NBSIR 80-2100

THERMAL RESISTANCE MEASUREMENTS

OF A BUILT-UP SYSTEM

Library, t-01 Admin. Bld|.

FEB 2 7 1981

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no.

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S. J. Treado

Center for Building Technology

National Engineering Laboratory

National Bureau of Standards

U.S. Department of Commerce

Washington, DC 20234

October 1980

Prepared for Department of Energy

Architectural and Engineering

Systems Branch

Buildings Division

Office of Buildings and

Community Systems

Washington, DC 20585

U.S. DEPARTMENT OF COMMERCE, Philip M. Klutznick, Secretary

Luther H. Hodges, Jr., Deputy Secretary

Jordan J. Baruch, Assistant Secretary for Productivity, Technology, and Innovation

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

ABSTRACT

This report describes factors which affect the thermal performance of

built-up roof systems, and a technique for making in-place measurements

of thermal resistance. This measurement technique utilizes a combination

of infrared thermographic imaging, surface heat-flow meters and surface

thermopiles. The thermal resistance of the roof system is computed based

on temperature differences across the roof and the measured heat flow

through the roof

A field test of the measurement procedure is detailed, along with an examination of the time period required to perform a roof thermal

resistance measurement, as related to the thermal time lag for heat flow

through the roof due to the effect of the thermal mass of the roof.

Roof thermal resistance determinations performed according to this measurement procedure are found to be very accurate, if measurements are performed over a sufficient time interval, the minimum interval being

dependent upon the thermal mass of the roof system.

Key Words: Built-up roofs, measurement technology; moisture accumulation, nondestructive tests; thermal resistance.

iii
ACKNOWLEDGMENTS

The author is grateful to D. M. Burch for the assistance concerning

the experimental procedure and analysis, to B.A. Peavy for his help

with the mathematical modeling and computer simulation, and to

R.G. Mathey for the information provided about the roofing systems.

This investigation was done under Oak Ridge National Laboratory subcontract No. DE-A105-780R061 13 and the U.S. Department of Energy and is

part of the National Program for Building Thermal Envelope Systems and

Insulating Materials.

iv
TABLE OF CONTENTS

Page

1. INTRODUCTION 1

2. FACTORS AFFECTING THE THERMAL PERFORMANCE OF BUR SYSTEMS 2

3. BUR THERMAL PERFORMANCE ASSESSMENT PROCEDURES 4

4. THERMAL RESISTANCE MEASUREMENTS 7

4.1 Thermographic Survey

4.2 Sensor Installation

4.3 Measurements

5. COMPUTER SIMULATION OF BUR SYSTEMS 11

6. CONCLUSION 13

REFERENCES 14

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1. INTRODUCTION

Buildings are among the largest energy consumers in the United States

comprising approximately one-third of the total annual energy consumption [1]. The roof of a building is a major contributor to heat flow

through the building envelope. Bituminous built-up membrane roofing

(BUR) is widely used in building construction in the United States.

A conservative estimate of the amount of low-slope roofing in place in

the United States is 25 billion f t^ (230 x 10^ mO . About 3 billion

ft^ (280 X 10^ m^) is constructed each year [2]. The average thermal

resistance of these roofs is less than 7 °F »f t^ «h (1.23 K»m^/W), and

Btu over 40 percent of them contain no insulation at all [3] .

The uninterrupted design of low-slope roofs makes them readily accessible

providing a significant potential to conserve energy expended for heating

and cooling through the utilization of retrofit measures to reduce heatflow through the roofing materials. Increasing the thermal resistance

of the Nation's 25 billion ft^ of low-slope, built-up roofs by an additional 10 °F *f t^ «h (1.76 K*m^/W), would result in a savings of 155,000

Btu

barrels of oil per day (based on a 5000 heating degree day climate

during the heating season) . Seven hundred million kilowatt hours could

be saved during a cooling season requiring 1,000 equivalent operating

hours

Many factors affect the thermal performance of roofing systems, including

design, workmanship, materials, age, weathering, and moisture intrusion.

The actual thermal performance of built-up roofs may differ substantially

from the design or expected performance [4]. With increasing concern

for conserving energy in buildings, there is a need to develop a reliable

field measurement technique for assessing the thermal resistance of

built-up roof systems. Such a technique would provide information to

determine if the thermal resistance of these systems meets design expectations, and possibly lead to improved design procedures. Also, roofs

having inadequate thermal resistance could be Identified and treated as part of a thermal retrofit program.

The objective of this report is to identify factors which affect the

thermal performance of existing built-up roof systems, and to describe

a technique for measuring the thermal resistance of roof systems. This

measurement technique utilizes a combination of infrared thermographic

imaging, surface heat flow meters and surface thermopiles. The thermal

resistance is computed based upon the measured inside-to-outside surface

temperature difference across the roof and the measured heat flow through

the roof. A field validation of the measurement system was performed

and the results examined and reported. The technique for measuring the

thermal resistance of roof systems described in this paper can be used

to assess the thermal performance of many different types of roof systems

1
2. FACTORS AFFECTING THE THERMAL PERFORMANCE OF BUR SYSTEMS

The principle function of a roof is to protect the enclosure beneath it

from the weather elements, and to maintain desired environmental conditions within the structure. In this regard, a roof must be able to

withstand a variety of wind pressures, temperature cycles, and moisture

conditions. Throughout a typical year, low-slope BUR systems are subjected to rain, snow, solar radiation, ice, and to wide variations in

surface temperature. A roof system must have structural integrity and

be durable and serviceable to withstand these conditions or exposures.

Increasing concern for energy conservation has added another factor to

the list of desirable characteristics for roof systems, that of thermal

efficiency. The methods utilized to achieve the goal of a thermally

efficient roof are not always consistent with the methods which produce

the most durable and serviceable roof, since the strongest materials may

not be good insulators, and the best insulators generally provide little

structural strength. However, some types of failures will affect the

integrity of the water-proofing membrane and insulation as well as the

thermal performance. An example of this is the splitting of the membrane

which could lead to water penetration and mechanical and thermal degradation of the insulation due to moisture intrusion.

Many factors can affect the thermal performance of a BUR system. Since

a low-slope roof is made up of several layers of composite elements, one

or more of these factors can affect its performance. A typical BUR

system will consist of a metal or concrete deck covered with insulation,

four piles of felt alternating with layers of bitumen, and a bitumen

asphalt coat topped with slag or crushed rock.

The primary factor, and one of the most important, affecting the thermal

performance of a BUR system is the initial design. The characteristics

of the materials used, such as the thermal conductivity, moisture permeance, thickness and durability, as well as the construction details

and design, play a large part in determining the thermal performance of a roof. Adequate slope of the roof and the location and design of drains

and flashing are important in preventing premature failure [5]. Utilization of larger thicknesses of insulation requires special attention

because of thermally and mechanically induced stresses in the roof membrane [6]. Some insulations may not be a stable substrate for the membrane, particularly if they are very thick. In addition, the membrane

is more likely to be punctured by foot traffic or by objects which are dropped on the roof by accident, since insulation is softer than deck

materials. Extreme temperature variations can occur in a black roofing

membrane over insulation exposed to the normal day and night solar

radiation cycle. Dally variations as large as 140°F (78°C) have been

recorded, as well as seasonal variations up to 250°F (139°C) [7]. The

stresses associated with the expansion and contraction of a roofing membrane in response to these large temperature fluctuations can contribute

to premature failure in some cases. An example is membrane splitting

due to large decrease in temperature to well below freezing over a short

period of time. Ultra-violet radiation and corrosive atmospheric

deposits can also contribute to degradation of a roofing system.

2
Similarly, workmanship and construction practice can strongly Influence

the long-term thermal performance of a roof. The exterior membrane is

constructed of a number of felt plies, bonded together with asphalt or coal tar pitch which are generally applied hot. Attachment problems can

occur due to rapid cooling of the hot asphalt [8]Incorrect installation of flashings or other roof penetrations can also contribute to

leakage or premature failure.

Moisture can have a serious effect on the themal performance of a roofing system. The intrusion of moisture into BUR insulation can cause

a reduction in the thermal resistance of a roof [9] and lead to premature failure of the roofing system. Moisture can invade a BUR system

from the exterior through leakage, from the interior as water vapor

generated within the structure migrates outward, or can even be present

during construction and entrapped upon application of the waterproof

membrane. Failure of the membrane for any reason can result in moisture

related problems.

Other factors which affect the thermal performance of a BUR system

include insulation joints, metal fasteners, roof penetrations and aging

and weathering of the membrane. Heat flow through the roof can occur at

insulation joints and penetrations and through metal fasteners which

attach insulation to the deck. Aging and weathering of the membrane can

be controlled to some extent through the utilization of adequate materials and design procedures and periodic maintenance, such as visual

inspections followed by repairs to prematurely failing areas, cracks

or penetrations.

The incidence of rain can temporarily alter the thermal performance of

a roof due to the cooling effect of the runoff as well as evaporation.

Similarly, a bed of snow on a roof can function as an additional insulation layer, changing its overall thermal resistance.

3
3. BUR THERMAL PERFORMANCE ASSESSMENT PROCEDURES

Several methods are currently utilized for detecting moisture in BUR

systems [1]. These methods include gravimetric techniques and non- destructive evaluation methods (NDE) such as nuclear backscatter, electrical capacitance and infrared imagery (thermography) [10]. Although

most accurate, the gravimetric technique has the disadvantage of being

a destructive test method. Data are not available on the accuracy,

validity, and reliability of NDE methods to quantitatively detect moisture in roofs.

Thermal resistance determinations for BUR systems have also been performed utilizing heat flow meters and thermocouples [11]. In using this

method, the heat flow through a roof and the temperature difference

between its interior and exterior surfaces are measured. The thermal

resistance of the roof is subsequently calculated from the relation:

To make an accurate thermal resistance determination, measurements must

be made over a sufficiently long period (p) to reduce the transient

effects which can occur due to heat storage in the roof and due to the

thermal time lag of the roof, in which case heat flow lags behind temperature difference. These effects are most noticable in roof systems

with large amounts of thermal mass, such as BUR systems with concrete

decks. The ability of a roof system to store heat rather than transmit

it with little time delay results in a time lag between temperature

difference and the resulting heat flow. This factor can be described

as the thermal capacitance of a roof.

The technique utilized in this study for assessing the thermal resistance of BUR systems consists of a combination of thermographic imaging

and local measurements using heat-flow meters and thermopiles.

Infrared (IR) thermography is based on the principle that all surfaces

emit energy in the form of electromagnetic radiation, in proportion to

their surface emittance and the fourth power of their absolute temperature. The principal components of a thermographic imaging system are

the infrared camera, black-and-v/hlte monitor, color monitor and temperature profile display monitor (See figure 1) [12]. Upon sensing the

radiation intensity from a surface, the IR camera produces a video

signal which is internally processed by the black-and-white monitor and

displayed as a thermal picture in which the gray tones in the picture

correspond to local "apparent surface temperature." The term apparent

surface temperature is used here because the thermographic camera is

b

( 1 )

where AT = temperature difference

Q

= heat flow

p = period of integration

4
sensing radiation emitted and reflected from a surface. Without knowing

the emittance of the surface, the actual surface temperature is not

established and the apparent surface temperature is that which would

correspond to a black body emitting an equivalent level of IR radiation.

Video signals are also fed into the color monitor, where different

apparent surface temperatures are displayed as individual colors.

The actual surface temperature of an object may differ from its apparent

surface temperature, due to the emittance of the surface and consequently

the amount of reflected radiation from the surface. The brightness of

the image of a surface on the black-and white monitor, or its color on

the color monitor, is dependent upon the intensity of electromagnetic

radiation sensed from the surface. This intensity of radiation is a function of the temperature and emittance of a surface. Two objects

with identical surface temperatures but different emittances could

appear to have different surface temperatures, due to reflected radiation

from surrounding surfaces which are at different temperatures. On the

thermal picture, or thermogram, the color temperature reference scale

is displayed at the bottom with colder to warmer levels running left to

right, or black to white.

In following the technique proposed in this study, an initial thermographic scan is performed on the roof surface to obtain an apparent temperature profile map of the top of the BUR system. This scan must be

performed during non-daylight periods, allowing enough time to elapse

following sundown to enable residual solar radiation effects to dissipate. At this time of day, the outdoor temperature is more steady than

at other times, and variations in surface temperature will predominantly

be due to differences in thermal resistance of the BUR system, and surface convection effects if wind conditions vary at different roof locations. In addition, sufficient interlor-to-exterlor temperature

difference must be present, approximately 20°F (11°C) minimum, to obtain

accurate results. Regions of the roof surface whose temperature appears

to vary from the majority of the roof surface are marked with spray paint

or otherwise identified for subsequent examination. Apparent temperature

variation of some portions of the roof surface may actually be due to

differences in emittance or special conditions (such as water, ice

patches or metal surfaces) . Variations in surface temperature can also

be due to hot air exhausting onto a roof from a vent, hot rooms directly

below a roof, differences in the amount of roof insulation, difference

in exposure conditions, differences in underlying construction, or wet

or otherwise defective insulation. Close examination of suspect regions

usually enables an accurate assessment of the cause of the apparent

temperature variation.

Those areas which are identified as regions of hotter or colder surface

temperatures are instrumented for thermal resistance measurements. This

is accomplished through the use of heat flow meters and thermopiles. An

actual measurement system is describe! in Section 4. A heat-flow meter

produces a millivolt signal proportional to the heat flux passing

through its body. When attached flush with a surface, the heat flowing
through that surface can be measured. A thermopile is 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.

The heat flow meters are attached to either the interior or exterior

roof surface, and the thermopile is attached to the interior and exterior

surfaces of the roof. Heat flow meters should not be attached directly

to a metal deck because the metal deck will act as a fin, possibly

disrupting the accuracy of the measurement due to the effect of twodimensional heat flow. For metal decks, the heat flow meters should be

installed on the exterior roof surface, rather than on the interior metal

surface. For concrete or wood decks, the heat flow meters can be

attached directly to the interior surface of the deck.

The thermal resistance value for a particular area of the roof is determined by dividing the integrated temperature difference, as measured by

the thermopile, by the integrated heat flow, as measured by the heat

flow meter

Areas of low thermal resistance can be examined for moisture content

through coring of roof samples followed by an oven-drying procedure, or by a non-destructive moisture detection procedure.

6
4. THERMAL RESISTANCE MEASUREMENTS

Measurements were made on the roof of Building 226 at the National

Bureau of Standards. The building was constructed in 1963 with a 40,000 ft^ (2716 m^) BUR system consisting of a concrete deck, glass

fiber insulation and built-up roofing. A photograph of the roof

surface is shown in figure 2. Details of the construction design of

the BUR system were utilized to compute its thermal resistance.

The components of the BUR system and their thermal resistances are listed in table 1.

Table 1. Computed Thermal Resistance of Test Roof

Thermal Resistance

Component h*ft^*F/Btu (m^»K/W)

5-in. (12.7 cm) Concrete 0.40

Deck*

Glass Fiber Insulation

~ll/161n.(27mm) 4.17

(Conductance < 0.24 - fi-tU-. (h«ft^*F)**

or 1.36

^

m"^‘K

Built-Up Roofing* 0.33

° Asphalt Primer

° 4 Plies of Felt

° 3 Layers Asphalt

° Asphalt Flood Coat

° Slag

Total Thermal Resistance 4.90

0.07

0.73

0.06

.86

* From ASHRAE Handbook [13].

From Design Specification.

The actual thermal resistance of the roof may vary from the computed

thermal resistance if different insulation thickness is used instead of

the design thickness, or if the thermal properties of the actual materials differ from handbook values. Variations may also occur due to

local irregularities in construction materials, assembly techniques,

workmanship, or the presence of moisture.

7
4.1

THERMOGRAPHIC SURVEY

A thermographic scan of the roof surface was performed to obtain an apjarent temperature map. The scan was performed on a cold night, 29°F

(-1.6°C) during December under partially overcast sky conditions. Wind

speed was less than 5 mph (8 kh) and the average roof surface temperature

was 25°F (-4°C) . The major portion of the roof surface appeared to be

fairly uniform in temperature, varying by less than 0.9°F (0.5°C) (See

figure 3). Roof surface areas surrounding the large central vents are seen to be wanner than the majority of the roof. This is more apparent

in figure 4. In the color thermograms, the warmest area of the roof

surface appears to be 1.8°F (1°C) warmer than the remaining areas. Visual inspection of the warmer portions of the roof surface did not

reveal any obvious differences in roof construction materials from the

majority of the roof surface, and no differences in emittance were believed to be present which might be causing warmer apparent surface

temperature. One of the warmer areas of the roof surface was chosen as

the location for the installation of the heat flow meter and thermopile.

This location is shown in figure 5. The cylindrical object in the

center of the thermogram is a liquid nitrogen Dewar flask being used

as a marker

4.2

SENSOR INSTALLATION

A heat-flow meter was spot-glued to the interior surface of the concrete

roof deck. The meter consisted of a thin cylindrical wafer containing

an imbedded thermopile. The millivolt signal generated from this

imbedded thermopile is proportional to the heat flow passing through

the wafer. The heat-flow meter was connected to an analog Integrator,

for the purpose of recording hourly averaged values of heat flow, and a data logger which recorded instantaneous values at adjustable time

intervals. The temperature difference between the interior and exterior

roof surfaces was measured with a copper-constantan thermopile consisting of three pairs of junctions attached to the surfaces. The three

exterior thermopile junctions were attached to the outer surface of the

roof with a room-temperature vulcanizing silicone adhesive. The cor- responding interior thermopile junctions were attached to the inner roof

surface (concrete deck) with tape. The thermopile was connected to an analog integrator and to the data logger.

4.3

MEASUREMENTS

The thermal resistance value for the instrumented location of the roof was determined by dividing the hourly integrated temperature difference

by the hourly integrated heat flow, as measured by the heat-flow meter.

Measurements were made for several weeks to obtain an average value for

the thermal resistance. Figure 6 shows the data obtained during a typical nine day measurement period.

The hourly average thermal resistance of the roof, RI, was determined by

dividing the hourly average temperature difference by the hourly average

8
heat flow in discrete hourly increments. The values AT (temperature

difference), Q (heat flow) and R1 (hourly thermal resistance), are plotted in figure 6 at hourly Intervals. RAV, noted in figure 6, is the

cumulative average of RI or:

RAV

Z Rli

i=l

t

RIj ^

RI2 + ... Rl^

t

( 2 )

where t = elapsed time (hours) or number of readings.

The Instantaneous hourly value of the thermal resistance is seen to vary

strongly with the temperature difference across the roof. Maximum heat

flow is seen to lag behind maximum temperature difference by approximately 12 hours, due to the thermal capacitance of the roof.

The measured overall thermal resistance at this location of the roof was

O O found to be 5.06 h »f t »F /0.89 m *K \ , based on the nine days of measureBtu W

^ ment . This is 3.3% higher than the design value of 4.90 (.86). The RAV

line gives a good example of how long measurements must be made to negate

the effect of random daily temperature fluctuations on the average thermal

resistance value. After 67 hours of measurement the cumulative average

thermal resistance, RAV, stayed within 10% of the final value (5.06 +

.506). After 133 hours, RAV stayed within 5% of the final value (5.06 + .253) . These time factors would vary according to roof type and temperature conditions, but for roof of this type, three to six days of data

collection would be necessary to obtain a representative value for the

thermal resistance. The time required for cumulative averaging will vary

from one roof to another depending on the thermal capacitance (thermal

mass) of the roof. Shortening this time span could result in inaccuracies due to random fluctuations or variable daily temperature cycles.

The close agreement between measured and calculated thermal resistances

for the roof indicates that the thermal performance of the BUR system was

as would be expected based on design parameters. Moisture intrusion or other factors affecting thermal conductance were not believed to be present, since the roof thermograms indicate fairly uniform surface

temperatures, and since the measured thermal resistance at the warmest

spot on the roof showed no reduction from design value.

To investigate the actual moisture content of the roof, samples of insulation and membrane were taken from a location near that of the thermopile. The insulation was fiberglass and was about one inch thick. An

exact measurement of its thickness could not be made since it was damaged when cut from the roof. Since the roofing specification called

for insulation to have a conductance of 0.24 Btu/h»ft^»F (1.36 W/m^ *K) , it is assumed that the insulation was 1 1/16 in (27 mm) thick. The

moisture content of the insulation was determined gravimetrically and

was found to be very low, only 0.2 percent.

9
From the sample taken from the roof it was found that the membrane con- sisted of four plies of asphalt saturated organic felt with a surfacing

of slag embedded in an asphalt flood coat. The moisture content of the

membrane was also determined gravimetrically and was found to be 3 g of

water per ft^ of area (.09 m^). The average amount of interply asphalt

in the sample was about 10 Ib/lOOft^ (^.5 kg/9 m^). The Interply

asphalt thickness was measured using a machinist’s microscope.

10
5. COMPUTER SIMULATION OF BUR SYSTEMS

A computer simulation procedure [14] was utilized to examine the effect

of the thermal mass of the roof on the time period required for accurate

measurement of thermal resistance using the technique described in the

previous sections of this report. In the case of the BUR system used

for the field test, the thermal mass of the concrete deck contributed

to a long thermal response time. If the deck had been constructed of

steel, response time would have been much shorter, due to the reduction

in roof thermal mass.

A mathematical model was used to calculate heat flow through the roof

system, using the measured hourly indoor and outdoor surface and air

temperatures observed during the field test as input parameters. A

response factor technique was employed to first compute surface heat

transfer coefficients and subsequently calculate heat flow through the

roof system. To validate the model the BUR system used for the field

test was modeled, and heat flow through the roof was calculated and

compared to actual measured values. A typical four-day period is shown

in figure 7. Computed heat-flow is plotted beginning with the ninth

hour, to allow start-up time for the model (since heat flow lags behind

the driving force, temperature differential).

Agreement between measured and calculated heat flow is satisfactory,

considering that the input temperature parameters were in hourly increments, thus imparting some scatter to the calculated heat flow.

Next, the concrete deck was replaced with a steel deck in the model,

and heat flow through that roof system was calculated for the same temperature parameters. Figure 8 presents the inside-to-outside temperature difference conditions used for the simulation period, as well as calculated heat flow through the roof system with steel deck. Very

little time lag is seen for this roof system, as maximum heaf flow

follows maximum temperature difference within one hour.

As previously discussed, hourly values of thermal resistance (RI) were

computed based on the ratio of the measured hourly temperature difference and the calculated hourly heat flow. The cumulative average of

the thermal resistance (RAV) was also calculated. RI and RAV for the

BUR system with steel deck are also plotted in figure 8.

Since the steel roof deck provides negligible thermal resistance [13],

the thermal resistance of the BUR system modeled with the steel deck

would be expected to be the value for the BUR system with concrete deck

less the thermal resistance of the concrete deck, or 4.90 minus 0.40

equals 4.50 (.79 m^ *K/W) * Btu

At the end of the four-day simulation period the cumulative average for

the thermal resistance of the steel deck BUR system is seen to be

4.58 h»f t^«F (0.81 m^*K/W), or 2 percent higher than the design value.

Btu

11
RAV stays within 10% of its final value (4.58 + ,46) after 5 hours

elapsed measurement time, and within 5% (4.58 + ,23) after only 7 hours

elapsed measurement time. R1 is seen to approach RAV for long periods

of time during night hours, and in general RI varies much less for the

BUR system with steel deck as compared with the concrete deck. This

indicates that temperature conditions were fairly near steady-state

relative to the thermal response time of the BUR system with steel deck,

during night hours. RI variation for the steel deck is less than for

the concrete deck since the short thermal response time of the roof means that heat flow will fluctuate more closely in response to temperature conditions, therefore hourly values of heat flow and temperature

difference are more closely related, causing their ratio to more accu- rately reflect the actual thermal resistance. RI deviates from RAV the

greatest amounts during periods in which the temperature difference is

changing rapidly.

This analysis indicates that the thermal resistance of BUR systems with

metal decks could be accurately measured using this technique in a fairly short time period, and probably within one day, depending on weather conditions.

Based on the thermal resistance measurements performed on the BUR system

with a concrete deck, and the results of the simulation of a BUR system

with a steel deck, the required elapsed measurement time is presented

in figure 9 as a function of roof system time lag. Time lag for the

steel deck system was estimated to be 3/4 of an hour, although an exact

determination was difficult due to the fact that measurements were stepped in hourly increments.

The thermal time lag of a BUR system would be dependent upon the materials constituting the roof, especially heavy-weight decks or large

amounts of Insulation, as well as weather conditions.

Most BUR systems would probably fall somewhere between the two types

examined here with respect to thermal mass and consequently, thermal

time lag. Additional information is needed concerning the thermal time

lag of various different BUR constructions to enable more complete

criteria for required elapsed measurement time to be developed.

12
6. CONCLUSION

Many factors can influence the thermal performance of a built-up roofing

system, including design, materials, workmanship, exposure conditions

and moisture.

Thermally inefficient roofs can be identified through the utilization

of roof thermal asssessment procedures. A procedure utilizing a thermographic imaging system, in conjunction with local measurements using

heat flow meters and thermopiles, can be valuable in determining the

thermal resistance of BUR systems. The thermal resistance can be calculated using the measured integrated temperature difference across the

roof and the measured integrated heat flow through the roof, provided

data are obtained over a sufficiently long time period to negate

inaccuracies due to the thermal capacitance of the roof. A field test

of the measurement procedure yielded a measured thermal resistance value

for a BUR system within 3.3 percent of the design thermal resistance for

that roof. The 1 1/16 in (27 mm) thick glass fiber insulation contained

0.2 percent moisture by weight.

The thermal mass of a BUR system strongly affects the amount of elapsed

measurement time required to determine its thermal resistance accurately. Analysis indicates that a typical BUR system with a concrete

deck will require 3 to 6 days of measurement time, while a similar BUR

system with a steel deck would require less than one day.

13
REFERENCES

1. Seidel, M., Plotkin, S., Reck, R., Energy Conservation Strategies,

Environmental Protection Agency Report, EPA-R5-73-021 , p. 12 (July

1973) .

2. Busching, H., Mathey, R., Rossiter, W., Cullen, W., Effects of

Moisture in Built-Up Roofing; a State-of-the Art Literature Survey,

NBS Technical Note 965 (July 1978) .

3. "Project Pinpoint," National Roofing Contractors Association,

(1977).

4. Tobiasson, W., Korhonen, C., Dudley, T., Roof Moisture Survey,

Cold Regions Research and Engineering Laboratory Report 77-31,

(Dec. 1977).

5. McCurdy, C., The Importance of Design Criteria for Built-Up Roof

Flashings, NBS Thermally Efficient Roofing Systems Workshop, (May

1978) .

6. Perrine, E., Problems Associated with Thermally Efficient Roofs,

NBS Thermally Efficient Roofing Systems Workshop, (May 1978).

7. Garden, G. Fundamentals of Roof Design, Canadian Building Digest

67, Div. of Build. Res., Nat. Res. Council, Ottawa, Canada (July

1965) .

8. Dupuis, R., Attachment Characteristics of Field Applied Asphalt,

NBS Thermally Efficient Roofing Systems Workshop, (May 1978).

9. Powell, F., Robinson, H., The Effect of Moisture on the Heat

Transfer Performance of Insulated Flat-Roof Constructions, NBS

Building Science Series (Oct. 1971).

10. Tobiasson, W., Korhonen, C., VandenBerg, A., Hand-Held Infrared

Systems for Detecting Roof Moisture, Symposium on Roofing Technology, NBS/NRCA, (Sept. 1977).

11. Hedlin, C., Orr, H., Tao, S., Thermal Resistances of Experimental

Flat Roof Systems Measured with Heat Flow Meters, ASTM C-16 Thermal

Insulation Conference, (Oct. 1978).

12. Burch, D., Hunt, C., Retrofitting an Existing Wood-Frame Residence

for Energy Conservation - An Experimental Study, NBS Building

Science Series 105 , (July 1978).

13. ASHRAE, 1977 Fundamentals Vol., New York (1977).

14. Peavy, B., Determination and Verification of Thermal Response Factors for Thermal Conduction Applications, NBSIR 77-1405, April 1978.

14
SI CONVERSION CHART

Physical Quantity From To Multiply by

Area ft2 m^ 9.29 (10’2)

Thermal Resistance h»f t^ ‘F/Btu K«m2/W 1.76 (10"1)

Temperature F C tc » (Tf-32)/1.8

Length in m 2.54 (10"2)

Length ft m 3.05 (10*1)

Heat or Energy Flow Btu/h *f t^ W/m2 3.15

Thermal Conductance Btu/h *f t^ »F W/m2 .K 5.68

15
16

Figure

1.

Thermographic

Imaging

System

Figure 3. Roof thermogram Figure 4. Roof thermogram near vents

IH
A

T,

Insidf-Outsidv

Temperature

Difference

{**F)

Thermal

Resistance

Solar

Radiation

d

Haat

Row

at

Inside

Roof

Surface

(Btu/h

ft^)

(h

ft2

•

F/Btu)

(Btu/h

• ft2)

Bapead Maasuremant Time (kaurs)

Figure 6. Thermal resistance measurement of BUR

with concrete deck

system

(w/m^
iloliV

((» «/«lD MOliiVW

20

roni,Mifed

ver

measured

heat

flow

for

BUR

sys

with

i'onoret(

leek
IfcliV

8 « II « «

HMMl ( (« ' «VMN

21

Comj'urpd

heat

flow

and

thermal

resistance

for

HIH

i^vstem

wltli

steel

deck
TME LAG (HOURS)

Figure 9. Required elapsed measurement time as a function of ' BUR system thermal time lag '

)

)

22
NBS*1I4A (REV. 8-7B)

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO. NBCIR 80-2100

4. TITLE AND SUBTITLE

THERIvIAL RESISTANCE OF BUILT-UP ROOFING SYSTEMS

5. Publication Date October 1980

; Code

7. AUTHOR(S)

S. J. Treado

8. Performing Organ. Report No. NBSIR 80-2100

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, DC 20234

11. Contract/Grant No.

12. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS CSfree/. City, st^te, ZIP)

Department of Energy

Washington, D.C. 20585

13. Type of Report & Period Covered Final

15. SUPPLEMENTARY NOTES

1 1 Document describes a computer program; SF-185, FIPS Software Summary, is attached.

16, ABSTRACT (A 200~word or iess factual summary of most significant information. If document includes a significant bibliography or

literature survey, mention it here.)

This report describes factors which affect the thermal performance of built-up

roof systems, and a technique for making in-place measurements of thermal resistance.

This measurement technique utilizes a combination of infrared thermographic imaging,

surface heat-flow meters, and surface thermopiles. The thermal resistance of the

roof system is computed based on temperature differences across the roof and the

measured heat flow through the roof.

A field test of the measurement procedure is detailed, along with an examination of the

time period required to perform a roof thermal resistance measurement, as related to

the thermal time lag for heat flow through the roof due to the effect of the thermal

mass of the roof.

Roof thermal resistance determinations performed according to this measurement pro—

cedure are found to be very accurate, if measurements are performed over

time interval, the minimum interval being dependent upon the thermal mass

system.

a sufficient

of the roof

17, KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name;

separated by semicolons)

Built-up roofs 5 measurement technology; moisture accumulation;

nondestructive tests ; thermal resistance |

18. AVAILABILITY Unlimited 19. SECURITY CLASS

(THIS REPORT)

21. NO. OF

PRINTED PAGES

Oil For Official Distribution. Do Not Release to NTIS unclassified 27

U J Order From Sup. of Doc., U.S. Government Printing Office, Washington, DC

20402, SD Stock No. SN003-003-

20. SECURITY CLASS

(THIS PAGE)

22. Price

py'l Order From National Technical Information Service (NTIS), Springfield,

VA. 22161 UNCLASSIFIED $6 .00

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