate based upon the previous calculated infiltration rate in order to keep

the concentration in the test space within a particular target range. Infiltration rate is calculated using equation 4.5. In practice, equation 4.5 is solved

numerically by the microcomputer using a search algorithm that finds the set Q,

and V having maximum likelihood consistent with the measured values of C and

F for the time Interval.

A useful application of the constant flow technique, which eliminates the

problem of exceeding the detection limit of the detector, is the average infiltration monitor (AIM), to permit simple, unattended measurement of the long

term infiltration rate of a building. This monitor was developed at LBL [41].

The AIM minimizes both inconvenience to building occupants and the technical

68
skills required to install the system. It consists of two small outwardly

identical suitcases: the injector and the sampler. Each case contains a small

positive-displacement solenoid pump that is pulsed at a rate controlled by an internal timer. Each pump, connected to a gas sample bag, is either slowly

emptied, injecting tracer gas into the space to be tested, or filled, sampling

the mixture of tracer gas and room air present in the space. The concentration

is determined after the desired interval, which can be a period of days. Using

equation 4.13, the average air infiltration rate can be determined from the con- centration and from F, which is calculated from the duration of the test and

the total volume of injected tracer gas.

Theory—Constant Flow Technique

The equation that governs the constant flow technique is obtained from equation

4.5 by setting Cq = 0;

C(t) = (1-e -It) F/q (4.14)

Previous Use—Constant Flow Technique

Honma [25] used an electric heating element to release CO2 at a constant

rate, in succession in each room of one apartment at a time, in Swedish tall

buildings. He assumed that a room is only affected by other rooms separated

at most, one removed from each other. His method is a combination of constant

flow and tracer decay because the gas was released for a significant interval

and its concentration then permitted to decay. There are no reports of multisector measurements beyond the small number of rooms, as performed by Honma.

4.4 PROGRAM RECOMMENDATIONS

The tracer gas technique is extremely useful for the determination of the

actual air infiltration rates for large buildings. Except for the most complex

multi-sector buildings, the air bag sample method is the easiest to implement

and will most likely yield the necessary information needed to assess the air

infiltration characteristics of the buildings. The air bag sample technique

could be used to easily collect air infiltration data on a large number of

federal buildings at a reasonable cost.

If very accurate measurements are needed for legal reasons, for example,

certification of air infiltration rates of a new building, or if air bag

sampling data cannot be interpreted, then an automated system should be used.

4.5 RESTRICTIONS TO IMPLEMENTATION

There is no general restriction to the use of the tracer gas technique for

federal buildings if SF^ is used as a tracer. For large buildings (greater

than 100,000 sq. ft) the other tracers discussed in this chapter are not suitable due to the large quantities of gas required. This test can be performed

at all times of the year and times of the day when the air circulation system

is operating.

69
1 REFERENCES

[1] Grot, R. A. A Low-Cost Method for Measuring Air Infiltration Rates in a Large Sample of Dwellings, pp. 50-59 in Hunt, C. M. , King, J. C. , and

Trechsel, H. R. , eds.. Building Air Change Rate and Infiltration Measurements, American Society for Testing and Materials Special Technical

Publication 719, Philadelphia, 1980.

[2] Hunt, C. M. Air Infiltration; A Review of Some Existing Measurement

Techniques and Data, pp. 3-23 in Hunt, C. M. , King, J. C. , and Trechsel,

H. R. eds.. Building Air Change Rate and Infiltration Measurements,

American Society for Testing and Materials Special Technical Publication

719, Philadelphia, 1980.

[3] Lagus, P. L. Air Leakage Measurements by the Tracer Dilution Method—

Review, pp. 36-A9 in Hunt, C. M. , King, J. C., and Trechsel, H. R. eds..

Building Air Change Rate and Infiltration Measurements, American Society

for Testing and Materials Special Technical Publication 719, Philadelphia,

1980.

[4] Kusuda, T. Control of Ventilation to Conserve Energy While Maintaining

Acceptable Indoor Air Quality, ASHRAE Trans. 82(1), 1169-1182, 1976.

[5] Penman, J. M. An Experimental Determination of Ventilation Rate in

Occupied Rooms Using Atmospheric Carbon Dioxide Concentration, Building

and Environment 15: 45-47; 1980.

[6] Tamura, G. T. Carbon Dioxide Measurement in Open-Classroom School with

Outside Air-Supply Damper Closed to Conserve Energy, National Research

Council Canada Publication No. 169, Ottawa, 1980.

[7] National Institute for Occupational Safety and Health (NIOSH) , Conference

on Occupational Health Issues Affecting Clerical/Secretarial Personnel,

Cincinatti, July 21-24, 1981; Proceedings, NIOSH, Cincinatti.

[8] Spengler, J. D. , Hollowell, C. D. , Moschandreas , D. J., and Fanger, P. 0.

eds.. Indoor Air Pollution—Proceedings of the Symposium Held at the

University of Massachusetts, Amherst, MA, Oct. 12-16, 1981; Pergammon

Press, Elmsford, NY, 1982.

[9] Sundell, J. Nordic Guidelines for Building Regulations Regarding Indoor

Air Quality, in ref. [12].

[10] The American Society of Heating, Refrigerating, and Air Conditioning

Engineers, Inc. (ASHRAE), Ventilation for Acceptable Indoor Air Quality,

ASHRAE Standard 62-1981, Atlanta, 1981.

[11] Sandberg, M. What is Ventilation Efficiency?, The National Swedish

Institute for Building Research, 1980.

70
[12] Sandberg, M* , and Svensson, A. Measurements of the Ventilation Efficiency

by Using the Tracer Gas Technique, The National Swedish Institute for

Building Research, Stockholm, 1980.

[13] American Society for Testing and Materials (ASTM) , Standard Practice for;

Measuring Air Leakage Rate by the Tracer Dilution Method, ASTM Standard

No. E741-80, Philadelphia, 1980.

[14] Honma, H. Ventilation of Dwellings and Its Disturbances, FAIBO Graflska,

Stockholm, 1975.

[15] Howard, J. S. Building Scl. 1, 251-257, 1966.

[16] Hunt, C. M., and Burch, D. M. ASHRAE Trans. 81(1); 186-201; 1975.

[17] Grlmsrud, D. T., Sherman, M. H. , Janssen, J. E., Pearman, A. N. , and

Harrje, D. T, An Intercomparison of Tracer Gases Used for Air

Infiltration Measurements, ASHRAE Trans. 86(1); 258-266; 1980.

[18] Bassett, M. R. , Shaw, C. Y. , and Evans, R. G. An Appraisal of the Sulfur

Hexafluoride Decay Technique for Measuring Air Infiltration Rates In

Buildings, ASHRAE Trans. 87(2), 1981.

[19] Slnden, F. W. , Wind, Temperature and Natural Ventilation-Theoretical

Considerations, Energy and Buildings 1(3), 275280, 1978.

[20] Sepsy, C., McBride, M. F., Blancett, R. S., and Jones, C. D. , Fuel

Utilization In Residences, Electric Power Research Institute Report

No. EA-894, Palo Alto, 1978.

[21] Tamura, G. T. , and Shaw, C. Y., Studies on Exterior Wall Air tightness

and Air Infiltration of Tall Buildings, ASHRAE Trans. 82(1); 122-134;

1976.

[22] Shaw, C. Y. , and Tamura, G. T., The Calculation of Air Infiltration Rates

Caused by Wind and Stack Action for Tall Buildings, ASHRAE Trans. 83(2);

145-158; 1977.

[23] Condon, P. E., Grlmsrud, D. T., Sherman, M. H. , and Kamerud, R. C., An

Automated Controlled-Flow Air Infiltration Measurement System, pp. 60-72

in Hunt, C. M. , King, J. C., and Trechsel, H. R. , eds.. Building Air

Change Rate and Infiltration Measurements, American Society for Testing

and Materials Special Technical Publication 719, Philadelphia, 1980.

[24] Sherman, M. H. , Air Infiltration in Buildings, Lawrence Berkeley Laboratory

Report No. LBL-10712, Berkeley, 1980.

[25] Hill, J. E. , May, Jr., W. B., Richtmyer, T. E., Elder, J., Tibbott, R. L.

,

Yonemura, G. T., Hunt, C. M. , and Chen, P. T. , Performance of the Norris

Cotton Federal Office Building for the first 3 years of operation. National

Bureau of Standards Report .No. Building Science Series 133, Washington,

1981.

71
[26] Etheridge, D. W. Experimental Techniques for Ventilation Research,

pp. 45-71 in First Air Infiltration Center (AIC) Conference: Air Infiltration and Measuring Techniques, AIC, Bracknell, UK, Oct. 6-8 , 1980.

[27] Etheridge, D. W. , and Alexander, D. K. The British Gas Multi-Cell Model

for Calculating Ventilation, ASHRAE Trans. 86(2): 808-821; 1980.

[28] Sinden, F. W. Multichamber Theory of Air Infiltration, Building and

Environment 13, 21, 1978.

[29] Hunt, C. M. , and Treado, S. J. A Prototype Semi-automated System for

Measuring Air Infiltration in Building Using Sulphur Hexafluoride as a Tracer, National Bureau of Standards Technical Note 878, 1976.

[30] Harrje, D. T., Hunt, C. M. , Treado, S. J., and Malik, N. Automated

Instrumentation for Building Air Infiltration Measurement, Princeton

University Center for Environmental Studies Report No. 13, 1975.

[31] Harrje, D. T. and Grot R. A. Automated Air Infiltration Measurements and

Implications for Energy Conservation, in Energy Use Management,

Proceedings of the International Conference, Vol. I: 456-464; Pergamon

Press, New York, 1977.

[32] Harrje, D. T., and Grot, R. A. Instrumentation for Monitoring Energy

Usage in Buildings at Twin Rivers, Energy and Buildings 1(3): 293-299;

1978.

[33] Grot, R. A., Hunt, C. M. , and Harrje, D. Tracer Gas Automated Equipment

for Complex Building Studies, pp. 103128 in First Air Infiltration

Center (AIC) Conference: Air Infiltration and Measuring Techniques,

Proceedings; AIC, Bracknell, UK, Oct. 6-8 , 1980.

[34] Hartmann, P., and Muhlebach, H. Automatic Measurements of Air Change

Rates (Decay Method) in a Small Residential Building without any ForcedAir Heating System, pp. 87-101 in First Air Infiltration Center (AIC)

Conference: Air Infiltration and Measuring Techniques, Proceedings; AIC,

Bracknell, UK, Oct. 6-8 , 1980.

[35] Silberstein, S. Air Leakage Measurements of an Unpartitioned Mobile

Home, National Bureau of Standards Interagency Report 80-2105, Washington,

1980.

[36] Grot, R. A., and Clark, R. E. Air Leakage Characteristics and

Weatherization Techniques for Low-Income Housing, pp. 178-194 in Thermal

Performance of the Exterior Envelopes of Buildings, American Society of

Heating, Refrigerating and Air-Conditioning Engineers, Inc. Publication

No. SP-28, Atlanta, 1981.

[37] Kumar, R. , Ireson, A. D., and Orr, H. W. An Automated Air Infiltration

Measuring System Using SF5 Tracer Gas in Constant Concentration and

Decay Methods, ASHRAE Trans. 85(2): 385-395; 1979.

72
[38] McNally, T. Building Department, Institute of Technology, Taastrup,

Denmark, Personal Communication.

[39] Petterson, B. Swedish Council for Building Research, Stockholm, Personal

Communication.

[40] Dumont, R. S. National Research Council Canada, Division of Building

Research, Saskatoon, Personal Communication.

[41] Sherman, M. H. , Grimsrud, D. T., Conddon, P. E., and Smith, B. V. Air

Infiltration Measurement Techniques, First Air Infiltration Center (AIC)

Conference; Air Infiltration and Measuring Techniques, Proceedings;

AIC, Bracknell, UK: 9-44; Oct. 6-8, 1980.

[42] San, N. I. Dangerous Properties of Industrial Materials, Third Edition,

Van Nostrand, New York, 1968.

[43] Dick, J. B. Measurements of Ventilation Using Tracer Gas Technique,

Heating, Piping and Air Conditioning Journal 23(5): 131-137; 1950.

[44] Colbentz, C. W. , and Achenbach, P. R. Design and Performance of a Portable Infiltration Meter, ASHRAE Trans. 69: 358-365; 1963.

[45] Prado, F., Leonard, R. G., and Goldschmit, V. W. Measurement of

Infiltration in a Mobile Home, ASHRAE Trans. 82(2): 151-166; 1976.

[46] Porter, K.

,

and Volman, D. F. Flame Ionization Detection of Carbon

Monoxide for Gas Chromatographic Analysis, Analyt. Chem. 34(7); 748-749;

1962.

[47] Hill, J. E., and Kusuda, T. Dynamic Characteristics of Air Infiltration,

ASHRAE Trans. 81(1): 168-185; 1975.

[48] Drivas, P. J., Simmonds, P. G. , and Shair, F. , H. Experimental

Characterization of Ventilation Systems in Buildings, Environmental

Science and Technology 6(7): 577-666; 1972.

[49] Lidwell, 0. M. The Evaluation of Ventilation, J. Hyg. 58; 297-305; 1960.

[50] Occupational Exposure to Anesthetic Gases and Vapors, National Institute

for Occupational Safety and Health (NIOSH), Washington, 1977.

[51] Ellins, H. R. , and Wensman, C. W. Natural Ventilation of Modern Tightly

Constructed Homes, Presented at American Gas Association-Institute of Gas

Technology Conference on Natural Gas Research and Technology, Chicago,

February 28-March 23, 1971.

[52] Janssen, J. E., Torborg, R. H. , and Bonne, V. Measurement of Heating

System Dynamics for Computation of Sensoral Efficiency, ASHRAE Trans.

83(2): 159-169; 1977.

73
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5. FAN PRESSURIZATION TECHNIQUES

A building may be pressurized by a fan and the air flow rate measured to

estimate envelope tightness. At a fixed pressure difference, the smaller the

air flow rate, the tighter the envelope. In order to overpower any natural

pressure differences, and to obtain measurable air flow rates, large pressure

differences are usually induced. For even medium-sized buildings this may

require the use of an enormous fan or the simultaneous use of a number of fans.

Figure 5.1 shows a fan mounted in a tall building [1]. A way to avoid the

necessity for using large fans is to measure isolated wall and roof areas [2].

This step method also yields permeabilities of individual building surface

components. To estimate the overall envelope permeability a calculation can be

made based upon the assumption that the building as a whole shows permeability

similar to the measured ones [1-4]. This procedure is useful in estimating

naturally occurring air flow rates from pressurization data.

Figure 5.1 Tall building installation of large fan

74
H,- ' & to 't^-f ^ ’^'f^

.is} e^iK .4.0J (.,.»/ »'»^% ' *i n>»Jij(>(|i^ i^'v#^'»t>.5s,ii

^fiit-Hii l^l•1. •^^W'^ttiai iii,;f^ f,U }a>»(WP#^iiA gii5«,i9j!%r

V. f ,H» (: , ..

ftf io-4i<«> ‘J, iM^i- 9, 'ifejsti,

J, \i.«ar4*.,,r»tlKi|1|ai;^*;’,i«ait;,.«^

. • '^-

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. ^. '^ '‘I

' ' >,

-iift - ,. ”

«» .4* . ^o* ’• SiicaiR .

' ’

.

< •i' i • . i w
Another method is to use the building's air handling system to pressurize the

building. Only overall permeabilities for the combined effects of walls and

top and bottom separations are obtained by curve fitting. This method cannot

be used to derive permeabilltes of individual components such as windows or doors.

In this section the basic pressurization technique individual component testing,

and use of the building's air handling system to calculate overall surface

permeabilities is described.

5.1 SUMMARY

The fan pressurization technique [5-7J measures envelope tightness directly,

independent of weather conditions unless they are extreme, and includes the

contributions from all leakage openings. The building is pressurized or depressurized by a fan and the inward or outward air flow (as the case may

be) is measured. Buildings can be directly compared by expressing the induced

air leakage at a standardized pressure difference, usually 50 Pa [7]. When

this type of test can be applied it is much easier to perform than a tracer

study; a fan mounted in an airtight "blower door" assembly is mounted either

in a window or doorway, and the measurement can be accomplished in minutes.

The potential problem of inadequate mixing tracer gas measurements does not

represent an important factor (e.g., stagnant pockets or air) with fan

measurements.

The fan pressurization method has the advantages of simplicity, direct

comparability with other buildings and times, intrinsic meaningfulness without

reference to temperature and wind conditions, usefulness in locating leakage

openings when used in conjunction with infrared thermography (see chapter 2)

[5,8,9], and the ability to assess the effectiveness of retrofit measures

applied one at a time. The pressure differences induced by fans are so large

that by comparison weather has hardly any effect. A disadvantage under some circumstances is that pressure differences are 10 to 100 times as great as those that occur naturally, and consequently, leakage pathways may be different. However, Taraura and Shaw devised methods [3,4,10,11] to calculate air

leakage rates under natural conditions for large buildings. Several models

permit this to be done for houses as well [12-17], with poor to fair

predictability.

An alternative pressurization technique that which presumes that pressures

closer to naturally occurring ones are developed is the infrasonic [18], or

"AC fan" [19], method. A piston totally inside the building, or mounted in a wall, alternately causes air to leak in and out. If the driving frequency is

low enough, there is little compression and decompression [20]. The frequency

also provides a means of distinguishing induced from natural effects; the

former can be electronically filtered. This method has not been widely used

yet and it may prove as difficult to apply to large buildings as the usual fan

pressurization test.

Infrasonic methods may be helpful in simulating natural conditions since they

produce lower pressure differences than fans, but their relationship to natural

75
conditions has not yet been determined. The positive displacement of the

piston is capable of overcoming the effects of external (wind driven) pressure

distributions which would normally be induced by natural weather conditions.

This method probably indicates leakage openings in a manner closer to faninduced pressure differences.

5.2 DESCRIPTION OF EQU1PMF.NT

1. Air moving equipment; a fan, blower, or blower door assembly (illustrated

in figure 5.2), which is capable of establishing indoor-outdoor pressure

differences in the range 10-70 Pa. The air flow should be constant at each

fixed pressure for the time required to read the air flow rate or velocity.

2. Pressure-measuring device: manometer or pressure indicator capable of

measuring pressure differences to within 2.5 Pa.

Figure 5.2 Schematic of blower door assembly used by Princeton University

76
3. An air flow or velocity measuring system should be capable of measuring the

air flow rate to within 6 percent of its average value. The Instrument

should be calibrated according to the manufacturer's instructions or in a calibrating wind tunnel.

4. Wind speed measuring device accurate to 1 km/h or 0.3 m/s.

5. Temperature measuring device accurate to 1®C.

6. Air flow regulating system: a device such as a damper, or variable motor

speed control, which will regulate and maintain air flow within specific

limits

.

7. Ductwork should accommodate both pressurization and depressurization.

8. The size of the air duct and the capacity of the fan or blower should be

matched so that the linear flow velocity within the air duct is within the

range of measurement of the air flow meter.

Table 5.1 Approximate Fan Capacities Required for Various Spaces

Building Type Building Volume (m^) Fan Capacity (m^/s)

Approximate

Cost

Schools 7500-20,000 23.6 $20,000

5-Story

Apartment

House not specified 23.6 $20,000

Single-Family

Houses "normal" 1-1.5 $2000 to $16,000

Single-Room

Office "normal" 0.7 $2000

Wall Section

in Room "normal"

0.04 $500 to $1000

A blower door assembly (figure 5.1) is a convenient way to perform the tests.

Components unique to this assembly are;

1. A door mount for a fan or blower adjustable to fit common door openings.

2. A fan or blower with a variable speed DC motor capable of establishing an

air flow rate of 5000 m^/h. The fan or blower should be calibrated according

to the manufacturer's instructions or with a set of standard flow orifices.

77
3. Fan speed controls (with display Indicator) for fan speed; if calibrated

it can be used for measurement of flow rate.

5.3 THEORY

The fan pressurization method is a procedure which uses a fan to pressurize or depressurize a building relative to outdoor conditions [5]. Almost instantaneously, nearly stable air flow is induced through leakage openings. The air

flow rate through leakage openings must be equal to the flow through the fan.

The latter can be measured and is given by;

q

= C»A • AP^ (5.1)

where:

C = flow coefficient

A = surface area, m^

n = flow coefficient, a number between 0.5 and 1.0.

Caution; The relative air flow Induced in any given leakage opening may bear

little relation to the share of natural air infiltration through that opening.

5.4 MEASUREMENT GUIDELINES [5]

Tests are conducted as follows:

Step 1. Preparations. Make general observations of the condition of the

building, including windows, doors, walls, roof, and floors. Measure and record

the wind speed (direction is optional), and outdoor and Indoor temperatures.

Place the air-moving apparatus near the structure and connect the air duct or blower door assembly to the building envelope, using a window, door, or vent

opening. Seal or tape openings to prevent leakage.

If no other flow meters will be used it is necessary to calibrate the fan to

obtain air flow rates.

If a damper is used to control flow, it should be in fully closed position at

the beginning of the test and adjusted to obtain desired pressure differences.

Step 2; Measure air flow rates at pressure differences from 10 to 70 Pa at

10-Pa Increments.

Several flow meters are available; for example, orifice plates and pitot tubes;

these measure pressure differences from which air flows can be calculated:

q = k«(2»AP«A/T)^/2 (5.2)

where

A = cross-sectional area of measuring tube, m^

T = absolute temperature, ®K

k depends on the instrument.

78
1

Generally the indoor-outdoor temperature difference is not large enough for a correction term to be required. The correction term is given by:

for depressurization. For example, suppose that during a pressurization test

= 20*0 and Tq = -10°C. Then the flow rate only has to be corrected by 5

percent; the correction factor may safely be ignored.

Similarly, an atmospheric pressure correction term may be used in high altitude

areas

:

with pressure, P, in kPa.

This factor can also usually be neglected; for example, for Denver, where the

elevation is 1600 m; the correction term would correct the air flow rate by

less than 10 percent.

A uniform pressure should be maintained in the building to within 20 percent of

the measured indoor-outdoor pressure difference. The maximum pressure variation

due to stack or wind effect should be no more than 10 percent of the measured

pressure difference. Some researchers do not perform tests when wind speeds

are above 15 km/h or 4 m/s [21]. Kronvall [7] asserts that in a pressurization

test, a wind speed of 8 ra/s can create a 10 percent variation in the flow rate;

for depressurization, a 10 m/s wind speed is needed to produce the same effect.

The thermal stack effect may be disregarded for one-story buildings. Kronvall

calculates that for a single-story house the pressure contribution of the stack

effect for AT = 30°K is only 1 Pa. A rule of thumb for two or more stories is

that the stack effect results in a pressure difference of approximately 0.1

Pa/K per story [5], At a temperature difference of 20°K, for example, the

stack effect in a 10-story building would result in a pressure difference of

approximately 20 Pa, which is substantial when compared to the induced pressures.

The Swedish Council for Building Research recommends that tests not be performed

when the temperature difference exceeds 30°K [7]. Both pressurization and

depressurization tests are recommended.

A major practical problem with using the pressure method for large buildings is

that either an extremely large fan or a number of ordinary-sized fans must be

used. Table 5.1 lists approximate fan capacities reported for various types

of spaces that have been measured. This table cannot be used as an absolute

(T /T )l/2

i o (5.3a)

for pressurization, and the reciprocal;

(5.3b)

(P/101. 325)1/2 (5.3c)

79
guide, inasmuch as fan capacity is governed by building tightness as well as by volume. It may be found impractical to try to achieve a pressure difference

of 50 Pa for an especially loose building; one may have to settle for a pressure

difference on the order of 5 Pa.

Another major problem is that the cross-sectional area of the ducts may be too

large to conveniently permit air flow measurements. In this situation tracer

gas may be required. Because the air exchange rate during fan testing is so large, the tracer study would probably have to be operated in an automated mode

with frequent readings. Bassett et al., [21] report that in a school a number

of small fans mixed the air better than a single fan ; a single fan caused

imbalanced tracer-gas concentrations. This suggests that several small fans

result in better mixing than a single large fan, which causes a moving front of

air that may not have time to become mixed. If tracer gas is used to measure

flow, the possibility of having nonuniform concentrations may be an important

consideration.

Nylund [22] claims it is very difficult to Isolate a space from adjoining spaces.

He also indicates that it is difficult to pressurize adjoining spaces so as to

maintain the same pressure difference with outdoors as the space being measured.

He suggests an alternative method to correct for the flows. In this method,

the pressures (with respect to the outside) are measured within houses attached

to the one of interest, the cross-flows are then calculated from the pressurization curve of the measured house, and subtracted. There is a correction term

which must also be subtracted, but it is unclear how it is calculated. An

advantage of his method, (assuming adjacent space corrections are valid)

is that it requires only one fan. Unlike the case of tracer-gas measurements

different rooms may be measured at different times and the results may be

meaningfully combined. Whether this method can be applied in office buildings

where an office may be next to not only two other offices, but also to a hallway, (and over and under other offices) is unclear. Nylund [23] has developed

a "reciprocity" test for this situation, but it is very difficult to understand.

Also, Nylund [22] is working with houses where the indoor-outdoor pressure

difference in the adjoining houses is measured through the water trap of the

toilet. In an office building, this approach may be impractical and a method

of measuring pressure differences such as the one [20] used by Shaw et al.,

[24] may have to be used.

Interpretation of Results

Step 3: Analyze the data. Calculate air flow rates if these are not read

directly, e.g., from fan speed, rpm, or from pitot tube pressure differences.

For both pressurization and depressurization, plot air flow rates vs. pressure

differences on log-log paper. The slope of this line is the flow exponent.

Alternatively, the flow exponent, as well as the flow coefficients, may be

calculated by linear regression analysis.

Calculate, or read from the graph, the value of the air flow rate at a pressure

difference of 50 Pa [7,8].

80
Accuracy of Technique

ASTM [9] estimates the uncertainty of the measurements as about 10 percent.

Kronvall [llj uses the following equation to estimate uncertainty:

m = (Z (5.4a)

1

where

m probable measurement error of a step, percent

The m^^ are explained In figure 5.3. The total measurement error Is obtained

from;

o 1/2

“total * ^ (5.4b)

From the data shown In figure 5.3 he calculates that uncertainties are less

than 10 percent.

Figure 5.3 Schematic of errors In fan pressurization measurements

81
Previous Use

There are only a very small number of reports of fan pressurization measurements

of large buildings in their entirety [2,3,25]. For example, Shaw and Jones [3]

measured the air leakage characteristics of schools using fans with variablepitch blades which can be manually adjusted to obtain flow rates between 0 and

23 m^/s. The fan was connected by ducts to a door that replaced the schools'

entrance door. This procedure is similar to the "blower doors" [5] used for

smaller buildings. Measurements were obtained only when wind speeds were less

than 15 km/h. The volumes were as high as about 20,000 m^. Pressure difference measurements were first taken with the air handling systems operating and

sealed. This reading, which indicates the amount of pressurization resulting

from an imbalance between outside air supply and exhaust rates of the airhandling systems, was then subtracted from the pressure difference readings

produced by the fan. Tests were run with various components, both sealed and

unsealed, to determine their contribution to air leakage. It was found that

the air handling system ducts themselves contributed 15 to 45 percent of the

schools' air leakage rates.

5.5 COMPONENT TESTING BY FAN PRESSURIZATION

Summary

An alternative to using many fans in a tall building is to sample parts of the

building that one hopes are representative of the entire building and then use Tamura and Shaw's [1,4] equations. The drawback of this approach, however,

is that there is no assurance that all of the wall sections of the building

have equal permeabilities. In such a study, a section of wall needs to be

isolated from the rest of the building.

Description of Equipment

An example of using the above system follows. A portable fan with a maximum

capacity of 200 1/s at 2 kPa was used. The fan was fitted with a manual damper

to control the flow rate. The test chamber surrounding the wall assembly was built of plywood panels covered with polyethylene sheets and sealed with tape.

Theory

The Tamura-Shaw equations [1,4] for stack action are:

qg = C«S (0.0342‘Y*P(4T^/TQ))^(6H)^''’V(nH) (5.5)

where

Y

= ratio of actual to theoretical pressure difference (thermal draft

coefficient)

S * ratio of* height of neutral pressure level above ground to building

height

82
H = building height, m

n = flow exponent,

Subscript "s” refers to stack action.

The coefficient y arises because the floors obstruct air flow. It is estimated

to range from 0.63 to 0.88.

The equation is simplified for practical purposes by assuming P =101.325 kPa,

Ti = 294®K, n = 0.65 and 6 = 0.5. The equation then becomes:

q = 0.96 C‘S(y*AT/Tq)®*^^ (5.6)

For wind action, the equation is:

q„ = 0.0925* a • (5.7) *w s where

:

a is a factor depending on wind direction.

L = length of wall facing wind.

Vg = wind speed at weather station.

Subscript "w" refers to wind action.

Shaw and Tamura [4J suggest combining wind and stack action as follows:

q = [1 + 0.24 (q3^/q^^g)3.3j

where:

qirg = larger of q^ and qg

9sml “ smaller of q^, and qg

Measurement Guidelines

Shaw [2] described 2 methods (direct and indirect) for measuring induced air

leakage rates of walls, windows and doors of multistory apartment buildings.

The direct method for measuring an entire wall assembly is illustrated in fig- ure 5.4. The test chamber was slanted towards the wall in order to facilitate

sealing the top to prevent leakage. Each wall component can also be measured

by a similar technique (figure 4.13). However, since it is difficult to obtain

a tight seal in this case, the adjoining areas are also pressurized, as illustrated in figure 5.4 for the direct method. Shaw [2] states that these pressures could be set to within 2.5 Pa of each other. The Indirect method is

performed identically to the direct wall assembly in question but the measurements are performed twice: with the component of interest sealed and unsealed.

The difference in flow is the leakage attibuted to the component.

83
Figure 5.4 Direct method for testing a wall

Accuracy of Technique

It was found that the air leakage rate of entire wall assemblies was within

15 percent of the summed leakage rates of the components.

5.6 USE OF BUILDING'S AIR HANDLING SYSTEM

Summary

The stack effect was usefully employed by Shaw et al., [24] in calculating flow

coefficients (i.e., permeabilities) of building walls, and top and bottom

separations. They used the air handling system to pressurize a building. All

typical floor spaces between the ground floor and the top mechanical floor of a building were pressurized using 100 percent outside supply air, with return and

exhaust systems shut down. Supply air rates were varied and pressure differences across the enclosure were measured. To ensure stable pressure

differences, tests were conducted in the absence of strong winds.

Theory

Under steady-state conditions, the rate of outside supply air equals the sum of

the leakage rates through the exterior walls of typical floors and bottom and

top separations

N

q = E [A^.(AP^)'^]. + C^.Au-(AP^)''^ + C. -A^- (AP. (5.9)

j=i

where

84
N “ total number of floors with typical wall construction and subscript;

"w" ** exterior wall

"b" = bottom separation

"t" = top separation

Measurement Guidelines

Note that q, AP^^, AP|j and APj. are measured quantities. The total air leakage

rate for the three components depends upon the values of pressure differentials

across each separation. The tests should be conducted when outdoor temperatures are lower than indoor temperatures, creating a stack effect that will

produce different pressure differences for the three components.

Fresh air supply rates were measured using total pressure averaging tubes [24],

a method with accuracy better than 6.5 percent. Pressure differences were

measured with a pressure recording instrument located on the top mechanical

floor. One side of the meter was connected to the rooftop pressure tap which

served as a reference pressure; the other side of the meter was connected to a pressure selector switch, from which plastic tubes were installed vertically

in a stair shaft ending at various floor spaces and to ground level outdoors.

Interpretation of Results

The data were analyzed by computer. The unknowns can be estimated by a trial

and error technique used in conjunction with the method of least squares. The

three flow exponents were assigned combinations of values between 0.5 and 1.0

(as this is the range of accepted values). The flow coefficients were then

obtained by the method of least squares for various combinations of exponents.

The set of values of flow coefficients and exponents were selected that gave

the lowest value of estimated error.

Accuracy of Technique

This method has not been validated for an actual building but a computer

simulation revealed that the lowest standard error did not necessarily yield

the values of the coefficients specified by the simulation. The authors state

that this is because the standard error of estimate is related to overall air

leakage rates. The wall leakage rates calculated from the computed wall flow

coefficients were within 10 percent of the specified wall air leakage rates.

Previous Use

Tamura and Shaw [1,4] conducted investigations to correlate natural flow rates

with weather conditions. In those studies the values of the flow coefficients

were determined from test data and solution of equation 5.9.

85
REFERENCES

[1] Tamura, G. T., and Shaw, C. Y. Studies on Exterior Wall Air Tightness

and Air Infiltration of Tall Buildings, ASHRAE Trans. 82(1): 122-134;

1976.

[2] Shaw, C. Y. Methods for Conducting Small-Scale Pressurization Tests and

Air Leakage Data of Multi-Story Apartment Buildings, ASHRAE Trans. 86(1):

241-250; 1980.

[3] Shaw, C. Y., and Jones, L. Air Tightness and Air Infiltration of School

Buildings, ASHRAE Trans. 85(1): 8595; 1979.

[4] Shaw, C. Y., and Tamura, G. T. The Calculation of Air Infiltration Rates

Caused by Wind and Stack Action for Tall Buildings, ASHRAE Trans. 83(2):

145-158; 1977.

[5] American Society for Testing and Materials (ASTM) , Proposed Standard

Practice for: Air Leakage Measurement by the Fan Pressurization Method,

ASTM, Philadelphia, 1981.

[6] Kronvall, J. Testing of Houses for Air Leakage Using a Pressure Method,

ASHRAE Trans. 84(1): 72-79; 1978.

[7] Kronvall, J. Airtightness—Measurements and Measurement Methods, Swedish

Council for Building Research Publication No. ISBN 91-540-3201-6, Stockholm,

1980.

[8] Petterson, B., and Axen, B. Thermography—Testing of the Thermal

Insulation and Airtightness of Buildings, Swedish Council for Building

Research Publication No. ISBN 91-540-3171-0, Stockholm, 1980.

[9] Stewart, M. B. , Jacob, T. R. , and Winston, J. G. Analysis of Infiltration

by Tracer Gas Technique, Pressurization Tests, and Infrared Scans, pp.

138-148 in Thermal Performance of the Exterior Envelopes of Buildings,

American Society of Heating, Refrigerating and Air-Conditioning Engineers,

Inc. Publication No. SP-28, Atlanta, 1981.

[10] Shaw, C. Y. A Method for Predicting Air Infiltration Rates for a Tall

Building Surrounded by Lower Structures of Uniform Height, ASHRAE Trans.

85(1); 72-84; 1979.

[11] Shaw, C. Y. Wind and Temperature Induced Pressure Differentials and an Equivalent Pressure Difference Model for Predicting Air Infiltration in

Schools, ASHRAE Trans. 86(1); 268-279; 1980.

[12] Sherman, M. H. , Grimsrud, D. T., Condon, P., and Smith, B., V. Air

Infiltration Measurement Techniques, pp. 9-44 in First Air Infiltration

Center (AIC) Conference: Air Infiltration and Measuring Techniques,

Proceedings; AIC, Bracknell, UK, Oct. 6-8, 1980.

86
[13] Tamura, G. T. The Calculation of House Infiltration Rates, ASHRAE Trans.

85(1): 58-71; 1979.

[14] Sherman, M. H. , and Grimsrud, D. T. Infiltration Pressurization

Correlations: Simplified Physical Modelling, ASHRAE Trans. 86(2):

778-807; 1980.

[15] Shaw, C. Y. A Correlation between Air Infiltration and Air Tightness for

Houses in a Developed Residential Area, ASHRAE Trans. 87(2), 1981.

[16] Blomsterberg, A. K. , Sherman, M. H., and Grimsrud, D. T. A Model

Correlating Air Tightness and Air Infiltration in Houses, pp. 122-137 in

Thermal Performance of the Exterior Envelopes of Buildings, American

Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc.

Publication No. SP-28, Atlanta, 1981.

[17] Warren, P. R. , and Webb, B. C. The Relationship between Tracer Gas and

Pressurization Techniques in Dwellings, pp. 245-276 in First Air

Infiltration Center (AIC) Conference: Air Infiltration and Measuring

Techniques, Proceedings; AIC, Bracknell, UK, Oct. 6-8, 1980.

[18] Card, W. H. , Sallman, A., Graham, R. W., and Drucker, E. E. Infrasonic

Measurement of Building Leakage: A Progress Report, pp. 73-88 in Hunt, C.

M. , King, J. C. , and Trechsel, H. R. , eds.. Building Air Change Rate and

Infiltration Measurements, American Society for Testing and Materials

Special Technical Publication 719, Philadelphia, 1980.

[19] Sherman, M. H. , Grimsrud, D. T. , and Sonderegger, R. C. The Low Pressure

Leakage Function of a Building, pp. 94-121 in Thermal Performance of the

Exterior Envelopes of Buildings, American Society of Heating, Refrigerating and Air Conditioning Engineers, Inc. Publication No. SP-28, Atlanta,

1981.

[20] Ma, W. Y. L. The Averaging Pressure Tubes Flowmeter for the Measurement

of the Rate of Airflow in Ventilating Ducts and for the Balancing of

Airflow Circuits in Ventilating Systems, Inst, of Heating and Ventilating

Engrs., Feb., 1967.

[21] Bassett, M. R. , Shaw, C. Y. , and Evans, R. G. An Appraisal of the Sulfur

Hexafluoride Decay Technique for Measuring Air Infiltration Rates in

Buildings, ASHRAE Trans. 87(2), 1981.

[22] Nylund, P. 0. Tightness and its Testing in Single and Terraced Houses,

pp. 157-170 in First Air Infiltration Center (AIC) Conference: Air

Infiltration and Measuring Techniques, Proceedings; AIC, Bracknell, UK,

Oct. 6-8, 1980.

[23] Nylund, P. 0. The Application of Reciprocity in Tightness Testing, pp.

143-155 in First Air Infiltration Center (AIC) Conference: Air

Infiltration and Measuring Techniques, Proceedings; AIC, Bracknell, UK,

Oct. 6-8, 1980.

87
[2A] Shaw, C. Y. , Sander, D. M. , and Tamura, G. T. Air Leakage Measurements

of the Exterior Walls of Tall Buildings, ASHRAE Trans. 79(2): AO-48;

1973.

[25] Hunt, C. M. , and Treado, S. J. Air Exchange Measurements in a High-Rise

Office Buuilding, pp. 160-177 in Thermal Performance of the Exterior

Envelopes of Buildings, American Society of Heating, Refrigerating and

Air Conditioning Engineers, Inc. Publication No. SP-28, Atlanta, 1981.

88


6 . TEMPERATURE MEASURING SPOT RADIOMETERS

6.1 SUMMARY

For this technique* a hand-held infrared thermometer (spot radiometer) is used

to measure the inside surface temperature of a building component and the inside

air temperature. Another measuring device is used to measure the coincident air

temperature at the exterior surface of the component. These three measured

temperatures are subsequently substituted into simple mathematical relations for

estimating the apparent thermal resistance of the component. This technique is

considered inaccurate under typical transient thermal conditions to which

buildings are subjected. A suitable application is to determine qualitatively

whether a wall is insulated or if insulation voids or other thermal defects

are present. This technique is not accurate enough to permit an R-13 wall to

be distinguished from an R-15 wall.

6.2 DESCRIPTION* SPECIFICATION AND COST OF EQUIPMENT

A spot radiometer is a hand-held device used to radiometrically measure the

equivalent blackbody temperature of a relatively small area of a surface. It

is generally small, light weight* and frequently gun-shaped (see figure 6.1).

Spot radiometers are calibrated by pointing the device at a surface of known

temperature and adjusting a knob on the device such that the device radiometrically determines the correct temperature of the reference surface. Manufacturers frequently provide reference surfaces having approximate blackbody

characteristics (i.e.* reflecting little radiation from surrounding surfaces).

Interior surfaces of buildings generally have emittances which range from about

0.80 to 0.95. Consequently* the apparent surface temperature determined using

a spot radiometer differs from the actual surface temperature by a small amount.

This problem may be partially avoided by using a reference surface which has a surface emittance more nearly equal to that of the subject surface.

Figure 1. Spot radioaeter Figure 6.1 Spot radiometer

89
The device is pointed at a surface area of interest and depressing the on-off

trigger. The device senses the total infrared radiation over a particular

wavelength band emanating from the surface, including both the self-emitted

surface radiation and reflected radiation from surrounding surfaces. The

device is calibrated to read out the apparent radiance temperature on either a digital or meter display. The apparent radiance temperature is defined as the

temperature of a perfectly black surface which would radiate the same amount

of thermal radiation as a real surface at the same temperature and having a surface emittance different from unity.

Typical specifications for spot radiometers include a temperature measurement

resolution within + 0.5°F, although a device with a reported resolution of

within + 0.1°F is now commercially available. Spot radiometers generally have

their principal spectral response in the range of 10 microns, and a response

time of less than 2 seconds. Since contact with the surface to be measured is

not necessary with these devices, it is possible not only to minimize the error

due to disturbing the thermal regime of the subject surface, but also to determine the surface temperature of otherwise Inaccessible objects, such as secondstory walls. The target size increases with increasing distance from the

object to the instrument at a typical ratio of 1-inch diameter target per 15-

inch operating distance. A spot radiometer indicates the average apparent

temperature for the surface being measured over its entire field of view.

Therefore, operation close to a wall will indicate apparent surface temperature

of a small region, while operation at a distance will indicate apparent surface

temperature for a larger region. Spot radiometers which measure temperature

are made by approximately five firms in the United States. They vary in price

from $300 to $1500.

Temperature

gradient

Interior Exterior

Figure 6.2 Exterior wall schematic

90
Spot radiometers may be equipped with an audio device which can be adjusted to

produce a change in audible tone when the radiometer is panned across a thermal

anomaly of the building envelope such as an air infiltration path or a place in

the wall where Insulation is missing. A spot radiometer equipped in this fashion can be used to quickly pan the envelope of a building, readily detecting

gross thermal defects.

6.3 THEORY

Consider the exterior wall schematic shown in figure 6.2.

The instantaneous heat-loss rate (q) at the inside surface is given by:

q = hx • (Ti-T^) (6.1)

where hj is the overall heat-transfer coefficient at the inside surface, Tj

is the interior air temperature, and T^ is the interior surface temperature.

Under steady-state conditions, the heat-loss rate through the wall may also be

determined using the relation:

q = (Ti-To)/R (6.2)

where R is the air-to-air thermal resistance and Tq is the exterior surface

temperature. Under steady-state conditions, we may combine Eq. (6.1) and (6.2)

to give:

R =(Tx-To)/hi.(Ti-T^) (6.3)

If this model is valid, the thermal resistance of an exterior wall can be

determined based on the three pertinent temperature measurements, together

with an assumed value for the Interior surface heat-transfer coefficient.

The overall heat transfer coefficient (hx) at the inside surface is equal

to the sum of the radiative and convective heat transfer coefficients. The

radiative heat transfer coefficient (h^.) can be estimated by the relation:

h = 4.a*T^ (6.4)

where a is the Stef an-Boltzmann constant and Tg is the average absolute

temperature of the surroundings.

Under typical indoor conditions, a representative value for h^. is

1.0 Btu/(h«ft2 . °F). The convective component of the overall surface heat

transfer coefficient (h^,) is a complicated function of: 1) the surface-to-air

temperature difference; 2) large-scale convective air movement patterns present

in a room; and 3) the height above the floor. In the absence of large-scale

convective air movement, a laminar f ree-convection boundary layer will exist

at the top of the wall. This boundary layer will grow in size, passing through

a transition region, and then turn into a turbulent boundary layer farther down

the wall. Generally, the convective component will vary between 0.2 and 0.4

91
Btu/(h»f t^°F) . It is recommended that the following convective heat transfer

coefficient be used to represent the average conditions existing on the wall;

h^ = 0.22(Tj-T^)°’^^ (6.5)

where (Tj-T^) is the surface-to-air temperature difference in degrees Fahrenheit.

This relation is applicable for natural convection in the turbulent regime.

6.4 GUIDELINES FOR CONDUCTING MEASUREMENTS

Spot checks to determine the apparent thermal resistance of a surface should be

carried out no sooner than 3 hours after sunset in order to avoid solar loading

effects on exterior surfaces and to obtain a minimum variation of the outdoor

air temperature [1]. Another requirement is that the indoor-to-outdoor temperature difference shall exceed 18°F in order to produce measurable temperature

differences between the Inside surface of a building component and the indoor

air [1]. It is also recommended that the building heating plant be turned off

for approximately 30 minutes prior to carrying out measurements in order to

minimize the effect of surface-to-air temperature fluctuations due to the

cyclic operation of the heating plant. Once the foregoing conditions have been

met, the interior shading devices (such as blinds and draperies) should be

closed in order to minimize the effect of radiation exchange between Interior

window surfaces and other interior room surfaces.

Next, the spot radiometer is calibrated by pointing it at a reference surface

of known temperature. The spot radiometer is adjusted to read the temperature

of the reference surface. The spot radiometer is subsequently aimed at a spot

on a building surface and the surface temperature, T^, is recorded. The Indoor

ambient temperature, Tj, is then determined by pointing the spot radiometer at

a low-heat-capacity object having an emlttance comparable to that of the subject surface. The object should be in front of the subject surface and in

thermal equilibrium with the surrounding air. A spray-painted polystyrene

board with an attached thermocouple and associated digital readout device will

serve both as a reference surface for calibrating a radiometer and also as a low-heat-capacity object for measuring the indoor air temperature. The average

temperature, Tg, of the surrounding surfaces may be estimated by attaching a highly radiometrically reflective surface onto the subject building surface. A

spot radiometer aimed at such a surface will sense predominantly reflected radiation from surrounding surfaces and therefore will essentially measure the

equivalent blackbody temperature of the surroundings. The outdoor air temperature, Tq, should be measured with a conventional thermometer in order to

avoid inaccurate measurements of the spot radiometer at low temperature. The

measured values for Tg, Tj, T^, and T^ are substituted into eq. (6.3), (6.4),

and (6.5) to calculate the thermal resistance, R, of the building component.

6.5 INTERPRETATION OF RESULTS

If the thermal resistance, R, as determined by the foregoing technique is

found to be less than 7, then the wall should be considered to be uninsulated.

Otherwise the wall is insulated.

92
6.6 ACCURACY OF THE TECHNIQUE

One of the main contributing factors to inaccuracies of this technique is the

inability to accurately resolve the temperature difference (Tj-T^) required

in Eq. (6.3). Most commercially available spot radiometers are able to resolve

temperatures to within +0.5 °F. For this tolerance, the maximum uncertainty

in measuring (Tj-T^) is + 1.0 ®F. Consider an insulated wall (R = 15) and an uninsulated wall (R = 5) both subjected to a 40 ®F inside-to-outside temperature difference. The resulting heat-flow rates as given by Eq. (6.2) are 2.7

Btu/(h»ft2) for an insulated wall, and 8.0 Btu/(h»ft2) for an uninsulated wall.

A representative value for the inside surface heat transfer coefficient is 1.2

(Btu/h»f t2.°F) . Using Eq. (6.1), the air-to-surface temperature difference

(Tj- T^) is estimated to be 2.3 ®F for an insulated wall and 6.8 °F for an uninsulated wall. Hence, the error in estimating (Tj-T^) is 1.0/2. 3 = 43 percent for an insulated wall and 1.0/6. 8 = 15 percent for an uninsulated wall.

Using a spot radiometer with a resolution of + 0.1 °F, which is now commercially

available, would reduce these errors by a factor of 5 (8 percent and 3 percent,

respectively). The uncertainty in measuring (Tj-T^) in Eq. (6.3) is also about

+ 1.0 °F, and the uncertainty in specifying the overall heat transfer coefficient (h) is probably about + 0.2 Btu/(h«f

t

2 *°F) . The foregoing uncertainties

are summarized in table 6.1.

Table 6.1 Summary of Uncertainties in Measuring the Thermal

Resistance of Insulated (Uninsulated) Walls

Uncertainty

Parameter Normal Range % Error

Tl-Tw

Tl-To

+ 0.2 Btu/h.ft2.®F + 17%

+ 1.0°F (+ 0.2°F) + 43% (+ 8%)

+ 1.0°F + 2.5%

It has been shown in ref. [2] that when these uncertainties are introduced

into Eq. (6.3), the net uncertainity (Ej.) is given by the relation;

®r

" ^y^ (6.6)

where E^^, Ey, and are the uncertainties given in table 6.1. Substituting

the uncertainty values, the net uncertainty in determining the thermal resistance of an insulated wall is 46 percent. The uncertainty in determining the

thermal resistance of an uninsulated wall is found to be 23 percent. The

foregoing error analysis is optimistic because the effect of time-dependent

heat flow due to diurnal temperature fluctuations was not considered.

93
6.7

AVAILABILITY OF EQUIPMENT AND ENERGY AUDITS WITH SPOT RADIOMETERS

Temperature-measuring spot radiometers are manufactured by approximately six

firms in the United States. In the DoE survey of firms providing infrared

services, no firm used only a spot radiometer [5]. There are indications that

that energy audit firms and insulation contractors do use these devices, but

detailed information describing such usage is unavailable.

6.8

PREVIOUS USE

The major previous use of this technique has been for quality control purposes

[5] and made by firms engaged in retrofit installations of blown and foam-inplace insulation.

6.9 TRAINING REQUIREMENTS FOR PERSONNEL USING SPOT RADIOMETERS

These devices are simple to use and require little special training if used for

quality control. If they are used in energy audits, the responsible person

should be trained in energy audits and also have training to level I for a paraprof essional as indicated by the training program of Public Works Canada

(see section 2.8).

6.10 PROGRAM APPLICATION RESTRICTIONS

This technique should be restricted to finding gross deficiencies in smaller

federal buildings in colder climates. It is advisable that spot radiometer

measurements should be made in conjunction with thermographic surveys.

94
REFERENCES

[1] Application of Infrared Sensing Devices to the Assessment of Building

Heat Loss Characteristics, ASHRAE Standard 101, 1981.

[2] Treado, S. J. and Burch, D. M. Evaluation of Hand-Held Infrared

Thermometers for Wall Thermal Resistance Determinations, NBSIR 79-1736,

National Bureau of Standards, July 1979.

[3] Berman, H. L. Direct Radiometric Estimates of Heat Flow and Thermal

Resistance Using Spot Radiometers, Proceedings of Thermosense IV,

Society of Photo-Optical Instrumentation Engineers, 1982.

[4] Hamoen, D. W. J. A New Low-Cost Residential Energy Audit Technique,

Proceedings of Thermosense IV, Society of Photo-Optical Instrumentation

Engineers, 1982.

[5] Shumaker, D. L. , Snow, F. L. R. , Barthle, C. , Culp, T. L. and Wood, J. T.

Status of Thermal Imaging Technology as Applied to Conservation, Department of Energy Report HCP/M 4228-01, 1979.

95

1



7. RADIOS ITY MEASURING SPOT RADIOMETERS

7 .

1

SUMMARY

In this technique a hand-held infrared spot radiometer is used to measure the

radiosity emanating from a surface and the radiosity incident upon the surface

to obtain the net radiative heat flow from the surface. The total heat flow

from the surface is then estimated by assuming standard values for the ratio

of the convective heat transfer coefficient to the radiative heat transfer

coefficient. By measuring the inside-to-outside temperature difference, an estimate of the thermal resistance can be made if the building element is close

to steady-state conditions. The spot radiometer used for this technique indicates radiosity directly in energy flow units of (Btu/Cft^.h) or watts/m^ and

is referred to as an energy meter.

7.2

DESCRIPTION AND COST OF EQUIPMENT

The device used for this type of measurement is basically a spot radiometer

which does not have the linearization circuitry to convert from radiosity to

temperature. The device is calibrated to indicate radiosity in energy flow

units, either Btu/(ft2«hr) or watts/m^. The sensitivity of these devices

is approximately +0.1 Btu/(f t^»hr) . This equipment has a spectral response

in the 8-14 micron band, a response time of 3 seconds and a spot size of 4 in

at 6 ft (a ratio of 1 in target diameter to 18 in operating distance).

At present one manufacturer makes this type of spot meter for approximately

$1000. An additional temperature-measuring device is required for use with the

equipment if thermal resistance measurements are to be made.

7 .

3

THEORY

The theory of operation for the energy meter version of the spot radiometer is

based on the equation for heat flow from the surface of the wall to be measured

q = qr + qc

= qj. (1 + qc/qr) (7.1)

= qj. (1 + hc/hj.)

where

q is the total instantaneous heat loss rate,

qj. is its radiative component,

q^ is its convective component,

he is the inside convective surface heat transfer coefficient, and

hr is the inside radiative surface heat transfer coefficient.

Since qj. is the net radiative heat transfer from the surface, it is the

difference between the radiosity, Jx, from the surface and the irradiance (or

the incidence radiation on the surface), Gx,

96
• — »

Ji:.

'5fiV

fknwB

^ '"^f .’^ ''.KW> '(tf'..-*tpi.,,-».^- h^;ki4\r i^ff^Qihm^ ^jt^7 ,,

NrtJiJ ?ordbit:tt fec^ iS H»>t|UT

W4!.- . Jtdrf eij'l' .p-<»i''^.-*j #Ka-,s«OTi 'V^'> ^ -.i 3,^fT bt' T i i.,»>*w i>?*

;^rf4 b‘ieihmi>i l^f ’ !W>r,3l> -{! *!o-< >

"Iftod «v ‘5i»fi_ib?-'-3jj,«>o

?• V«i5*r . f

. v» vaifjtt^n'? i^fcp !i|»U'^wor&^~»J^V4>U *-/ ,'i^f 3 ^»m[»-v'

^ ‘ -»#i5 <njij nj#Mni.tw»

ft'iff'litotOrfi iiii.J 74i5 &9(ui 1.(^7 •'7i^„;J.U'"'Ai*|.srp> » ^»\R3ri.i. i« 0%s,n - .1«; »,oXl iti !xi^aa:^io t fi'«>llyi.7

• Tf-iW' Yaiiff^ r.i, It II o’ Wi minr «i

^-rt J •

0? ntrii, tij TJ3 f ti irf3 ^ iSVP^' I?6i3 ii-t vil^ai -rt |^»3*adiis:3 ^

•«©;U''36 ,,^j;>iu

^.tf ft ^ \i|3^ X ,Q 1 ^ r. la osts fi ^a« 0teaa®#< tiKM

• ,

- ^nftfcie;| ti}-M. ui l io otim- ^

ao 9^3 «id^l isn^Tjaa / jj^j.-tf >*j^

^ 9£ii f^.xHr upttr tff’tiifp»i bI ?v<Mb ta,o?#iy /ttttR pA

.-ijae -id »5b ‘-iv^fc-'-i-TfiiRiiiart I:

K^

"I I a #> !.5

9

iriiifi/ilJasii XMpj 10 nvk%t9v ^rfi aor lu V7o&j' t int

•fcsXuoia* .dci .v^r^i4i.»» vfi la v?pi> 3iiV|

(i.\?

ai’

. -AV. .

' • t(' • V

' 1 lA' I'V?.-'

S..;iii'w 6S

t»vjr3!iiifea'ygRai

^ 4!* > . . If -, *'.

i ^

*'::. 5iv3«Mfl’4’5 ,’J«'»4? ' '> 1 ./r"

'''

3/R.r>^ -«S^4^Ai5>{7 i.d a 1 • . - ' ''

' v^.^T .

' ' • M • . •; *® =s» 1^!^ «:.> I®

hui, «#: %:?^..;::^?i5;j>v

__B— (I'iO- ,.tTp#>tiW '»tx'n^-6uy 1^ j

". ''

' a .

•' '".iiffii 'll.
qr

* Ji - Gi (7.2)

The radioslty measuring spot radiometer (obtained by pointing at the wall

surface) measures Jj directly in energy units. The measurement of Gj is

obtained by one of two techniques.

The simplest method is to point the spot radiometer at the interior surfaces

of the room and measure their radiosity (k = 1 .n).

The incidence radiation can be determined by the relation

n Gj = Z Aj^ (j)|^ (7.3)

k=l

where A]^ is the area of the interior surfaces, and (j>]^ is the view factor

between the interior wall and the spot being measured.

If the radiosities are approximately equal, then

n Gi = Ji( E = Ji (7.4)

k=l

since E = 1 for a closed room. In Eq. (7.4) is the average of and

usually taken to be the radiosity of the wall opposite to that being measured [ 1 ]

.

A second method is to place a small reference pad [2] in the field of view near

the surface being measured (see figure 7.1). If the pad is constructed of

insulating material so that the net heat flow is zero then it can be shown that

ref ref ref

q net = 0 =

‘^i " ^1 + ^c

where

is the radiosity of the reference pad and q^^^^ is the convective heat

flow between the reference pad and the room air.

If 0, then measuring gives the incident radiation on the

reference pad which is the same as that incident on the wall:

Gj « (7.6)

By performing a heat balance on the reference pad, it can be shown that

qcref

I I h h

r c

^r +

^c

(To - Ta) (7.7)

97
i i 1 1 1 1 1 1 1 1 i i

Figure 7.1 Use of reference pad for measuring incident radiation [2]

where

is the room air temperature,

Tg is the average temperature of the interior surfaces, and

I t h^ and h^ are the radiative and convective surface heat transfer

coefficients of the reference pad.

t If the reference pad is constructed from low emissivity material, then h^ « 0

and relation (7.6) would be valid. The thermal resistance of the wall is then

determined from the equation

R = ^ = at (7.8)

q (1 + hr/hc)(Ji - Ji)

where

Jx is the radiosity of the wall being measured and is either the

radiosity of the reference pad or of the interior walls.

98
7.4

GUIDELINES FOR CARRYING OUT MEASUREMENTS

As with temperature indicating spot radiometers the determination of the

thermal resistance of a building element using radiosity-measuring spot radiometers should not be carried out sooner than 3 hours after sunset in order to

avoid solar loading effects on exterior surfaces and to obtain minimum variations of the outdoor temperature. The indoor-to-outdoor temperature difference

should be greater than 10°C (18°F). It is recommended that the building heating

system be turned off 30 minutes prior to performing the measurements. The spot

radiometer is pointed at the exterior element and its radioslty is measured.

The radiosity of the interior surfaces is then measured using either the

reference pad or following the description with Eqs. (7.2) and (7.4). For rooms that have more than one exterior wall, an exterior celling, an uninsulated

ground floor or radiant sources of energy, such as hot pipes or radiators, the

reference pad should be used. It is important that sufficient time be allowed

after positioning the reference pad to permit it to come to equilibrium with

the surroundings. The inside and outside temperatures should be measured with

appropriate conventional thermometers.

7.5

INTERPRETATION OF DATA

If the thermal resistance, R, is found to be less than 7, then the walls should

be considered to be uninsulated. Otherwise, the wall is insulated.

7.6

ACCURACY OF THE TECHNIQUE

If the Eq. (7.8) is used to determine the resistance of a wall, then the

probable error in the value of R is + 21 percent for an insulated wall and + 10

percent for an uninsulated wall. The errors of each component are given in

table 7.1. No independent laboratory or field verification study of these

error estimates has been performed; the data are those reported by the manufacturer of the equipment. The error estimate does not include errors due to

transients or errors in measuring other than Instrument error.

7.7

AVAILABILITY OF EQUIPMENT AND PREVIOUS USE

At present this equipment is available from only one manufacturer. Due to the

newness of this equipment there are no public data on how many energy audit

service firms use this equipment. Reference [2] does describe the use of the

radiosity measuring radiometer in an instrumented audit and claims that such

an instrumented audit can be undertaken for only a little more cost and effort

than non-instrumented energy audits of buildings.

7.8

TRAINING REQUIREMENTS

This equipment is relatively simple to use. However, there are no data to

indicate exactly what training requirements would be required. It is safe to

assume that the personnel who use this technique should be trained as energy

auditors and also have training comparable to that given by Public Works of

Canada for level I personnel (see section 2.8).

99
Table 7.1 Summary of Uncertainty in Measuring Thermal Resistance of

Insulated (Uninsulated) Wall

Parameter Uncertainty

Normal Range % Error

t^c + 0.1 Btu/h.ft^«“F 00 +1

"r

+ 0.5 Btu/h*ft^.“F + 2%

Jl-Ji + 0.2 Btu/h-ft^^^F + 20%

Ti-To 1+ • o

o + 2.5%

7.9 PROGRAM APPLICATION RESTRICTIONS

It is considered that this technique initially should only be used for smaller

buildings in colder climates and then only to assess where gross deficiencies

exist in the building.

100
REFERENCES

[1] Hermann, H. L. Direct Radiometer Estimate of Heat Flow and Thermal

Resistance Using Spot Radiometers, Proceedings of Thermosense IV,

Society of Photo-Optical Instrumentation Engineers, 1982.

[2] Hamoen, D. W. A New Low-Cost Residential Energy Audit Technique,

Proceedings of the Thermosense IV, Society of Photo-Optical

Instrumentation Engineers, 1982.

101


. A
8. HEAT FLOW METERS

8.1 SUMMARY

For this technique a heat flow meter is spot-glued to a representative location

on the interior surface of a building component, and the temperature-sensing

probes are attached to the inside and outside surfaces at the same location.

The measured thermal resistance of the component is determined by dividing the

average surface-to-surface temperature difference (for a sufficient period of

time) by the average heat flow measured (during the same time interval).

This technique has been shown to be accurate to within 6 percent when the heat

flow meters are accurately calibrated. The thermal resistance measurement is

applicable to only the local spot where the heat flow meter is mounted onto

the component. However, coupled with a thermographic survey, this technique

becomes an extremely viable tool for assessing the thermal performance of

building components.

8.2 DESCRIPTION AND COST OF EQUIPMENT

Heat Flow Meters

A heat flow meter consists of a thin flat wafer (comprised of a series of pairs

of thermocouple junctions) either circular or rectangular in shape. The wafer

contains an embedded thermopile which produces a voltage (millivolt) signal

proportional to the rate of heat flow passing through the wafer (see figure

8 . 1 ).

The constant of porportionality relating the millivolt output of the sensed

heat flow rate is called the sensitivity of the device. It is expressed in

millivolts per Btu/(h»ft^). The sensitivity of a heat flow meter is a function

of its average temperature. In selecting a heat flow meter for a particular

application, it is important that the heat flow meter provide a signal sufficiently large for measurement and resolution by the readout device at the

lowest expected heat flow rate. Heat flow meters cost approximately $100 to

$200 each.

Consider the example where it is desired to measure the thermal resistance of an insulated wall (R = 13). The expected temperature difference is 10°F,

giving a heat flow rate of 0.77 Btu/(h«ft^). If the read-out device has a resolution of + 5 microvolts, then the heat flow meter must produce a signal of

20 times this level or 0.100 millivolts in order to provide a 5 percent resolution. The required sensitivity is determined by taking the ratio of the millivolt output to the heat flow rate at the lowest heat flow condition, or 0.10/

0.77 = 0.14 mv/Btu/(h«f t^)

.

Another important characteristic of a heat flow meter is the time required for

the device to respond to changes in heat flow rate. It is desirable for the

heat flow meter to respond to fluctuations in heat flow caused by diurnal variations in the outdoor air temperature; but it is not desirable for it to respond

102
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Figure 8.1 Schematic drawing of heat flow meter

Figure 8.2 Example of commercially available heat flow meter

103
to high frequency fluctuations at the inside surface due to small-scale

convection and cyclic operation of the building heating/cooling plant. Good

results are obtained by using heat flow meters having a thickness of about

1/8 inch. Heat flow meters having extremely small thicknesses introduce the

problem of responding to high frequency fluctuations in heat flow.

Temperature Sensors

It is recommended that the inside and outside surface temperatures be monitored

with premium grade 24-gage or higher copper-constantan thermocouple wire. Other

larger temperature-sensing probes should not be used, since the temperature

sensed with the larger probes may not be indicative of the actual surface temperature. An alternate approach is to locate a thermopiles on the interior and

exterior (across the building component). Here, the thermopile consists of a series of pairs of thermocouple junctions which are attached to opposite surfaces of a roof or wall to measure the temperature difference between the two

surfaces. The thermopile will develop a voltage proportional to the temperature

difference being measured.

Read-Out Devices

The output signals from heat flow and temperature-sensing probes can be read out

at hourly intervals using data loggers, strip-chart recorders, or analog integrators. In all cases, the read-out device must have sufficient sensitivity

to resolve the signal at its lowest signal level. Analog integrators have the

advantage of averaging out fluctuations in signals due to the cyclic operation

of the heating plant. In the case of monitoring signals from thermocouples, an ice-point reference must be provided external to the read-out device, or internally, using an electronic ice-point referencing system.

The total cost of an Instrument system suitable for measuring the heat flow and

temperature differences at ten locations in a building will cost approximately

$7000 to $10,000.

8.3 CALIBRATION OF SENSORS

Heat Flow Meters

The problem that has contributed to a poor reputation for heat flow methods has

been the lack of accuracy in the calibration of commercially manufactured com- ponents. Precision laboratory assemblies have provided precisely calibrated

heat flow meters. The accuracy of data reported in this section is limited to

the accuracy to which the heat flow meters are calibrated. The following procedure should be utilized to calibrate heat flow meters; The heat flow meters

to be calibrated are first sandwiched into a composite assembly having heat

transfer properties comparable to those of the heat flow meters. This composite assembly is subsequently sandwiched between two insulating boards and

inserted into a thermal conductivity measuring device capable of receiving

large specimens. Inside this apparatus, the embedded heat flow meters are exposed to a uniform heat flux at the desired mean temperature. After a 24-hour

104
conditioning period, the sensitivity of each of the heat flow meters is

determined by taking the ratio of millivolt output to the heat flux rate.

When heat flow meters are calibrated, they must be at a temperature very close

to the temperature of their intended application, since their sensitivity is

temperature dependent. If the heat flow meters are intended for exterior

applications, the above calibration must be repeated at mean temperatures

corresponding to those experienced during the test.

Temperature Sensors

The accuracy of the temperature sensors can be verified by inserting the sensors

into known temperature baths. Often the read-out device has adjustments for

nulling out temperature errors.

8.4 GUIDELINES FOR CARRYING OUT HEAT FLOW METER MEASUREMENTS

In assessing the thermal performance of a building component, the measured

performance at locations between structural members is probably of much greater

interest than that at locations on the structural members, since the focus of an in situ measurement is often aimed at the performance of insulation placed

between structural members. It may be desirable to use a spot radiometer or infrared thermographic system to select a measuring location which is representative of the bulk performance of the component. Since small defects in the

envelope Insulation do not contribute largely to the overall performance of the

building envelope providing these defects are few in number, it is desirable to

mount heat flow meters at locations free of minor insulation defects. When the

measuring location is selected, attach the heat flow meter to the inside surface

using industrial contact cement. It is imperative that the heat flow meter be

held in intimate contact with the inside surface at all locations of the device.

Substantial errors will occur if the bonding breaks loose at certain locations

on the heat flow meter. After the heat flow meter is bonded to the surface, it

should be covered with the same (or similar) paint system used on other parts of

the surface, so that the radiative exchange between the metered location and

other surfaces in the room will be comparable to that of the subject surface.

Mounting heat flow meters onto the exterior surface of a component is poor practice because the sensitivity of the heat flow meter can no longer be treated as

a constant, owing to changes in the mean temperature of the device. Heat flow

meters mounted on the underside of metal decks may produce erroneous results

because metal decks act as a fin to conduct heat laterally.

Thermocouple junctions are mounted (using epoxy) to the inside and outside

surface of a building component. After the epoxy dries, it is a good practice

to cover the dried epoxy with the same paint as used on the subject wall in

order to equalize the solar absorptance and long-wave emittance of the measuring

location to that of the subject surface. It is also good practice to run thermocouple leads at least two feet along the subject surface prior to departing from the surface in order to minimize heat conductance along the thermocouple wires themselves. Such heat conduction could conceivably alter the

temperature at the measuring location. After the sensors are installed, they

can be "hooked up" to the read out devices. The thermal resistance, R, of the

105
component is determined by divided the average temperature difference by the

average heat flow over a sufficiently long period of time. This may be

expressed mathematically by the relation:

t

/ ATdt

R = — (8.1)

/ Qdt

0

where Q

AT

t

= the heat flow rate

= the temperature difference

= time required for the measurement

The time, t, required for the measurement will be dependent on the thermal

capacity of the component. Light-weight components (l.e., wood-frame walls)

require short periods, while heavy-weight components (i.e., masonry walls)

require long periods of time. During the course of a measurement, the outdoor

conditions must be sufficiently cold to always keep the temperature difference

(AT) above 10°F. Required measurement times, t, are correlated with respect

to time lags for building components in table 8.1. Here "time lag" tefers to

the elapsed time (phase lag) between peak heat flow rate and the maximum temperature difference driving force when a building component is exposed to typical

outdoor temperature variations.

Table 8.1 Typical Phase Lags for

Component

Various Wall and

Phase Lag

(hours)

Roof Constructions

Required

Measurement

Time (days)

Built-up roof, concrete deck 12 6

Built-up roof, steel deck 1 0.5

Wood-frame cavity wall 2 1.0

Masonry walls 2.5 - 9.5 1 - 5

Metal curtain walls 0.2 <0.5

Typical phase lags for various wall and roof construtlons along with estimated

required measurement times, are summarized in table 8.1. The indicated measurement times should ensure an accuracy of 5 percent. The phase lag values were

taken from ref’s. [1,2]. When the required measurement time is less than one

day, it is recommended that a measurement period of a full day be used. In

summary, wood-frame walls, built-up roofs having steel decks, metal curtain

walls require a measurement time of one day. Masonry walls require a period

varying from one to five days depending upon the thermal capacity of the wall,

and built-up roofs with concrete decks require about six days.

106
8.5

ACCURACY OF TECHNIQUE

When the heat flow meter technique has been used in field and laboratory studies

to determine the thermal resistance of walls and roofs, the agreement between

measured thermal resistance and the corresponding predicted thermal resistance

calculated, using steady-state heat transfer theory, has agreed to within 6 percent [1,3-5] when the material composition of the wall was known accurately.

For the predicted thermal resistance values, heat transfer coefficients from

engineering handbooks were used in the calculations. Therefore, it could not

be determined whether the difference between the measurement and the theory

was due to the use of incorrect values of the material properties or to

inaccuracies associated with the measurement technique.

Significant sources of errors may be introduced due to the envelope

construction and location of heat flow meters on wall surfaces over studs or on conductive wall coverings. Details of construction and materials should be

reviewed by examination of blueprints and specifications. Whenever multiple

heat flow paths exist then the normal heat flux measurement inherent with

these devices provides only a fraction of the total.

This technique has accuracy sufficient for verification that the performance

of the building component meets thermal specification required of building

contractors for either new buildings or retrofit installations of insulation.

8.6

INTERPRETATION OF DATA

The thermal resistance calculated by this method is sufficiently accurate to

allow the accurate prediction of the long term heat loss through the component.

It is important that a sufficient number of measurements on different points of

the building be made so that representative values will be obtained. If the

more sophisticated analytic methods developed by the researchers at Lawrence

Berkeley Laboratory are used [6,7], the dynamic response of the component can be

determined from the data obtained when using this technique.

8.7

PREVIOUS USE

Heat flow meters have been used for measuring building heat flow for more than

30 years. However, due to the lack of good calibration procedures and insufficient understanding of the dynamics of the thermal response of building, the

technique developed a bad reputation. Recent research has corrected these

deficiences. A large scale application of this technique for measuring, in

situ, the thermal resistance of buildings has been carried out in Sweden [8].

8.8

TRAINING REQUIREMENTS

Qualified technicians are required to carry out this procedure. The field

personnel should be experienced with techniques for making proper low-level

electrical measurements and also have an understanding of the fundamentals of

building heat transfer. If the dynamic response of the building component is

to be determined, graduate level training in mathematics is required.

107
8.9 PROGRAM APPLICATION RESTRICTIONS

This technique should be used only when accurate determination of the thermal

performance of the building is required. It can only be used during periods

when there is no reversal of direction in the heat flow. It should not be

used on glass surfaces.

108
REFERENCES

[1] Treado, S. J. Thermal Resistance Measurements of a Built-Up Roof System,

NBSIR 80-2100, National Bureau of Standards, 1980.

[2] Arumi, F. N. Thermal Inertia in Architectural Walls, DOE Report,

University of Texas, Austin, Texas, 1978.

[3] Burch, D. M. , Contraras, A.G., and Treado, S. J. The Use of

Low-Moisture-Permeability Insulation as an Exterior Retrofit System - A

Condensation Study, ASHRAE Transactions, Vol. 85: Part 2 ; 1979.

[4] Flanders, S. N. Time Constraints On Measuring R-Values, CRREL Report

80-15, 1980.

[5] Flanders, S. N. , Marshall, S. J. Measuring Building R-Values for Large

Areas, Proceedings of Photo-Optical Instrumentation Engineers, Vol. 254,

1981.

[6] Sherman, M. H. , Sanderegger, R. C. , and Adams, J. W. The Determination

of the Dynamic Performance of Walls, Submitted for presentation at ASHRAE

winter meeting, 1982.

[7] Adams, J. W. , Sherman, M. H. , and Sanderegger, R. C. Dynamic Measurement

of Wall Thermal Performance, Proceedings of DoE/ASTM Conference, Thermal

Insulations, Materials, and Systems for Energy Conservation in the '80's.

December 1981, Clearwater, FL.

[8] Hammersten, S. , and Pettersson, B. Testing of Retrofits in Building,

Swedish Council for Building Research, Stockholm, 1981.

109
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9. PORTABLE CALORIMETER BOXES

9 . 1 SUMMARY

Recently, the Building Research Division of the National Research Council of

Canada (BRD/NRCC) developed a portable calorimeter (guarded hot box) for

measuring in-situ heat transmission through building components.

The calorimeter consists of a five sided insulated box, the open side of which

is sealed against the subject building component. An electric heater located

inside the box is thermostatically controlled so that the inside box temperature is equal to the indoor temperature of the building enclosure. Since the

reverse heat loss through the box and the edge loss where the box edge contacts

the metered surface are essentially nulled to zero, the metered electric energy

supplied to the electric heater is essentially equal to the heat transmission

through the metered area. This technique has the advantages that the measurement provides a minimum disturbance to the measured heat transmission and a sufficiently large surface area is metered to be considered representative of

the bulk performance of the building component. The accuracy of the technique

is reported to be about 5 percent.

9.2 DESCRIPTION OF EQUIPMENT

Calorimeter Box

The construction details for the BRD/NRCC calorimeter are given in figure 9.1a.

The size of the metering area is 1.2 x 2.1 meters. The walls consist of two

layers of 72 kg/m^ glass-fiber insulation board glued together with an exterior

6 mm plywood covering to provide structural support. The thermal resistance of

the walls is 2.8 m^»K/W. The edge portion of the calorimeter which contacts

the surface of the building component contains a gasketing material for providing an air tight seal. A 150-watt electric heating wire is suspended in a zigzag fashion within the calorimeter as shown in figure 9.1b. A multijunction

thermopile is placed across the rear surface of the calorimeter. The controller

shown in figure 9.1b modulates the electrical energy supplied to the heating

element such that the thermopile is nulled to zero. The electric energy

supplied to the box is metered with a conventional watt-hour meter. Air

circulation within the calorimeter box is achieved through natural convection.

Surface Temperature Sensors

It is recommended that the inside and outside surface temperature be measured

with one of the following type sensors;

i. premium-grade, 24-gauge or higher, copper-cons tantan thermocouples.

ii. multi-junction copper-constantan thermopiles fabricated from premium

grade, 24-gauge or higher, copper-constantan thermocouple wire.

iii. precision bead-type thermistors

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Figure 9.2 Illustration of measurement technique

111
Read-Out Device

The electrical energy supplied to the calorimeter box should be metered with a watt-hour meter equipped with a demand metering device such as a contact closure

or pulse generating device. The generated contact closures or pulses should be

accumulated and printed out at prescribed time Intervals using commercially

available pulse counting and recording systems. With regard to the electric

energy measurements, a sufficient number of contacts or pulses should be developed to permit adequate resolution of the energy consumption (approximately 1 pulse per watt-hour). Thermocouple, thermopile or thermistor signals may be

read out at hourly intervals using data loggers, continuous strip chart

recorders, or if the signal is a linear function of temperature, averaged over hourly intervals using analog integrators. In all cases, the readout device

must have sufficient sensitivity to resolve the signal at its lowest level.

When monitoring signals from thermocouples, an ice-point reference must be provided either externally to the readout device, or internally, using an electronic ice-point referencing system.

9.3 MEASUREMENT TECHNIQUE

Prior to conducting a calorimeter box measurement of a building component, a representative measuring station should be selected. While thermal anomalies

such as missing insulation need to be located, usually one is more interested

in the performance of a building component in areas free of such defects, unless

the purpose of the survey is to measure the effect of such thermal anomalies on

the envelope performance.

The calorimeter box is subsequently sealed to the measurement station as shown

in figure 9.2. It is very Important that a good seal is provided continuously,

along the edge seal gasketing, in order to prevent convective air exchange

between the calorimeter and the room air.

The calorimeter box is then turned on and permitted to equilibrate with respect

to the room air. An equilibrium condition is deemed to exist when the box loss

measurement with the thermopile approaches zero.

After an equilbriura condition is reached, then the calorimeter box is operated

over a measurement period, t, and the thermal resistance, R, of the building

component is computed using the relation:

T / AT.dt

R = .2 (9.1)

W^

where

W-p = electrical energy measured with the watt-hour meter system

A-p = measured surface-to-surface temperature difference across the

building component.

112
Required measurement periods, t, for various building components are summarized

in table 9.1.

Table 9.1 Required Measurement periods (t) for Various Building

Components (based on information in ref*s. [2,3])

Component

Measurement Time ( t

)

days

Built-Up Roof, Concrete Deck

Built-Up Roof, Steel Deck

Wood-Frame Cavity Walls

Masonry Walls

Metal Curtain Walls

6

0.5

1.0

1 - 5

>0.5

The foregoing measurement periods should insure an accuracy of 5 percent in

determining the thermal resistance, R.

9.4 GUIDELINES FOR CONDUCTING MEASUREMENTS

Calorimeter box measurements should be carried out only during periods when

the outdoor to indoor temperature difference (AT) is always greater than 10°F.

Solar loading on walls in the winter season may frequently produce AT's less

than 10°F. During the period of the measurement, the indoor temperature must

be thermostatically controlled at a constant level in order to minimize differences in the temperature between the calorimeter box and the room. In addition,

solar radiation (into the interior) and conditioned air from warm air supply

registers must not be permitted to strike the calorimeter box.

9.5 SPECIFICATIONS FOR THE CALORIMETER BOX

In specifying a calorimeter box, it is desirable that the reverse heat loss

through the box be minimized. That is accomplished by designing the thermal

resistance of its walls as high as practical, minimizing the box surface area

which is exposed to room air, and minimizing the temperature difference that

exists across the box wall during its operation. With regard to minimizing the

box surface area, it is desirable to reduce the depth of the box to the point

where natural convection patterns within the box begin to depart from typical

room conditions. The metering area should be as small as practical, while still

measuring a representative section of the building component. For instance,

for a wood-frame cavity wall the minimum interior wall surface to be metered

would be a width of 3 stud spaces and a height of 6 feet. The edge loss where

the box is sealed to the wall is minimized by reducing the average temperature

difference across the box wall around the edge perimeter. The surface emittance

of the interior of the box should be selected to be comparable to that of typical indoor room surfaces, so that the radiation heat transfer coefficient at

the metered surface will be comparable to its value under natural room

conditions

.

113
The pattern for the zigzag electrical heating within the calorimeter box should

be designed to produce uniform temperatures within the box. For example,

calorimeter boxes used to measure vertical walls should have closely spaced

wires at the bottom of the box in order to offset vertical temperature gradients.

It is desirable that the exterior covering for the calorimeter box have a low

emittance in order to minimize the radiation exchange between the box and

other surfaces of the room. This will help to maintain the exterior box

temperature more closely to the room temperature.

9.6 ACCURACY

The accuracy of the BRD/NRCC calorimeter was checked by measuring the heat

transmission rate through a test wall of known thermal resistance. The thermal

resistance of the test wall had previously been measured using a guarded-hot-box

apparatus. The field calorimeter was installed against the test wall with the

temperature conditions on both sides of the test wall maintained identical to

those prevailing during the hot-box test. The accumulated heat transmission

measured with the calorimeter box agreed within 5 percent of the corresponding

value predicted using the wall resistance value and the measured surface temperature difference. In order to achieve this accuracy it is important that

there be no lateral heat flow from the metered area (to adjacent areas)

especially by convection loops within cavities within the metered wall.

9 . 7 INTERPRETATION OF DATA

The methods described in section 8.6 for the Interpretation of data from heat

flow meters are applicable to data produced by this technique. In the case of

the portable calorimeter, the results are applicable to greater areas of the

building component.

9.8 PREVIOUS USE, AVAILABILITY OF SERVICE AND TRAINING REQUIRED

This technique has been used only by BRD/NRCC of Canada. They have constructed

about 10 portable calorimeters and have had private contractors construct other

portable calorimeters using their specification. The cost of construction is

estimated to be from $800 to $1000. No data are available describing training

requirements; however, they should be equivalent to those described for heat

flow meters in chapter 8.

9.9 PROGRAM APPLICATION RESTRICTIONS

The comments made in section 8.9 also apply to this technique.

114
REFERENCES

[1] Brovm, W. C. , and Schuyler, G. D. A Calorimeter for Measuring Heat Flow

Through Walls, Proceedings of the ASHRAE/DoE-ORNL Conference, Thermal

Performance of the Exterior Envelop of Buildings, ASHRAE Special

Publications SP 28, 1981.

[2] Treado, S. J. Thermal Resistance Measurements of a Built-Up Roof System,

NBSIR 80-2100, National Bureau of Stadards, October 1980.

[3] Arumi, A. N. Thermal Inertia in Architectural Walls, DoE Report,

University of Texas, Texas, February 1978.

115



10. ENVELOPE THERMAL TESTING UNIT

10.1 SUMMARY

The envelope thermal testing unit (ETTU) was developed by the Lawrence Berkeley

Laboratory in order to evaluate the in-situ transient thermal performance of

walls. It consists of two blankets which are attached to opposite sides of a wall and through which the heat flux to opposite wall surfaces can be con- trolled [1]. This unit was designed to overcome some of the difficulties in

using heat flow meters and calorimeters for the in-situ evaluation of building

components. It differs from those devices in that it is the heat flow that is

controlled and not the temperature.

10.2 DESCRIPTION OF EQUIPMENT

The physical arrangement of the ETTU is shown in figure 10.1. The unit

consists of two identical "blankets" which are placed in close thermal contact

with opposite surfaces of a subject wall. The term "blankets" is used because

they cover the wall section under test and are slightly flexible, so that they

can be made to conform to slight irregularities in the wall surfaces. Placing

the blankets in direct thermal contact with the wall eliminates complications

associated with air film and considerably reduces the bulk of the unit.

Each blanket consists of a pair of large area electric heaters separated by a low-thermal-mass insulating layer. Embedded in each heater layer is an array

of temperature sensors. Heat drive is provided by the primary heaters according

to the time-dependent program which covers the relevant frequency spectrum.

Each electric heater is designed to provide a heat output that is uniform over

the whole area. The actual heat output is controlled by adjusting the voltage

applied to the heater.

The guard function in the transverse direction is accomplished by the secondary

heaters. The secondary heaters are driven by a servo-control which drives the

measured secondary temperature Tg toward the measured primary temperature Tp.

This reduces the temperature difference across the guard insulation and minimizes the portion of the drive heat that flows into the blanket and does not

contribute to driving the wall. A microprocessor-controlled data-acquisition

system is used to drive the system and to record the system response.

10.3 MEASUREMENT TECHNIQUE

The blankets are installed across a wall section as shown in figures 10.2 and

10.3. The microprocessor is programmed to give the schedule of heat fluxes to

which tne wall is to be subject. Since the guard function in the transverse

direction limits multipath heat flow and when uniform heat flow, i.e., power

is attained, the data and analysis applies to the central test section where

normal heat flux is developed. The data is analyzed in the manner as the

data obtained from heat flow meters and calorimeters if the thermal resistance

of the wall is to be calculated. In particular, the periods of measurement

noted in the sections on those methods must be observed.

116
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Figure 10.2 Outside blanket Figure 10.3 Inside blanket

117
10.4

ACCURACY OF THE METHOD

After calibration of the envelope thermal testing unit, it is reported that the

thermal resistance of the building element can be determined within + 5 percent.

If a suitable heat flux schedule is programmed into the computer, the dynamic

reponse of the building element can be accurately modeled using a simplified

thermal parameter model which determines the thermal conductance, the time

constant of the wall and a series of inside and outside surface thermal storage

coefficients 12, 3J.

10 . 5 PREVIOUS USE, COST, AVAILABILITY OF EQUIPMENT AND TRAINING REQUIRED

This equipment has been only used in research situations and must still be

considered as being under development. The training of the personnel who performed the measurements would probably be comparable to that required of personnel who perform portable calorimeter measurements (section 9.8). This

equipment is estimated to cost between $5,000 and $10,000.

10.6 PROGRAM APPLICATION RESTRICTIONS

This measurement technique can be used where accurate determination of the

thermal characteristics of the building envelope are required. Its high

accuracy makes it suitable for the verification of thermal specifications in

new and retrofit buildings. Since this technique requires the application of

both an exterior and interior blanket, it may be difficult to use on the upper

stories of tall buildings. It cannot be used in below grade applications.

This technique should be applicable to both cold and warm climates. Where

possible it should be used after a thermographic survey of the building has

been performed.

118
REFERENCES

[1] Condon, P. E. , Carroll, W. L. , and Sonderegger, R. C. A New Measurement

Strategy for In Situ Testing of Wall Thermal Performance, Proceedings of

the Thermal Performance of Exterior Envelopes of Buildings, ASHRAE, 1981.

[2] Sherman, M. H., Sonderegger, R. C. , and Adams, J. W. The Determination

of the Dynamic Performance of Walls, Submitted for presentation at ASHRAE

winter meeting, 1982.

[3] Adams, J. W. , Sherman, M. H., and Sonderegger, R. C. Dynamic Measurement

of Wall Thermal Performance, Proceedings of DoE/ASTM Conference, Thermal

Insulation, Materials and Systems for Energy Conservation in the '80's,

December 1981, Clearwater, FL.

119
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11. GLOSSARY

Aerial infrared survey - see survey, aerial infrared.

Aerial thermography - see survey, aerial infrared.

Air change rate - air leakage in volume units per hour divided by the building

space volume with identical volume units (normally expressed in air changes per

hour, ACH or ACHP).

Air flow or velocity measuring system - a device or assembly of components to

measure air flow within the + 6 percent of the average value (air flow or velocity measuring system calibrations shall follow the manufacturers instructions, and be recorded as such; alternatively a calibrating wind tunnel may be

used, and recorded as such). Sensors and displays may vary from simple instruments (based on air pressure changes) to complex arrangements of sensing elements with electronic interfaces for amplification and analysis of signals with

displays and data recording systems.

Air leakage rate - the volume of air movement per unit time across the building

envelope. This movement includes flow through joints, cracks, and porous

surfaces, or combination thereof. The driving force for such an air leakage in

service can be either mechanical pressurization and evacuation, natural wind

pressures, or air temperature differentials between the building interior and

the outdoors, or combinations thereof.

Air leakage graph - the graph which shows the relationship of measured air flow

rates to the corresponding pressure differences.

Air moving equipment - a fan, blower, or blower door assembly, which is capable

of moving air into or out of the building envelope at required flow rates under

a range of test pressure differences. The system shall provide constant air

flow at fixed pressure for the period required to obtain readings of air flow

rate or velocity, and at each incremental pressure difference.

Air sample containers - non-absorbent, inert, low permeability containers (e.g.,

sample bags, syringes with needle caps, or plastic bottles) used to collect

and store air samples from buildings under test.

Anomalies , thermal - the heat loss characteristics of a structure which are not

in accordance with intended design characteristics.

Apparent radiant temperature - see temperature, apparent .

Aspect ratio - see ratio, aspect .

Background - the irradiance at the entrance aperture of the infrared sensing

system which is not radiated directly from the object being investigated.

120

Blackbody - An Ideal thermal radiator (emlsslvlty 1.0), which emits and

absorbs the maximum theoretically available amount of thermal radiation at a given temperature.

Bridge, thermal - a low thermal resistance path connecting two surfaces.

Building enclosure - the aggregate of materials and components that make up

the exterior shell of the building.

Building envelope - the exterior shell enclosing the interior space.

Building heat loss characteristics - see characteristics, building heat loss .

Building performance evaluation - an investigation of the appropriateness and

adequacy of the building enclosure as an environmental separator between indoor

and outdoor environments.

Building science - fundamental laws of physics and chemistry and their effects

on complex assemblies of building materials.

Building science thermologlst - a specialist who has fundamental knowledge of

the cause and effect for certain building enclosure problems and who has the

skills of a thermologlst.

Building space - the volume of a building that exchanges air with outside

ambient air. In most cases, this volume is the deliberately conditioned space

within a building, generally not including the attic space, basement space, and

attached structures, unless such spaces are connected to the heating and air

conditioning system, such as a crawl space plenum.

Characteristics, building heat loss - those properties of a structure which

pertain to the loss of energy through the building envelope.

Coefficient, convective film - constant of proportionality relating the

convective rate of heat transfer at a surface to the temperature difference

across the air film on that surface.

Coefficient, radiation - constant of proportionality which relates the

radiation energy exchange between a surface and its surroundings to the

temperature difference between the surface and its surroundings.

Convective film coefficient - see coefficient, convective film.

Copy, hard - see record, hard copy.

Detector, infrared - A device which transduces the infrared irradiance

incident upon it into some other form of energy, in most cases electrical.

Device, Infrared sensing - A wide class of instruments used to display and/or

record information which is proportional to or equivalent to the thermal radiation from any object surfaces viewed by the Instrument. The Instrumentation

121
varies in complexity from simple spot radiometers which measure only one "spot" or area to full two dimensional thermal Imagers which provide photographic type quality pictures which map the scene radiosity.

Emittance, spectral band - The ratio of radiant energy from a surface at a given temperature to the corresponding radiant energy from an ideal surface

(blackbody) at the same temperature for a portion or spectral band of the

electromagnetic spectrum.

Equilibrium, thermal - Condition where heat flow through a structure has

reached a steady state condition. For building diagnostic purposes, this

condition is approached when the environmental conditions remain approximately

steady over a period of time.

Equipment-use services - Services provided by companies that use infrared

sensing devices to perform heat loss surveys of structures.

Exf lltratlon - Air flow outward through a structure.

FOV - See Field of View.

Field of View (FOV) - The total angular dimensions within which objects can be

imaged, recorded and displayed by an imaging device when pointed in a fixed

direction.

Fissures - For the purposes of this report , narrow openings or cracks in

Insulation having dimensions of at least 125 mm (5 inches) by 25 mm (1 inch)

and less than the dimensions specified for a void.

Fundamental principles of building science - These principles can be generally

divided into the categories of the physics , materials and enclosures with the

following subcategories:

A. Physics

1. heat transfer and radiation

2. psychrometry

3. air movement

4. fluid dynamics

5. structure

6 . sound

B. Materials

1. nature of materials

2. durability of materials

3. deterioration of materials

4. strength of materials

5. combustibility and resistance

C. Enclosure

1. heat transfer

2. water and moisture penetration

122
3. air movement

4. structural stability

5. fire prevention and control

6. sound control

7. performance of enclosure elements

roofs

walls

windows

foundations

Heat capacity - the quantity of heat needed to cause a described temperature

change in a known mass of material.

Heat transfer - the movement of heat energy from a location of higher

temperature to a location of lower temperature. This can occur by conduction,

convection, or radiation depending on the characteristics of the material

involved and the energy transfer.

Imaging line scanner - See scanner, imaging line.

Imaging system - See system. Infrared imaging.

Infiltration - Air flow Inward through a structure

Infrared sensing device - See Device, infrared sensing.

Interior space - See building space.

Irradlance - The radiant power per unit area Incident on a surface.

Lag, thermal time - The phase difference in hours between the exterior and

interior surface temperatures when the exterior surface is subjected to a temperature change.

Line scanner - See scanner, line.

Minimum resolvable temperature difference (MRTD) - See MRTD.

MRTD - Minimum, resolvable temperature difference - A measure of the ability

of an Infrared imaging system to allow the human observer to recognize periodic

bar targets on the display. The MRTD is the minimum temperature difference

between a standard periodic test pattern (7:1 aspect ratio, 4 bar) and its

blackbody background at which an observer can resolve the pattern as a four

bar pattern. Unlimited viewing time and optimization of instrument level and

gain controls are allowed.

NADIR - The object point which is directly beneath the aircraft flying an airborne imaging survey

123
NEAT - Noise, equivalent temperature difference , - A measure of the noise of an infrared imaging system or line scanner. It is the target to background temperature difference between a low spatial frequency blackbody target and its

blackbody background at which a ratio of one is obtained between the peak-topeak-signal and RMS noise at the output of the detector processing electronics

of the sensor in question.

Noise equivalent temperature difference - See (NEAT).

Object plane resolution - See resolution, object plane.

Partial voids - Areas of similar size to voids which are Insulated but where

the insulation thickness is less than the full specified insulation

Pixel - A picture element on a video display.

Pressure difference - The actual pressure difference across the building

envelope; expressed as Ibf/ft^, Pascals (Newtons/m^) , inches of H

2

O, or inches

of Hg.

Pressure measuring device - manometer or pressure indicator to measure pressure

difference, with an accuracy of + 0.01 inches of H2O (+ 2.5 pascals).

Pump - A non-contaminating air sample pump, either manual or powered, used to

fill air sample bags. Plastic bottles can be filled by hand squeezing.

Qualitative test - Applied to determine the occurrence, the type and the

distribution of a property.

Quantitative test - Applied when a numerical value of a property is required.

Radiance - The total radiant flux per unit solid angle per unit projected area which emanates from a surface. It includes the self-emitted radiation plus

reflections from sources other than the object of interest, as intercepted from

the direction of measurement. See Radiosity.

Radiation Coefficient - See Coefficient, radiation.

Radiometer, spot - An apparatus which measures the average radiosity or average

apparent temperature of the surfaces subtended by its field of view, (since the

spot radiometer has no scanning mechanism its IFOV is the same as its FOV).

Radiosity - The total radiant flux which leaves a surface per unit area. The

radiosity is the sum of the radiant flux emitted and reflected by the surface

plus any radiant flux transmitted through the surface. See Radiance.

Ratio, aspect - The ratio of the length to the width of a rectangle. Also, the

ratio of length to width to height of a building shape.

Record, hard copy - Any permanent record. Typically, the record will consist

of photographs, magnetic tapes or some data log documentation.

124
Reflection, thermal - That portion of the radioslty from a surface which is

contributed by reflection.

Resistance, thermal - A measure of the insulating quality of a building envelope

component based on steady-state conditions.

R.esolution element - See Field of View.

Resolution, limiting - The highest spatial frequency of an object space target

which an imaging sensor is able to resolve. The spatial frequency of the target

is determined by taking the inverse of the ratio of two bar widths divided by

the distance from sensor to target. This number is generally multiplied by

1,000 and expressed in cycles per milliradian.

Resolution, object plane - The size in the object plane which corresponds to

the product of the system's instantaneous field of view in radians and the

distance from the system to the object.

Resolution, spatial - The planar angle defined by the instrument instantaneous

field-of-view, i.e., the resolution element.

Resolution, thermal - A measure of the capability of an infrared sensing device

to distinguish the apparent radiance temperature difference between two

blackbodies near the same temperature.

Sampling network - A network consisting of tubing, tubing junctions, a pump,

and possibly an aspirator. This network is used to draw samples from remote

locations within a structure, blend them, and bring the blended sample to a convenient place for analysis. In general, it is best to avoid plasticized

tubing such as vinyl; use copper, stainless steel, or possibly polypropylene

or nylon. The experimenter should be aware that surface absorption within the

sampling network can be a major source of confusion in any concentration decay

measurement

Scanner, imaging line - An apparatus which scans in a single dimension and is

moved perpendicular to the scan direction to produce a two-dimensional thermal

map of the scene. See also Survey, aerial infrared and Scanner, line.

Scanner, line - An apparatus which scans along a single line of a scene to

provide a thermal mapping of a discrete width or slice of the scene. See also

scanner, imaging line.

Sensing device, infrared - See Device, infrared sensing.

Shading - The percentage of total radiation from the surrounding hemisphere

which does not irradiate a structure surface because of an obscuration.

Spatial resolution - See Resolution, spatial.

Spot radiometer - see Radiometer, spot.

125
Surface emlttance - See Emittance, spectral band.

Survey, aerial infrared - The generation of thermograms of building structures

as viewed from an overhead platform using an imaging line scanner.

Survey, Imaging exterior - The generation of thermograms of building outside

surface.

Survey, imaging interior - The generation of thermograms of portions of a building as viewed from the building interior.

Survey , non-imaging - The generation of a set of apparent temperature measurements

or building surfaces (usually the Interior) obtained with a non-imaging thermal

sensing device.

Syringes - Disposable syringes may be used to inject gas samples when the gas

monitor is the gas chromatograph. The use of a plastic bottle containing tracer

gas or tracer gas air mix can also be used.

System, infrared imaging - An apparatus which converts the two dimensional

spatial variations in infrared radiance from any object surface into a two

dimensional thermal map of the same scene in which variations in radiance are displayed in gradations of gray tone.

Temperature , apparent - The temperature of an object as determined from the

measured radiance (see definition of Radiance).

Temperature, blackbody equivalent (apparent temperature) - The apparent

temperature of an object as determined from the measurement of its radiance and

the assumption that it is an ideal blackbody with emlssivity of 1.0.

Thermal anomalies - See Anomalies, thermal.

Thermal Bridge - See Bridge, thermal.

Thermal pattern - A gradient representation Indicating regions of temperature

variation.

Thermal resolution - See Resolution, thermal.

Thermal time lag - See Lag, thermal time.

Thermogram - A photograph or two dimensional record of an image which maps the

apparent temperature of the scene as sensed by an infrared Imaging system.

Thermographer - An infrared camera operator who is able to adjust controls to

provide useful thermal images. Thermographers also have the ability to produce

high-quality thermograms plus isotherm Images as well as to use single or dual

isotherms. A thermographer has the skill necessary to adjust all Images, grey

scale or step scale. A thermographer is able to complete a standard recording

format for work completed and is expected to provide a preliminary report that

126
identifies problems, interprets the mechanisms of the problems, records the

exterior microclimate and orientation of the building, room plan indications,

and location and direction of internal thermal imagery.

Thermography - The process of generating a thermogram by using an Infrared

imaging system, usually with some means of temperature calibration.

Thermography, Aerial - See Survey, aerial infrared.

Thermologlst - A camera operator who is competent in all the skills of a thermographer . In addition, a thermologlst must have a greater knowledge of

building construction, building materials, and basic knowledge of building

science principles, especially those related to temperature and psychrometry

In addition, a thermologlst must be capable of interpreting causes and effects

of common building problems, and proposing appropriate solutions.

Thermology - is a combination of thermography, building science and building

technology.

Time lag, thermal - See Lag, thermal time.

Time Lag - See Lag, thermal time.

Tracer gas - A gas that can be mixed with air and measured in very small

concentrations, making it possible to detect air movements and measure air

change rates. Typical gases are SF^, N2 O, CO, C02«

Transmittance - The ratio of the radiant energy transmitted through a body to

that incident upon it.

Void - For purposes of this standard, a void is defined as any localized area

of the building envelope which has a significantly different thermal resistance

from the area surrounding it.

127
NBS«) |4A (REV. 2«8C) U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET (See , n struct, on s)

1. PUBLICATION OR

REPORT NO.

NBSIR 82-2605

2. Performing Organ. Report No

4. TITLE AND SUBTITLE

Publication Date November 1982

MEASUREMENT METHODS FOR DIAGNOSTIC PROCEDURES IN EVALUATION

OF THERMAL INTEGRITY OF BUILDING ENVELOPES

5. AUTHOR(S)

Grot, Richard A., Burch, Douglas M. , and Silberstein, Samual

6. PERFORMING ORGANIZATION (If joint or other than NBS, see instructions) 7. Contract/Grant No.

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE 8. Type of Report & Period Covered

WASHINGTON, D.C. 20234

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street, City, State, ZIP)

Public Buildings Service

General Services Administration

Washington, D.C. 20405

10. SUPPLEMENTARY NOTES

13^ Document describes a computer program; SF-185, FIPS Software Summary, is attached.

11. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here)

This report presents reviews of various measurement and inspection techniques

appropriate for the development of detailed diagnostic procedure for assessing the

thermal performance of the exterior envelopes of federal buildings. The inspection

techniques include the use of ground-based infrared thermographic surveys, aerial

infrared surveys, tracer gas air infiltration measurement, pressurization tests for

measuring the tightness of the building envelope, and spot radiometer surveys for

detecting gross defects. Heat flow meters, a portable calorimeter, and a microprocessordriven envelope testing unit are also considered.

For each technique recommended procedures are provided; they include equipment

requirements, conditions under which the techniques can be carried out, calibration,

accuracy, and limitations. The detailed diagnostic procedures specific to small and

large federal buildings require further development from on-site field testing of

representive buildings.

An Executive Summary provides an overview of the Building Diagnostic Program of which

Phase I is covered in this report. Field test evaluations will be carried out in

Phase 2 and implementation under Phase 3.

I

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolons)

Air infiltration rates; envelope thermal performance; Infrared imaging; radiometers;

thermal bridges; thermographic surveys; tracer gas techniques.

13. AVAILABILITY 14. NO. OF

Un! imited

i 1 For Official Distribution. Do Not Release to NTIS

PRINTED PAGES

140

1 1 Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C.

15. Price 20402.

1^X1 Order From National Technical Information Service (NTIS), Springfield, VA. 22161 $13.50

USCOMM-DC 6043-P80

4