A111DS 43L223

NATL INST OF STANDARDS & TECH R.I.C.

All 102436223

Fang, Jin B/The assessment of accuracy o QCfOO .U56 NO. 86-3328 VI 986 C.2 NBS-PUB- The Assessment of Accuracy of

In-Situ Methods for Measuring

Building Envelope Thermal

Resistance

PUBLICATIONS

Jin B. Fang

Richard A. Grot

Hyo Soon Park

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Engineering Laboratory

Center for Building Technology

Building Physics Division

Gaithersburg, MD 20899

February 1 986

-QC

100

• U56

86-3328

1986

Building Energy Research and Development

artment of Energy

:on, DC 20585

NBS RESEARCH INFORMATION CENTER

NBSIR 86-3328

THE ASSESSMENT OF ACCURACY OF

IN-SITU METHODS FOR MEASURING

BUILDING ENVELOPE THERMAL

RESISTANCE

Jin B. Fang

Richard A. Grot

Hyo Soon Park

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Engineering Laboratory

Center for Building Technology

Building Physics Division

Gaithersburg, MD 20899

February 1 986

Prepared for: Office of Building Energy Research and Development

U.S. Department of Energy

Washington, DC 20585

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

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

ABSTRACT

A series of field and laboratory tests were conducted to evaluate the

accuracy of in-situ thermal resistance measurement techniques. The results

of thermal performance evaluation of the exterior walls of six thermal mass test houses situated in Gaithersburg, Maryland are presented. The wall

construction of these one-room houses includes insulated light-weight wood

frame, uninsulated light-weight wood frame, insulated masonry with outside

mass, uninsulated masonry, log, and insulated masonry with inside mass.

In-situ measurements of heat transfer through building envelopes were made

with heat flux transducers and portable calorimeters. A sufficiently long

period of measurements, depending on the thermal mass of wall structure, is

needed to provide reliable thermal resistance data. The comparisons of the

results from these field measurements with those derived from sections of

the same wall structures tested in a guarded hot box facility in a laboratory are presented. A wel 1-insulated, double-stud test wall was also

tested under simulated thermal conditions including steady-state and

periodically varying outdoor temperature using a calibrated hot box and the

in-situ measurement procedures.

From these test results, the in-situ methods are shown to provide thermal

resistance data within 9% of the hot box results. The extent of

variability in wall resistance values measured by a single calorimeter or heat flux transducer is found to range from 0.3 to 9% with an average of

4%.

Key Words: Building, exterior envelope, field test, heat flux, in-situ

measurements, temperature, thermal resistance, portable

calorimeter, wall.

l
TABLE OF CONTENTS

Page.

L Introduction ....... ......... . .. » 1

2 . Descriptions of Test Buildings ................. . ...........

.

1

3. Instrumentation and Measuring Technique 2

4. Test Results 4

4.1 Field Measurements with Tests Buildings 4

4.2 Laboratory Measurements with Double-Studded Wall 5

5. Comparison of Test Results Between In-Situ Measurements

and the Hot Box Method 7

6. The Variability of Measured Thermal Resistance Values 7

7. Conclusions 8

8 . Acknowledgment 8

9. References 9

ii
LIST OF TABLES

Table 1. Construction Details of Exterior Walls of Test Buildings

Table 2. Comparison of Wall Thermal Resistances Measured with Portable

Calorimeters and Heat Flux Transducers to Corresponding Predicted

Values and Measured Results from Guarded Hot Box

Table 3. The Measured Average Air Temperatures Across Wall Structures

During Field and Laboratory Wall Thermal Resistance Measurements

Table 4. Comparison of Measured and Predicted Thermal Resistances of WellInsulated Double-Stud Wall Under Steady-State and Dynamic

Climatic Conditions

Table 5. The Variability of Wall Thermal Resistance Values Measured by a Single Portable Calorimeter or Heat Flux Transducer

Table 6. The Variations in the Measured Thermal Resistance Values Among

Heat Flux Transducers

LIST OF FIGURES

Figure 1. Construction Details of Small Sized Portable Calorimeter

Figure 2. Schematic of the Measurement /Control System for the Portable

Calorimeter

Figure 3. Construction Details and Sensor Locations on the WellInsulatedDoub le-Stud Test Wall

Figure 4. A Comparison Between the Wall Thermal Resistance Values

Measured with Portable Calorimeter and Values Obtained with

the Calibrated Hot Box

Figure 5. A Comparison of the Wall Resistance Values Measured by Heat

Flux Transducers and the Calibrated Hot Box Data

iii

1 . INTRODUCTION

The thermal performance of a building envelope has a great impact on both

heating and cooling fuel costs and on maintenance expenditure for a building. This necessitates increased attention to the design and

construction of more thermally efficient envelopes, and the retrofit of

existing buildings. Quantitative information about the thermal performance

of exterior envelopes under actual use conditions is required for various

purposes including assessing the effectiveness of energy conservation

measures, and estimating peak heating and cooling load for the sizing of

heating, ventilation and air-conditioning equipments.

Measured thermal resistance for building materials or components available

for design of building envelopes have been largely derived from carefully

prepared test specimens constructed for the sole purpose of obtaining

measurements in a laboratory facility under controlled conditions.

However, those test specimens may not adequately reflect conditions in the

field, such as substandard workmanship, defective insulation, and

deterioration of building components caused by materials aging and moisture

penetration. Also, the prescribed exposure conditions in the laboratory

tests may differ from the actual exposure in the field due to dynamic

variations of ambient temperature and solar radiation. Due to the

continuing development of construction techniques and new types of building

materials, there is a need to obtain data on thermal resistance of building

components under actual use conditions. Field thermal measurements can

also provide information about whether the thermal performance of building

components meets thermal design specifications and can be used for

comparison with theoretical predictions of computer simulation models.

Though the results of field measurements using heat flux transducers and

portable calorimeters have been reported in the literature [1-4], little

experimental work has been done to assess the accuracy of these in-situ

measurement techniques. This is primarily due to the lack of knowledge

regarding the heat flow characteristics of the wall sections in these

tests

.

This paper compares the results of in-situ methods for measuring thermal

resistance values of exterior walls of six test houses to the results

obtained in a guarded hot box on identical wall structures. The walls

tested are insulated light-weight wood frame, uninsulated light-weight wood

frame, insulated masonry with outside mass, uninsulated masonry, log, and

insulated masonry with inside mass. In addition, a highly insulated wall

was tested both by in-situ methods and in the NBS calibrated hot box.

2. DESCRIPTIONS OF TEST BUILDINGS

Field thermal measurements were carried out on six one-room test buildings

that had the same floor area and orientation, and were identical except for

the wall construction. These buildings, situated in Gaithersburg,

Maryland, had previously been used to investigate the effect of wall mass

on space heating and cooling loads and indoor comfort of residential

buildings [3]. Each building had a different wall structure as mentioned

before. A detailed description of the wall construction of these test

buildings is given in Table 1.

1
Each test building had a 20 x 20 ft. (6.1 x 6.1 m) concrete floor covered

with 2 in. (51 mm) thick polystyrene foam insulation board, and a pitched

roof with a gypsum board ceiling located 90 in. (2.29 m) above the floor.

In the attic space, 11 in. (280 mm) thick, R-34 h*ft^*°F/Btu (R-6.0 m^'K/w)

glass fiber blanket insulation was installed over the ceiling. Two double

hung, single glazed windows with exterior storm sash, norminal size 35 x 44

in. (0.89 x 1.12 m) were situated on both the south-facing and the northfacing walls, and a 19.5 ft^ (1.81 m^) insulated metal door was located on the east wall of each test building. The indoor air of each test building

was conditioned by a centrally located 4.1 kW electric forced air heating

plant equipped with a 13,000 Btu/h (3,800 W) split-unit vapor-compression

air conditioning system.

3. INSTRUMENTATION AND MEASURING TECHNIQUE

To obtain the wall thermal resistance data, measurements are required of

the heat flux through an exterior wall and the air-to-air temperature

difference across the wall. The heat flow rate was measured with heat flux

transducers and portable calorimeters. The heat flux transducer consisted

of a 4 in (102 mm) diameter, flat circular wafer containing an embedded

thermopile with its hot and cold junctions attached to the internal wafer

surfaces. The thermopile produced a voltage signal directly proportional

to the rate of heat flow through the wafer. The heat flux transducers were installed using masking tape on the interior surfaces of both the northfacing and the west-facing walls at the locations either midway between

wood studs or furring strips, or over these wall framing members.

Calibration of the heat flux transducers was accomplished prior to the

installation using a standard guarded hot plate apparatus described in

references [5, 6]. The heat flux transducers to be calibrated were

sandwiched between two insulation boards installed next to the hot and cold

plates of the apparatus, and exposed to a uniform heat flux at the mean

temperature corresponding to that observed during the field measurements.

After a 24-hour conditioning period, the sensitivity of each heat flux

transducer was determined by dividing the measured millivolt output by the

applied heat flux. The accuracy of the transducer calibration was estimated to be within + 1%.

The portable calorimeter was developed at the Building Research Division of

the National Research Council of Canada for in-situ measurement of heat

flow through building envelopes [1], Brown and Schuyler [2] used this

apparatus to quantify heat transmission through wood frame walls of singlefamily houses. In order to measure the thermal resistance of wall

structures, two different size portable calorimeters, similar to that

employed by Brown and Schuyler [2] were designed, fabricated and

instrumented. These portable calorimeters were used to determine the

overall thermal performance of masonry and metal panel faced exterior walls

of eight office buildings situated in different geographical and various

climatic regions [7-9],

The calorimeter is a five sided insulated box with an open side that is

sealed against the building wall under test. The construction details of

the smaller size portable calorimeter are given in figure 1. The

calorimeter walls were constructed from two layers of 2 in. (50 mm) thick

aluminum foil faced, semi-rigid glass fiber insulation boards glued

2
together with the foiled side exposed. The four exterior sides of the

calorimeter were covered by 1/2 in. (13 mm) plywood to provide structural

support. The back wall had an overall thermal resistance of R 17.4

ft^*h*°F/Btu (3.06 m^*K/w). In order to make an air-tight seal between the

calorimeter and the wall to be tested, a rubber foam gasketing material was installed along the edges of the open face of the calorimeter box. The

sizes of the metering area of these two calorimeters were 31 x 35 in. (0.79 x 0.89 m) and 46 x 75 in. (1.17 x 1.19 m), respectively.

Each portable calorimeter contained an electric resistance heater with an electrical power consumption rate of 90 W for the small size, and 140 W for

the large one. A thermopile, with its many thermocouple junctions

distributed evenly and attached on both the interior and the exterior

surfaces of the calorimeter back wall, was used to monitor the temperature

differential across the calorimeter wall.

The automatic measurement and control system used for the calorimeter is

shown schematically in figure 2. A voltage controller, using the

thermopile output as the feedback variable, was employed to control the

electrical energy supplied to the electric heater by maintaining a zero

temperature difference between the inside and the outside surfaces of the

calorimeter wall. A safety thermostat with a sensing element located in

the calorimeter box controlled the temperature of air inside the

calorimeter within the safety limit. The total electric energy consumed by

the heater was measured with a watt-hour meter equipped with an optoelectronic device consisting of a light-emitting diode and a detector.

This device generated an electric pulse each time 1.8 watt-hour of electric

energy was consumed. These pulses were totaled by an electronic counter.

Since the heat losses through the box walls and the edges that contacted

the metered surface were approximately nulled to zero, the electrical

energy supplied to the heater was essentially equal to the heat flow

through the metered area.

Bead shaped thermistors were used to measure the temperatures of the

outdoor and indoor air in the vicinity of heat flux transducers, and of the

air inside the portable calorimeters. Each thermistor consisted of an external network with a fixed precision resistor. This temperature sensor produced an output voltage proportional to the temperature over a temperature range between -22 to 122°F (-30 to 50°C). The measurement

accuracy of the thermistors was +. 0.5°F (0.3°C), based on the technical

data provided by the manufacturer.

In-situ heat flow measurements were conducted on the exterior walls of the

six test buildings from January to March, 1985. The duration of each test

ranged from 7 to 12 days. The indoor temperature of each test building was thermostatically controlled at 69 ± 0.6°F (20.6 +_ 0.3°C). During the field

measurements, the heat flux transducers were attached to the internal

surfaces of the north- and west-facing walls in each building using masking

tape. A stud finder was used to ensure that every heat flux transducer was mounted either on wall framing members or midway between them. The open

side of the calorimeter was sealed against the inner surface of the test

wall using duct tape. The outdoor air temperature was measured with a thermistor installed on a bracket extending 8 in. (203 mm) from the outer

surface of the north-facing wall. Each thermistor used for measuring

indoor temperatures was secured with its sensing element positioned 6 in.

3
(152 tnm) from the wall surface being measured. The output signals from the

heat flux transducers, the electric pulses from the watt-hour meters used

with the portable calorimeters, and the thermistors were recorded

simultaneously by a micro-computer based data acquisition system. This

system was capable of recording data from 15 thermistors, 15 heat flux

transducers and 6 calorimeters simultaneously. Continuous readings of all

transducers were taken at two second intervals. The computer averaged the

readings over one hour intervals, and recorded the hourly averages on a floppy disk for further processing and analysis.

4. TEST RESULTS

4.1 FIELD MEASUREMENTS WITH TEST BUILDINGS

Table 2 presents thermal resistance data for sections of the north-facing

wall in each test building as measured with both the large and small

portable calorimeters, and with the heat flux transducers. Heat flux was measured at locations where wall framing members exist and also at

locations midway between framings. The predicted steady-state resistance

and the measured results from the guarded hot box for wall specimens with

the identical construction as the building walls are also shown in Table 2. All of the measured data from the in-situ methods are mean values of the

data collected over a 24-hour period for 6 to 12 consecutive days. Error

due to transient effect can generally be minimized by averaging the air

temperature and surface heat flux data over a 24-hour period. The wall

thermal resistance was determined by dividing the temperature difference

between the indoor and the outdoor air across the wall structure by the

measured heat flow rate. Since there are no wall framing members contained

within the exterior walls built either from logs or masonry construction

insulated with inside mass, no data are presented.

The average R-value measured by heat flux transducers and the predicted

steady-state thermal resistance value were calculated using both the zone and the series resistance method [10]. The overall thermal resistance

value including the correction for wall framing effect can be calculated

from the following equation:

R"^ = Rp^ (% framing)/100 + R^ (100 - % framing)/100 (1)

where R is the average thermal resistance value, Rp is the thermal

resistance value measured at the framing member, and R^ is the resistance

value measured at the cavity between framings. For computing the predicted

values, the published data on thermal properties of the building materials

involved were used along with the assumed thermal resistance values of 0.68

ft 2 *h*°F/Btu (0.12 m

2 *K/W) and 0.17 ft 2 *h*°F/Btu (0.03 m

2 *K/W) for the air

films at the warm and cold wall surfaces. These film resistances represent

conditions when the room side of the exterior wall is exposed to still air

and the exterior side is subject to a 15 mph (6.7 m/s) wind.

As illustrated in Table 2, there is a fairly good agreement existing

between the measured wall thermal resistance values and the predicted

results. The percentage deviation between the predicted values and the

experimental results obtained from the portable calorimeters is found to

vary from 4 to 20% with an average of 12%. The wall resistance values

measured with heat flux transducers deviated from the predicted values by

4
an average of 13% with a range covering between 6% and 28%. The large

differences betweem the predicted and measured values occurred for

insulated and uninsulated masonry walls. Possible reasons for the

discrepancy include the uncertainties in the handbook values for thermal

resistances of the building components and the transient effects of massive

thermal masses for these building walls. In general, the thermal

resistances determined by the small sized calorimeter are lower than those

measured with the large calorimeter because its metering wall area contains

approximately 30% more wall framing area for framed walls. This results in

increased heat flow due to the presence of these conductive framing

members

.

The air-to-air thermal resistance measured with a guarded hot box apparatus

for wall specimens having the same construction as the walls of the six

test buildings are also presented in Table 2. Six 6x6 ft. (1.8 x 1.8 m)

wall specimens were tested in accordance with standard hot box test

procedures by an independent test laboratory. Each test wall was placed

between an environmental chamber and a metering box, which was maintained

at 70°F (21°C) inside a guarded hot chamber. The cold chamber was

controlled at 5°F (-15°C) until the test wall achieved thermal equilibrium,

and then raised to 35°F (2°C) for the six hour test. The airflow rates

parallel to the warm-side and cold-side surfaces of the test wall were 0.7

mph (0.3 m/s) and 0.4 mph (0.2 m/s), respectively. Thermocouples were used

to measure the hot and cold test wall surface temperatures, and the

temperatures of air in the proximity of the test wall on the hot and cold

sides. The heat flow through the test wall was determined by measuring the

total electrical power input to the heater and the fan in the metering box

using precision resistor networks. Overall thermal resistance of each test

wall was derived from these wall surface temperature and electrical energy

consumption data. The air-to-air thermal resistance values in Table 2 were based on the results of the calibrated hot box measurements along with the

estimated air film thermal resistances of 0.68 ft 2 *h*°F/Btu (0.12 m

2 *K/w)

at the warm surface and 0.40 ft 2 *h*°F/Btu (0.07 m

2 *K/w) at cold surface.

Table 2 shows that the experimental values obtained by the guarded hot box

are in good agreement with the thermal resistance results derived from

steady-state thermal resistance predictions. The average percent deviation

between the predicted and measured results from 'the guarded hot box for

each wall was approximately 8%. The average indoor and outdoor air

temperatures recorded during the field thermal resistance measurements are presented in Table 3 along with the observed air temperatures at the warm

and cold sides of the wall structures measured with the laboratory guarded

hot box. The air temperature difference across the building walls during

the in-situ measurements ranged widely from approximately 16 to 47°F with

an average of 33°F.

4.2 LABORATORY MEASUREMENTS WITH DOUBLE-STUDDED WALL

In order to check the overall performances of the portable calorimeters and

the heat flux transducers, and assess their accuracy levels, in-situ

thermal resistance measurements were also performed on a well-insulated

double-studded wall installed in the calibrated hot box apparatus at the

National Bureau of Standards, Gaithersburg, MD. This series of tests

allowed comparison of the thermal resistances measured using the

calorimeters and transducers with those determined with the calibrated hot

5
box test. The wall specimen measuring 15 x 8 ft. (4.57 x 2.44 m) high and

6.75 in. (172 mm) in thickness, had a 0.002 in. (0.051 mm) thick

polyethylene vapor barrier and 1/2 in. (12.7 mm) gypsum wa 1 lboard on the

inside, a double layer of R-ll ft^*h*°F/Btu (1.94 m^*K/w) glass fiber

insulation installed in the cavities formed between nominal 2x3 in. (51 x 76 mm) wood studs placed 16 in. (406 mm) on center, and a 3/4 in. (19 mm)

thick isocyanurate sheathing and a 1/2 in. (12.7 mm) wood siding on the

outside. The construction details of the wall specimen, and the locations

of the portable calorimeters and the heat flux transducers are shown in

figure 3. In order to accommodate the width of wood stud used, the glass

fiber blanket insulation in the stud cavities was compressed from 3.5 in.

(89 mm) to 2.5 in. (64 mm) thickness. The steady-state thermal resistance

of this double-stud wall was calculated using the series resistance method

[10], and found to be 23.75 ft *h*°F/Btu, in which a thermal resistance

value of 8.77 f t^*h*°F/Btu determined by the guarded hot plate apparatus

was used for the compressed glass fiber blanket insulation [11].

The NBS calibrated hot box can evaluate the thermal performance of

composite wall specimens having dimensions of up to 10 ft. (3.05 m) wide by

15 ft. (4.57 m) high by 2 ft. (0.61 m) thick. This apparatus is capable of

providing steady-state temperatures ranging from -40 to 65°C for the

climatic chamber, and from 10 to 65°C for the metering chamber. The

construction of this test facility has been described in detail in

references [11, 12], During the laboratory measurements, the double-stud

wall specimen was held in the test frame with its outer wall surface sealed

against the climatic chamber in which the temperature was either maintained

at a constant value or changed with time in a controlled manner to simulate

outdoor weather conditions. The interior side of the test wall was exposed

to ambient air in a large environmental chamber controlled at a fixed

temperature to represent indoor conditions. Both the large and small sized

portable calorimeters were sealed tightly against the inside surface of the

test wall by the duct tape and rubber foam gasket installed at the edges of

the calorimeters. The heat flux transducers were taped on the inside

surface of wall areas over both the wood studs and the thermal insulation

filled cavities between studs as shown in figure 3.

Two tests were performed, one test with the temperature of air in the

climatic chamber held at a constant value of 34.4°F (1.3°C), and the other

test with the chamber air temperatur-e varied periodically to simulate

diurnal air temperature conditions. The apparatus was programmed to

control the temperature of the air within the climatic chamber according to

the following cosine function:

T = 16.2 cos (irt/12 - tt/2) + 42.8 (2)

where T is the climatic chamber air temperature in °F and t is the time in

hours

.

Table 4 presents the average wall resistance values measured by the

portable calorimeters and heat flux transducers, and the air temperatures

as determined by thermistors, for the double-stud wall for both steadystate and dynamic climatic conditions. For the test wall exposed to a periodic temperature variation, it was found after completion of the test

that the duration time for each temperature cycle was 23.33 hours. The

values for constant outdoor temperature in the table are averages over two

6
consecutive 24-hour cycles and the values for the dynamic climatic

condition are averages over three successive 23-hour cycles. The wall

thermal resistance value measured by the calibrated hot box and the

corresponding predicted value calculated by the series resistance method

are also listed in Table 4 for comparison. The resistance value determined

by the calibrated hot box was obtained with the apparatus operated in a steady-state mode with the test wall in the test frame clamped between the

metering chamber and the climatic chamber. The thermal resistance value

obtained from the calibrated hot box measurements included estimated air

film resistances of 0.68 ft 2 *h*°F/Btu (0.12 m *K/w) at warm surface and

0.36 ft2 *h*°F/Btu (0.06 m

2 *K/w) at the cold surface based on a measured air

velocity of 1.9 mph (0.8 m/s).

Inspection of Table 4 indicates that the measured thermal resistance values

are generally in good agreement with the predicted results. The wall

resistance values determined by the large and small portable calorimeters,

the heat flux transducer and the calibrated hot box apparatus are within 6,

1, 6 and 4 percent of the corresponding predicted value, respectively.

Based on the results of these two laboratory tests involving in-situ

measurements. The dynamic climatic condition gave a slightly smaller value

of wall resistance than given by the steady-state condition. This is

probably a result of the transient effects of varying temperature on thermal conductivities of component materials in the test wall. More test

data are needed to evaluate these preliminary experimental observations.

5. COMPARISON OF TEST RESULTS BETWEEN IN-SITU MEASUREMENTS AND THE HOT

BOX METHOD

The thermal resistance values measured with the portable calorimeters for

the exterior walls of the six test buildings and the well insulated doublestud wall are plotted in figure 4 against the resistance values determined

by the hot box for the identical wall sections. A similar plot for

comparing the thermal resistance results from heat flux transducer

measurements with values obtained in the laboratory using the hot box test

is shown in figure 5. These plots show that a good correlation exists

between the wall resistance results obtained from the in-situ measurements

and the laboratory hot box data, because all the data points lie close to

the line of perfect agreement. The percentage deviation between the wall

resistances measured with the portable calorimeter and the resistance

values determined by the hot box apparatus was found to vary between 1% to

20% with an average of 9% for the seven walls. The greater departures were

found in the thermally massive insulated walls. The thermal resistance

values obtained from in-situ measurements with heat flux transducers

departed from the hot box data by an average of 8% with a range varying

between 1% and 15%.

6. THE VARIABILITY OF MEASURED THERMAL RESISTANCE VALUES

In order to determine the extent of variation in the values measured with

the portable calorimeters and heat flux transducers, calculations of the

coefficient of variation were made with the daily averaged wall resistance

data obtained from field and laboratory measurements. The coefficient of

variation is equal to the percentage of the ratio of the standard deviation

to the mean value. The results of the statistical analysis for each wall

structure are summarized in Table 5. It can be noted that there is a wide

7
variation in the measurement period for the field tests, from 3 to 12 days.

The measurement periods of several tests were shortened due to the

occurence of unseasonably warm weather, with the outdoor temperature

greater than 60°F (15.6°C) for a few days. In addition, some difficulties

were experienced with the temperature controller of the heating system

during the course of the test. Excluding one test that had considerable

variability of the measured resistance values attributed to malfunction of

the thermostat (insulated masonry wall with outside mass), the

coefficients of variation, which are a measure of the degree of variability

of the thermal resistance data within a single instrument, for the large

and small calorimeters, and the heat flux transducer are found on the

average to be 3.8%, 5.8% and 4.3%, respectively.

Table 6 lists the numbers of heat flux transducers installed between

framing members and over the wall framings, the range of measured wall

resistance values, and the coefficient of variation of the measured

resistance results between different heat flux transducers for each

exterior wall. As illustrated in the table, the coefficient of variation

varies significantly from wall to wall depending upon the wall mass and

thermal insulation property. These coefficients of variation were found to

range from 0.2 to 16% with an average of 6%. This average value

representing the between-transducer variability is slightly greater than

the corresponding within-transducer variability. However, all of the heat

flux transducers may be considered to be essentially in agreement because

the difference between these two variability values is small.

7. CONCLUSIONS

Both the portable calorimeter and heat flux transducer were found to be

reliable and practical tools for thermal resistance measurements of

exterior envelopes in the field. The performance evaluation of both

instruments involved the exterior walls of six thermal mass test houses and

a comparative test of calibrated hot box measurements and the in-situ

methods in a laboratory controlled environment. The exterior walls tested

had thermal resistance values ranging from 3.6 to 22.8 ft^’h*°F/Btu (0.6 to

4.0 m^°K/W) and wall mass varying from 4.2 to 83 lb/ft^ (21 to 405 kg/m^).

The portable calorimeter can measure a sufficiently large wall area to

provide a representative bulk performance of the wall with a minimum

disturbance to the heat transmission through the wall. Due to the dynamic

response of an exterior wall to diurnal variations in heat flow caused by

outdoor air temperature variation, a sufficiently long measurement period,

especially for a massive insulated wall system, is required to obtain

reliable thermal resistance data using the in-situ methods. Portable

calorimeters and properly calibrated heat flux transducers can produce wall

thermal resistance values with an average deviation of less than 9% in

comparison with values obtained in the laboratory using a calibrated hot

box. The extent of variability in the wall resistance values measured by a single calorimeter was found to range from 0.3 to 9% with an average of

3.8%, and varying from 0.3 to 7% with a mean of 4.3% for the heat flux

transducer measurements. To determine the overall thermal resistance value

for a framed wall, heat flux transducers must be mounted on wall areas

backed by both framing members and cavities between the framings.

Corrections then have to be made accordingly for the effect of framing

members. Based on the data from the tests, the variation in the measured

8
wall resistance values among the heat flux transducers varied between 0.2

and 16% with a mean value of 6%.

8. ACKNOWLEDGMENT

The work reported here was sponsored by the Office of Building Energy

Research and Development, U.S. Department of Energy through an interagency

agreement with the National Bureau of Standards. The authors would like to

thank Mr. Robert Zarr of NBS for his assistance in operating the calibrated

hot box test facility to obtain data used in this paper.

9 . REFERENCES

1. Brown, W. C. and Schuyler, G. D., "A Calorimeter for Measuring Heat

Flow Through Walls," in 'Thermal Performance of the Exterior Envelopes

of Buildings I,' Proceedings of the ASHRAE/DoE Conference on Thermal

Performance of the Exterior Envelopes of Buildings, Kissimmee, FL,

ASHRAE SP28, December 1979.

2. Brown, W. C. and Schuyler, G. D., "In-Situ Measurements of Frame Wall

Thermal Res is tance ." ASHRAE Transactions . Vol. 88, Part 1, pp. 667-

676, 1982.

3. Burch, D. M., Krintz, D. F. and Spain, R. S., "The Effect of Wall Mass

on Winter Heating Loads and Indoor Comfort - An Experimental Study, " ASHRAE Transactions . Vol. 90, Part IB, pp. 94-121, 1984.

4. Wilkes, K. E., "Dynamic Thermal Performance of Walls and

Ceiling/Attics," in 'Thermal Performance of the Exterior Envelopes of

Buildings II,' Proceedings of the ASHRAE/DoE Conference, Las Vegas,

Nevada, ASHRAE SP 38, December 6-9, 1982.

5. Powell, F. J. and Rennex, B. J., "NBS Line-Heat-Source Guarded Hot

Plate for Thick Materials," in 'Thermal Performance of the Exterior

Envelopes of Buildings II,' Proceedings of ASHRAE/DoE Conference, Las

Vegas , Nevada, ASHRAE SP 38, December 6-9, 1982.

6. Rennex, B., "Error Analysis for the National Bureau of Standards 1016

mm Guarded Hot Plate," NBSIR 83-2674, April 1983.

7. Fang, J. B. and Grot, R. A., "In-Situ Measurement of the Thermal

Resistance of Building Envelopes of Office Buildings," ASHRAE

Transactions . Vol. 91, Part IB, pp. 543-557, 1985.

8. Fang, J. B. and Grot, R. A., "Field Measurements of the Thermal

Resistance of Office Buildings," a paper presented at the ASTM

Symposium on Thermal Insulation, Materials and Systems, Dallas, Texas,

December 2-6, 1984.

9. Grot, R. A., Persily, A. K., Chang, Y. M., Fang, J. B., Weber, S. and

Galowin, L. S., "Evaluation of the Thermal Integrity of the Building

Envelopes of Eight Federal Office Buildings," NBSIR 85-3147, September

1985.

9
10. ASHRAE Handbook of Fundamentals, American Society of Heating,

Refrigerating and Air-Conditioning Engineers, Inc., Atlanta, GA, 1985.

11. Zarr, R. R., Burch, D. M., Faison, T. K. and Arnold, C. E., "Thermal

Resistance Measurements of a Well Insulated Residential Wall Using a Calibrated Hot Box," presented at the ASHRAE/DoE/BTECC Conference on Thermal Performance of the Exterior Envelopes of Buildings III,

Clearwater Beach, FL, December 2-5, 1985.

12. Jones, R. R., "Acceptance Testing of the NBS Calibrated Hot Box," in

'Thermal Performance of the Exterior Envelopes of Buildings II/

Proceedings of the ASHRAE/DoE Conference on Thermal Performance of the

Exterior Envelopes of Buildings, Las Vegas, Nevada, ASHRAE SP 38,

December 6-9, 1982.

10
Table 1

No. 1

No. 2

No. 3

No. 4

No. 5

No. 6

Construction Details of Exterior Walls

of Test Buildings

Insulated Lightweight Wood Frame

5/8 in. (16 mm) exterior plywood, nominal 2x4 in. (50 x 100 mm)

wood studs placed 16 in. (410 mm) on center with R-ll

h.f t 2.°F/Btu (R-1.94 m^*K/W) blanket glass insulation installed

between the studs, a 0.002 in. (0.05 mm) polyethylene film and

1/2 in. (13 mm) gypsum board.

Uninsulated Lightweight Wood Frame

Same as No. 1, except no insulation in the stud cavities.

Insulated Masonry (Outside Mass)

4 in. (100 mm) face brick, 4 in. (100 mm) 2-core hollow concrete

block at 105 lb/ft^ (1680 kg/m^), a 1/4 in. (6.4 mm) air space, 2

in. (51 mm) thick extruded polystyrene foam insulation placed

between 1-1/2 x 2 in. (38 x 51 mm) wood furring strips placed 24

in. (610 mm) on center, a 0.002 in. (0.05 mm) polyethylene film,

and 1/2 in. (13 mm) gypsum board.

Uninsulated Masonry

8 in. (200 mm) 2-core hollow concrete block of 105 lb/ft^ (1680

kg/m^), a 3/4 in. (20 mm) air space created by 2 x 1 in. (50 x 25

mm) wood furring strips placed 16 in. (410 mm) on center, a 0.002

in. (0.05 mm) polyethylene film, and 1/2 in. (13 mm) gypsum

board.

Log

7 in. (180 mm) square lodge-pole-pine logs (butt-jointed with a compressible foam backer-rod and a caulking compound applied to

interior and exterior joints).

Insulated Masonry (Inside Mass)

4 in. (100 mm) face brick, 3-1/2 in. (89 mm) perlite loose-fill

insulation in cavity, 8 in. (200 mm) 2-core hollow concrete block

at 105 lb/ft^ (1680 kg/m^), and a 1/2 in. (13 mm) plaster.

11
Table 2

Comparison of Wall Thermal Resistances Measured with Portable

Calorimeters (PC) and Heat Flux Transducers (HFT) to Corresponding

Predicted Values and Measured Results from Guarded Hot Box

Thermal Resistance (ft 2, h*°F/Btu)

Measured Value

Construction

Mass

( Ib/f t 2)

PC

Large Small

Between

Framings

HFT

At

Framing Avg.

Predicted

Value

Guarded

Hot Box

Insulated wood

frame

4.4 11.43 10.14 14.27 10.41 13.79 11.91 12.22

Uninsulated wood

frame

4.2 3.55 3.34 3.43 4.01 3.48 3.14 3.60

Insulated masonry

with outside mass

64.0 11.01 13.31 15.15 9.39 14.59 13.77 13.69

Uninsulated

masonry

42.0 4.41 4.18 4.95 5.08 4.97 3.89 4.63

Log 17.0 10.08 9.83 10.27 - 10.27 9.60 10.33

Insulated masonry

with inside mass

83.0 10.20 9.53 14.32 - 14.32 12.65 12.42

Note: 1 ft 2 *h*°F/Btu - 0.176 m

2 *K/W

1 lb/ft 2 = 4.882 kg/m2

12
Table 3

The Measured Average Air Temperatures Across Wall

Structures During Field and Laboratory Wall Thermal

Resistance Measurements

Air Temperature. °F

In -Situ Guarded

Wall Measurements Hot Box

Construction Indoor Outdoor Warm Side Cold Side

Insulated wood

frame

69.2 45.2 74.9 32.0

Uninsulated wood

frame

70.3 43.5 77.9 33.6

Insulated Masonry

with outside mass

69.9 54.2 73.3 24.2

Uninsulated

masonry

74.1 27.3 83.2 31.4

Log 68.4 26.8 83.6 37.4

Insulated Masonry 68.2 26.3 76.5 23.9

with inside mass

Note °C = 5/9 C°F - 32)
Table 4

Comparison of Measured and Predicted Thermal Resistances

of Well-Insulated Double-Stud Wall Under Steady-State

and Dynamic Climatic Conditions

Thermal Resistance (f t^*h *°F/Btu)

Measured Value

Air Temp . Calorimeter Flux Transducer

(°F

)

Calibrated

Climatic

Condition

Warm

Side

Cold

Side

Predicted

Value Laree Small

Between

Studs

At

Stud Avk.

Hot

Box

SteadyState69.5 34.4 23.75 25.25 24.07 26.37 20.98 25.16 22.84

Dynamic 70.0 42.1* - 23.86 23.56 25.30 20.47 24.23 -

Note: °C = 5/9 (°F - 32)

1 ft 2o h‘°F/Btu = 0,176 m

2 °K/W * The average value of periodically varying air temperatures ranging from

26.5 to 52 .2°F

14
Table 5

The Variability of Wall Thermal Resistance Values Measured by

a Single Portable Calorimeter or Heat Flux Transducer

Portable Calorimeter Heat Flux Transducer

Large Small Test C.V.* (%)

Wall Duration C.V.* Duration C.V.* Duration Between At

Assembly (dav) u> (dav) (%> (dav) Framings Framing

Insulated wood

frame

11 4.14 8 7.33 8 7.23

(7.2-7 .3)

7.01

(6.6-7 .4)

Uninsulated wood

frame

9 3.05 7 3.19 7 2.20

(2. 2-2. 4)

2.96

(2. 5-3. 4)

Insulated masonry

with outside mass

10 9.09 3 20.6 3 15.2

(13.4-18.2)

15.3

(14.9-15.7)

Uninsulated

masonry

12 3.13 5 7.67 5 7.02

(6. 6-7. 5)

7.92

Log 10 7.12 10 7.76 10 6.42

(6. 3-6. 5)

-

Insulated masonry

with inside mass

4 1.80 3 10.2 4 4.94

(4. 1-5. 8)

-

Double-stud

wood frame:

Steady-state

condition

2 0.34 2 3.44 2 0.27

(0.03-0.4)

0.74

(0.1-1. 4)

Dynamic case 3 2.03 3 1.30 3 1.85 2.83

(0.7-3. 2) (1.9-3. 8)

Coefficient of variation - (Standard Deviation/Mean) x 100.

15
Table 6

The Variations in the Measured Thermal Resistance

Values Among Heat Flux Transducers

Between Wall Framings At Wall Framing

Wall

Assembly

No . of

Sensors

Range of

R-Values

(ft2»h«oF/Btu)

Coefficient

of

Variation

____ (%)_

No . of

Sensors

R-Values

(ft2.h-oF/Btu)

Coefficient

of

Variation

(Z)

Insulated wood

frame

2 13.8 - 14.7 4.56 2 10.2 - 10.6 3.06

Uninsulated wood

frame

2 3.42 - 3.43 0.21 2 3.9 - 4.1 3.36

Insulated masonry

with outside mass

3 13.5 - 16.2 9.63 2 9.1 - 9.7 4.52

Uninsulated

masonry

2 4.7 - 5.2 6.00 1 5.1 -

Log 3 9.8 - 10.8 5.06

Insulated masonry

with inside mass

2 12.7 - 16.0 16.2

Double-stud wood

frames

Steady-state

condition

3 24.8 - 29.4 10.1 2 20.6 - 21.3 2.29

Dynamic case 3 23.6 - 28.1 9.79 2 20.1 - 20.9 2.63

16
34*(0.86m)

Rubber

foam

gasket

Safety

thermostat

sensing

element

Electric

heating

element

Thermopile

junctions

Rigid

foil faced

glass fiber

insulation

board

38'

(0.97m)

Plywood

frame

8

' (0.20m)

Figure 1. Construction Details of Small Sized

Portable Calorimeter

17
Figure 2. Schematic of the Heasurement/Control System

for the Portable Calorimeter

18
« “

©

0 2 U

© 3 1 • 2 a 5 J

& ?!

$

19

Figure

3.

Construction

Details

and

Sensor

Locations

on

the

Well

Insulated

Double-Stud

Test

Wall
30

25

20

15

10

5

0

(m2 -K/W) 12 3 4

Figure 4. A Comparison Between the Wall Thermal Resistance

Values Measured with Portable Calorimeter and the

Values Obtained with the Calibrated Hot Box

20

(m

2

K/W)
<m 2 *K/W)

e CME

Figure 5. A Comparison of the Wall Resistance Values

Measured by Heat Flux Transducers and the

Calibrated Hot Box Data

21

FORM NBS-1 14A (REV.11-84) U.s/OEPT. OF COMM. 1. PUBLICATION OR

REPORT NO.

2. Performing Organ. Report No. 3. Publication Date BIBLIOGRAPHIC DATA

SHEET (See instructions) NBSIR-86/3328 MARCH 1986

4. TITLE AND SUBTITLE

The Assessment of Accuracy of In-Situ Methods for

Measuring Building Envelope Thermal Resistance

5. AUTHOR(S)

Jin B. Fang, Richard A. Grot and Hyo Soon Park

6. PERFORMING ORGANIZATION (If joint or other than NBS, see instructions) 7. Contract/Grant No.

NATIONAL BUREAU OF STANDARDS -- U.S D&ARTMENT OF COMMERCE 8. Type of Report & Period Covered

GAITHERSBURG, MD 20899

9.

SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS ( Street, City . State, ZIP)

Office of Building Energy Research and Development

U.S. Department of Energy

Washington, DC 20585

10.

SUPPLEMENTARY NOTES

| | 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)

A series of field and laboratory tests were conducted to evaluate the accuracy of

in-situ thermal resistance measurement techniques. The results of thermal performance

evaluation of the exterior walls of six thermal mass test houses situated in Gaithersburg, Maryland are presented. The wall construction of these one-room houses includes

insulated light-weight wood frame, uninsulated light-weight wood frame, insulated

masonry with outside mass, uninsulated masonry, log, and insulated masonry with inside

mass. In-situ measurements of heat transfer through building envelopes were made with

heat flux transducers and portable calorimeters. A sufficiently long period of measure- ments, depending on the thermal mass of wall structure, is needed to provide reliable

thermal resistance data. The comparisons of the results from these field measurements

with those derived from sections of the same wall structures tested in a guarded hot'

box facility in a laboratory are presented. A well-insulated, double-stud test wall

was also tested under simulated thermal conditions including steady-state and periodically varying outdoor temperature using a calibrated hot box and the in-situ measurement procedures.

From these test results, the in—situ methods are shown to provide thermal resistance data within 9% of the hot box results. The extent of variability in wall

resistance values measured by a single calorimeter or heat flux transducer is found to

range from 0.3 to 9% with an average of 4%.

12. KEY WORDS (Six to twelve entries ; alphabetical order; capitalize only proper names; and separate key words by semicolon s) building; exterior envelope; field test; heat flux;

thermal resistance; 'portable“calorimeter; wall

13. AVAILABILITY

[~%~| Unlimited

in-situ measurements; temperature;

14. NO. OF

PRINTED PAGES

| | For Official Distribution. Oo Not Release to NTIS

| | Order From Superintendent of Documents, U.S. Government Printing Office, Washington,

20402.

[X] Order From National Technical Information Service (NTIS), Springfield. VA 22161

DC

27

15. Price

$9.95

USCOMM-DC 85-4006