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231

SOLAR HEATING, RADIATIVE COOLING,

AND THERMAL MOVEMENT -

THEIR EFFECTS ON BUILT-UP ROOFING

WILLIAM C. CULLEN

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
U. S. DEPARTMENT OF COMMERCE

Luther H. Hodges, Secretary

NATIONAL BUREAU OF STANDARDS

A. V. Astin, Director

THE NATIONAL BUREAU OF STANDARDS

Functions and Activities

The functions of the National Bureau of Standards include the development and maintenance of the national standards of measurement and the provision of means and methods for making measure - ments consistent with these standards; the determination of physical constants and properties of materials; the development of methods and instruments for testing materials, devices, and structures; advisory services to government agencies on scientific and technical problems; invention

and development of devices to serve special needs of the Government; and the development of standard practices, codes, and specifications, including assistance to industry, business and con- sumers in the development and acceptance of commercial standards and simplified trade practice

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of the back cover.

Publications

The results of the Bureau's research are published either in the Bureau's own series of publications or in the journals of professional and scientific societies. The Bureau itself publishes

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A complete listing of the Bureau's publications can be found in National Bureau of Standards

Circular 460, Publications of the National Bureau of Standards, 1901 to June 1947 ($1.25), and the Supplement to National Bureau of Standards Circular 460, July 1947 to June 1957 ($1.50), and Miscellaneous Publication 240, July 1957 to June 1960 (includes Titles of Papers Published in Outside Journals 1950 to 1959) ($2.25); available from the Superintendent of Documents, Government

Printing Office, Washington, D.C., 20402.
NATIONAL BUREAU OF STANDARDS

*€ecUnical 92ote 231

ISSUED DECEMBER 16, 1963

SOLAR HEATING, RADIATIVE COOLING,

AND THERMAL MOVEMENT - THEIR EFFECTS ON BUILT-UP ROOFING

William C. Cullen

NBS Technical Notes are designed to supplement the Bureau's regular publications program. They provide a means for making available scientific data that are of transient or limited interest. Technical Notes may be

listed or referred to in the open literature.

Reprinted with Errata - April 1964

For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C., 20402 - Price 25 cents
ERRATA

On page Aj line 11 now reads "purposes, solar radiation is

confined to the ratiation spectrum between"

Should read: "purposes, solar radiation is -confined to the

radiation spectrum between".

On page 7 , Table 1, under Weight Used now reads lbs/ft ,

Should read: lbs/100 ft 2

.

On page 7 , last line of Table 1 now reads Polyvinyl Fluroide

Film.

Should read: Polyvinyl Fluoride Film.

On page 14, 1st line now reads "Duplicate specimens, 12-in.

by 12-in.

,

were cut from the samples so"

Should read: "Duplicate specimens, 12-in. by 2 -in.

,

were cut

from the samples so".

On pages 26 and 27, the legends for Figures 7 and 8 should be

transposed.

On pages 30, 31, 32, and 33, for Figures 11 through 14,

+40 X 10~ 3 in the scale should read +40 X 10-5.

CONTENTS

Page

1. Introduction

2. Solar Heating and Radiative Cooling

3. Thermal Movement of Components of a Roof System ... 12

4. Summary and Discussion

1 9

5. References

li
SOLAR HEATING, RADIATIVE COOLING, AND THERMAL MOVEMENT

THEIR EFFECTS ON BUILT-UP ROOFING

William C. Cullen

Twenty different built-up roof construction specimens,

covered with five surfacing materials, were subjected to

natural solar heating and nighttime cooling. The temperatures and temperature changes observed during winter and summer exposures are discussed. The data indicate that the

temperature attained in a roof membrane is influenced by

the absorptance and emissivity of the surface as well as the thermal and physical properties of the substrate to

which the roofing is applied. The data show that roofings

placed over insulation may be heated to a temperature of

80°F. above ambient due to solar heating and may be subcooled as much as 20°F. below the ambient due to radiative

cooling. The thermal movements which occur in the components

of a roof system due to temperature change are discussed in

relation to built-up roof performance and failures. Thermal

expansion data are presented for some composite bituminous

membranes. The data show that these membranes undergo

greater thermal movements than most other components of a roof system and the rate of expansion is not linear but

decreases as the temperature is increased.

1 . INTRODUCTION

A major percentage of flat roof construction in the United States

consists of built-up roof systems. The built-up system may be a relatively simple system as a deck covered by roofing or it may consist of a

number of components such as: a roof deck, a vapor barrier membrane, a

thermal insulation, a waterproofing membrane and a protecting surface.

These components may involve many materials which exhibit many properties

that contribute to the performance of a roof system. As the complexity

of a roof system increases, the problems involving performance also increase and the effects of outdoor exposure on the system must be known.

For example, in recent years serious splitting and wrinkle cracking

1
failures have occurred in built-up roof membranes. These failures

appeared most frequently on membranes applied over insulation and after

periods of cold weather. Because of such failures questions have been

raised regarding the temperature cycles of roof systems and the expansion

and contraction of built-up membranes due to temperature change. Therefore, a program was conducted to study the effects of natural solar heating and radiative cooling on the temperature and thermal movements of

built-up roof systems.

Experience in both the laboratory and in the field has indicated

that high temperatures attained by a roof surface, rapid changes in temperature, and thermal movements resulting from solar heating and radiative

cooling are contributing factors in such other common failures in builtup roof systems as: blistering, buckling, membrane slippages, flashing

failures, chemical degradation of bitumen, and physical deterioration of

bitumen.

The purpose of this paper is to present temperature data on several

built-up roof systems when exposed to natural solar heating and radiative

cooling during summer and winter seasons in Washington, D. C, and to

discuss these data in terms of roof performance. Another purpose is to

furnish data on thermal expansive and contractive characteristics of a

composite built-up membrane and to discuss these in relation to the

performance of the roof membrane.
2. SOLAR HEATING AND RADIATIVE COOLING

The greatest known source of energy for the earth is the sun and

this energy is transmitted to the earth in wave form as infrared, ultraviolet and visible radiation. Roofs, per se, are exposed to solar radiation and the energy accompanying it. The law of conservation of energy

demands that the radiant energy impinging on a roof surface must be

quantitatively accounted for by three factors: reflection, transmission,

and absorption. For the purpose of this paper, prime consideration will

be given to the radiant energy which is absorbed and converted to heat.

It may be of interest to note that Billington [1]~~ reported that the

intensity of solar radiation (amount of energy falling on a plane surface perpendicular to the rays) outside the earth's atmosphere is about

420 BTU/sq. ft. (hr) . A good deal of this energy is absorbed and

scattered by the atmosphere and the maximum recorded at the earth's surface is about 320 BTU/sq. ft. (hr) . The effect of solar radiation on

building components was pointed out by Beckett [2] in 1935 when he

warned that there is a very real danger of structural failure due to

expansive movements resulting from heating by sunshine.

The effect of solar heating also contributes to the degradation of

the waterproofing bitumen protecting a roof membrane since this process

is essentially a photooxidative reaction which is accelerated by heat.

1/

Figures in brackets refer to literature references at the end of this

paper.
When a roof is exposed to solar radiation the heat absorbed causes

its surface temperature to rise above the ambient air temperature. As

the roof continues to absorb heat, it begins to lose heat by radiation

and convection to the surroundings. The temperature finally attained by

the roofing is a result of the balance of heat absorbed and the heat lost

by radiation and convection. Important material properties that influence

surface temperature are the solar absorption coefficient and the emissivity of the surface. Therefore, to characterize a surface during solar

heating, it is necessary to know its solar absorptance and its emissivity

over the spectral range represented by solar radiation. For practical

purposes, solar radiation is confined to the ratiation spectrum between

0.3 and 2.5 microns with the peak intensity at about 0.5 microns.

Another factor, not as well known as solar heating, but equally as

important when roof temperatures are considered, is radiative cooling

which, in effect, is the reradiation of energy from the roof surface to

the sky. The emission of radiation is not confined to luminous bodies as

generally believed. The radiation from a material at moderate temperatures is predominately in the form of long-wave infrared radiation which

cannot be seen, but which can sometimes be detected by its cooling effect

on the material. This outgoing radiation can have a significant influence on roof surface temperature. For example, on clear nights when the

roof radiates to the sky, the temperature of a roof surface can go below

the ambient air temperature.
Solar heating and radiative cooling may contribute substantially

to the magnitude of the thermal cycle of a built-up roof system.

Assuming a clear day, the roof surface temperature begins to rise under

exposure to the morning sun and it reaches its maximum about noon and

then decreases as the sun declines until night falls. If the night is

clear and still, the heat loss from the roof surface to the sky will

result in a sub-cooling (cooling below ambient) of the roof surface.

The magnitude and rate of the temperature increase or decrease will

depend on the following factors: intensity of solar radiation (direct

and diffuse), the absorptance of the surface for solar radiation, rate

at which surface emits radiation to sky and surroundings, unit convectlve conductance for heat transfer between air and roof surface including the effects of wind velocity, thermal properties of the roof system,

and ambient air temperature. The effects of wind velocity have been

discussed by Hendry and Page [4]. Their computations indicated that

the maximum temperature of a flat, dark, 6-inch concrete roof slab on

a still day would be 30°F higher than on a day with a 12 mph wind and

16°F higher than on a day with a 4 mph wind when exposed at a latitude

of 20°N.

2.1 Experimental

Ten, 18- by 18-in., built-up roof constructions were prepared on

2-in. thick concrete decks and ten similar constructions were prepared on

3/4- in. exterior plywood decks. A four-ply asphalt - asphalt-saturated

organic felt membrane was applied directly to the deck on five of the

concrete and on five of the plywood constructions. On the remaining five
of each type deck, two inches (two one-inch layers) of expanded polyurethane was used to separate the four-ply membrane from the respective

deck. Each construction contained copper-constantan thermocouples at

selected locations for recording temperatures continuously 24 hours a

day. Figure 1 is a schematic diagram of typical specimens.

The specimens were exposed on the roof of the Industrial Building,

National Bureau of Standards, Washington, D. C, in a horizontal position

on a platform about 3 feet above the roof surface. Each specimen was

supported on face bricks about 3 inches above the platform. This arrangement provided for the free circulation of air beneath a large portion of

each experimental slab. Figure 2 shows a general view of the outdoor

exposures.

2.1.1 Substrates

The materials which were used as substrates for the built-up membrane

were dense concrete, 3/4-inch plywood and expanded polyurethane insulation. These materials had measured densities of 140-, 40-, and 2-pounds

per cubic foot, respectively, as determined in the laboratory. These

densities were selected to represent the range of those of roof membrane

substrates used in service.

2.1.2 Protecting Surfaces

Five surfaces representing those commonly used on built-up roofs

were used in the experimental work. Table 1 lists the materials used

together with their pertinent properties.
TABLE 1.

SOME PROPERTIES OF SURFACINGS FOR BUILT-UP MEMBRANES

Material Color Weight Used

Estimated

Solar Absorptance

lbs/ft'

Asphalt Black 25

Stone (crushed) Gray 400

Scoria Maroon 150

Marble (crushed) White 200

Polyvinyl

Fluroide Film White .002" th

Coefficient'*

0.9

0.7

0.7

0.5

0.2

"Estimated from solar absorptances of similar materials reported in:

Thermal Radiation Properties Survey, 2nd Edition, p. 245, MinneapolisHoneywell Regulator Company (1960).

2.2 Experimental Results

2.2.1 Solar Heating

The influence of the substrate on the temperature and temperature

changes produced by the solar heating of a black, smooth-surfaced roof

membrane is shown in the time-temperature curves in Figure 3, The

maximum temperatures attained by the black asphalt-surfaced membranes

over concrete, wood and insulation on a summer day in August 1962 were

120°F, 145°F, and 165°F, respectively.

In respect to temperature fluctuations, the temperatures of the membranes over concrete, wood and insulation declined 15°F., 30°F., and 45°F.,

respectively, between the hours of 5:30 p.m. (1730) and 6:00 p.m. (1800),

due to a change in atmospheric conditions.

One might infer from this information that all insulated roof membranes are subjected to greater extremes in both temperatures and
temperature variations. This is apparently true if other factors are

similar. However, other variables come into play which limit these extremes. For example, Figure 4 shows the effect of surface color (solar

absorptance) on the thermal cycle of an asphalt surfaced and a white

polyvinyl fluoride smooth-surfaced built-up roof membrane placed over two

inches of polyurethane insulation. A comparison of curve 1 with curve 2

indicates a decline of 40°F. for the black and only 15°F. for the white

between noon (1200) and 1:00 p.m. (1300) when the sun was obscured by

clouds and then as the surfaces were again exposed to sunshine, the blacksurfaced membrane registered a gain of 90°F., while the white-surfaced

membrane was limited to a 25°F. rise between 1:00 p.m. (1300) and 3:00

p.m. (1500).

The influence of such other surfaces as crushed stone and marble

chips on the thermal cycles of membranes placed over insulation and over

concrete are illustrated in Figure 5. These results show that the lower

the solar absorptance of a roof surface, the smaller the temperature

difference between the insulated and non- insulated membranes, and also a

lesser total rise in temperature.

Figure 6 illustrates the effect of the protecting surface on the

temperature and temperature changes of membranes placed over insulation

on a sunny June day while Figure 7 gives time- temperature curves for

identical systems on a sunny January day with the ambient temperature

reaching a maximum of 30 o F. Although these curves represent specimens

placed over insulated concrete decks, similar curves were obtained on the

specimens placed over insulated wood decks. These observations
indicate that the roof temperature is not related to the type or kind of

deck when an efficient insulation is used. The time- temperature curves

for the scoria surfaced membranes approximated those of the crushed stone

and therefore have been eliminated from both the figures and following

tables.

The maximum temperatures which were recorded in membranes protected

with the selected surfaces during summer and winter exposures are listed

in tables 2 and 3, respectively.

TABLE 2

EFFECT OF SURFACE TREATMENT ON ROOF TEMPERATURE DURING SUMMER EXPOSURE

Surface

Asphalt (Black)

Crushed Stone

Marble Chips

Polyvinyl Fluoride (white)

Ambient

Temp arature Rise

Max „ Tempfr om Ambient

°F °F

166 81

148 63

118 33

93 8

85 --

TABLE 3.

EFFECT OF SURFACE TREATMENT ON ROOF TEMPERATURE DURING WINTER EXPOSURE

Temperature Rise

Surface Max, Temp„ From Ambient

°F °F

Asphalt (Black) • 75 45

Crushed Stone 55 25

Marble Chips 35 5 Polyvinyl Fluoride (White) 30

Ambient 30
It has been reported by Cullen and Appleton [5] that insulation

placed between the deck and the membrane may lead to the accelerated

deterioration of the built-up roof. On the other hand, if insulation is

omitted from the top of the deck or placed beneath the deck, the relative

thermal movement in the roof deck may be considerable due to solar heating

and radiative cooling. The question, therefore, arises as to the magnitude of the temperature changes which occur between a structural deck

that is separated from the membrane by insulation and one that is not.

Figure 8 shows the extent of the temperature changes by the time-temperature curve for both the built-up membrane and the concrete deck as recorded on both insulated and uninsulated specimens on a summer day during

August 1962. Incident ly, Curve 2 coincides with the ambient temperature.

The temperature fluctuations and temperature rise in the uninsulated

concrete deck were considerably less than those which occurred in a

built-up membrane placed over insulation.

A snow cover was an important factor in stabilizing membrane temperatures of the roof constructions used in the experimental work. During winter exposures in December 1962 and January 1963 when the specimens

were covered with snow, little variation was recorded between the ambient

temperature and that of the roof membrane either during daytime or nighttime exposures regardless of the type of substrate and surface treatment

employed. However, when there was an absence of snow the membrane temperature showed considerable variation both above and below the ambient

temperature due to solar heating and radiative cooling as shown in

Figure 7.

10
2.2.2 Radiative Cooling

The reradiation of energy from a roof surface to a clear night sky

often results in roof membrane temperatures less than ambient. The

degree of sub-cooling depends on a number of factors including the

properties of the substrate, and atmospheric conditions. Figure 9

serves to illustrate these points. The ambient temperature (Curve 1)

and that of uninsulated membranes protected with a black and white surface (Curves 2 & 3) and the membranes with similar surfaces insulated

from the deck (Curves 4 & 5) coincided with one another on a night when

the sky was cloudy until 1 :00 a.m. (0100 hours) at which time the cloud

cover lifted. This event was soon apparent by comparing the ambient

temperature with that of membranes insulated and not-insulated from the

concrete decks. The temperature of the membrane placed on insulation

was about 12°F cooler than those placed directly on the concrete deck

and about 20°F cooler than the ambient. In each case, the white

surfaced membranes (Curves 3 & 5) were slightly cooler than their black

surfaced counterparts.

The emissivity of most non-metallic roofing materials, unlike their

absorptance, are for all practical purposes independent of surface color

or texture [2]. A case in point is illustrated in Figure 10 where the

marked effect of solar heating on the black surfaced membrane as opposed

to the white was clearly indicated at 1:00 p.m. (1300), but at 7:00 p.m.

(1900) the temperatures of both the black and the white surfaced membranes were about the same.

11
3. THERMAL MOVEMENT OF COMPONENTS OF A ROOF SYSTEM

It is well known that with few exceptions the dimensions of

materials increase as the temperature of the material increases and

decrease as the temperature decreases.

Many of the common built-up roof failures which occur in the field

can be directly or indirectly traced to thermal movements within a

specific component of the roof system or by the differential movement

among the various components and, therefore, it is desirable to know

the extent of the movements.

The coefficients of linear expansion for many materials used in

the roof system has been determined and are reported in the literature.

Values for some materials of interest are given in Table 4. The

temperatures at which the determinations were made were not reported.

However, no published information was found for the thermal expansion coefficient of a composite membrane consisting of alternate layers

of bitumen and felt. Therefore, a program was undertaken to measure

this property of a number of composite membranes.

3.1 Experimental

The samples consisted of 12-in. by 12-in. sections of built-up

membranes constructed in the laboratory with alternate layers of felt and

hot bitumen in accordance with good roofing practice. Approximately 20

pounds/ 100 sq. ft. of bitumen were used between the plies and no surfacing was applied to the specimens. Table 5 identifies the membranes.

12
TABLE 4.

LINEAR EXPANSION OF MATERIALS USED AS COMPONENTS OF A ROOF SYSTEM

Material Coefficient of Linear Expansion

in.

in. °F

Roof Decks

Steel 6.7 x ]0~ 6 ^

Concrete 4.0 to 6.0 x 10 -' Wood 1.7 to 2.5 x 10" 6 £' (along grain)

Plywood 3.0 to 4.2 x 10" 6 $/

Gypsum 8.1 x 10~ 6 £/

Insulations

Foam Glass 4.6 x 10"*? -^ Polystyrene 35 x 10"^ -' Polyurethane 30 x 10~ 6 ^

Flashing Metals

Aluminum 13 x 10~ 6 S/

Copper 9.8 x 10"£ -'. Gal. Iron 6.7 x 10 -' Lead 15 x 10

•6 a/

-' N. A. Lange, Handbook of Chemistry, 6th Edition, 772 (1946). — L. S. Wells, W. C. Clark, E. S. Newman and D. L. Bishop, Building

materials and structures Report No. 121, National Bureau of

Standards, 34 (1951).

-' Wood Handbook No. 72, Department of Agriculture, 47-48 (1955).

AIA File No. 37-B (1963).

i' AIA File No. 37-0-1 (1963). - AIA File No. 37-D-1 (1963).

13
TABLE 5.

MEMBRANES FOR EXPANSION MEASUREMENTS

Sample No. No. of Plies Description

1 4 Asphalt - 15-lb asphalt sat.

organic felt

2 4 Asphalt - 6- lb asphalt impregnated glass mat (felt)

3 4 Coal-tar pitch - 15-lb coal-tar

saturated organic felt

4 4 Asphalt - 15-lb asphalt saturated

asbestos felt

Duplicate specimens, 12-in. by 12-in., were cut from the samples so

that the long dimension coincided with the machine direction of the felt.

Brass reference plugs, 1/4- in. in diameter, were inserted and sealed with

an epoxy adhesive into a hole drilled into the specimens near each end.

The plugs were spaced about 10 in. apart when measured at 70°F. The specimens were placed unrestrained on a flat surface dusted with talc to prevent sticking in the conditioning chamber. A copper-constantan thermocouple was inserted into a control specimen and the temperature of the

chamber was reduced to -60°F. The temperature in the chamber was gradually raised to 140°F in increments of about 10°F. and after the specimens

reached equilibrium at each increment the distance between the reference

plugs in each specimen was measured to the nearest ten thousandth of an

inch with a Whittemore Strain Gauge. The experiments were performed on

duplicate specimens of each built-up membrane three times, over the temperature cycle of -60°F. to 140°F. and back down to -60°F. The measurements over a complete temperature cycle required approximately 8 hours.
3.2 Results

The averaged results of the thermal expansion measurements are

reported in Table 6. The apparent expansion coefficient of each membrane over the temperature range indicated is expressed in inches per

inch per °F.

TABLE 6.

APPARENT EXPANSION COEFFICIENTS FOR BUILT-UP MEMBRANES

Sample Spec. Apparent Expansion Coefficient

No. Description No. in/in/°F

-60° to 0°F 0°F to 70°F 70°F to 140°F

-6

10"

-6

-6

3 x 10-6

3 x 10

-<S 3 x 10 -6

3 x 10

a/

a/ -6

2 x 10

° -6

2 x 10

Asphalt-Organic 1 18 x 10 , 5 x 10 , f

6

2 Asphalt-Glass 1 27 x 10 " 6 x 10

2 31 x 10"£ 6 x 10

3 Coal-tar-Organic 1 15 x

10~ 12 x

10~

2 14 x 10"g 13 x 10"^

4 Asphalt-Asbestos 1 11 x 10~ 5 x

10~

2 10 x 10" 6 x 10 _

Decrease in length observed.

The length-temperature curves for a typical specimen of each respective membrane over the complete thermal cycle are shown in Figures

11, 12, 13, and 14. The actual data points which were obtained at

10°F intervals are not shown. Data points did not depart from the

curves shown by more than ±5%.

15
4. SUMMARY AND DISCUSSION

The data which were obtained in this study indicated that:

1. Built-up membranes are heated by natural solar radiation.

The temperature increase depends on: a) the mass, density

and thermal properties of the substrate; b) thermal radiation properties of the exposed surface; c) atmospheric

conditions; and d) presence or absence of a snow cover.

2. Built-up roof membranes may be sub-cooled by night sky

radiation. The magnitude of sub-cooling is dependent on:

a) the mass, density and thermal properties of the substrate; b) the emissivity of the exposed surface; c) atmospheric conditions; and d) the presence or absence of a snow

cover.

3. Built-up roof membranes apparently undergo greater thermal

expansive and contractive movements than most other components of a roof system at low temperatures.

4. Thermal movement of a built-up bituminous membrane is not

linear with temperature and decreases markedly as the

temperature is increased.

The data for thermal movements of composite bituminous membranes

showed that the movements caused by temperature change are most critical

in the low temperature ranges as compared to the minimal movements observed in the higher temperature ranges. This behavior is caused, at

least in part, by the visco-elastic behavior of roofing bitumens. It is

apparent from data presented in Figures 11, 12, 13, and 14 that there was

little or no positive movement of the membranes at the higher temperatures

which suggest that plastic flow was occurring.

Field observations regarding splitting and membrane shrinkage indicated that thermal movement is a factor in many such failures. Solar

heating and radiative cooling can cause such failures; therefore, they

must be considered. Nighttime sub-cooling of a membrane appears significant, especially if the membrane is applied over a low density substrate.

16
In addition, the observations of the stabilizing effects of a snow cover

are in agreement with reports that the greater number of complaints involving roof splitting failures and membrane shrinkage are received

after extended periods of cold weather with no snow cover.

In comparing field experience to the data obtained in the laboratory,

it is apparent that other common failures of built-up roof systems observed in the field result directly or indirectly from solar heating,

radiative cooling and the resulting thermal movement. For example,

flashing failures at the junction of a bituminous membrane with metal

base flashings, gravel stops and other metal appurtenances may be caused

or accelerated by both differences in emissivity and by their different

apparent coefficients of linear expansion.

Temperature and temperature changes are factors to be considered in

the wrinkle cracking or buckling of roofing felts and in the formation

and subsequent growth of blisters beneath or between the plies of the

membrane especially if moisture is present in some component of the roof

system. Solar heating appears to be a most important factor when a slippage of roof membrane occurs.

In connection with the weathering of the roofing bitumen, it has

been demonstrated that the chemical degradation of asphalt is accelerated

by increased temperatures. In addition, experience has indicated that

the thermal shock due to rapid temperature changes may be responsible for

accelerating the physical deterioration of the bitumen.

17
Solar heating, radiative cooling, and thermal movement may also

result in insulation problems such as breakage, cupping or curling,

if the following conditions are present: insulation has a high coefficient of thermal expansion, adhesion of insulation to roof deck is

poor, and large thermal gradient exists within the insulation.

Thermal movement at low temperature may be a factor in relating

certain roofing failure to thermal shock properties of materials in

the roof system. McCawley L6J has used the term "thermal shock" to

describe a condition when a roof is subjected to a drop of 100°F or

more in a few hours. According to Marin [7] in discussing the mechanical behavior of engineering materials, the best materials for thermal

shock resistance were found to have good thermal conductance, low coefficient of exvansion, low modulus of elasticity and high tensile strength,

He equates the thermal shock resistance, R, to the above variables as

follows:

R = - ^ where:

E o^

k = thermal conductivity

S = tensile strength

E = Modulus of elasticity

oC = Expansion coefficient

Although the pertinent properties of built-up roof membranes,

other than thermal movement^ were not demonstrated experimentally in

this program, it is believed that the thermal shock concept may be

useful in predicting the performance of built-up roof membranes especially at low temperatures.

18
5. REFERENCES

[1] N. S. Billington, J. Inst, of Heat and Vent. Eng. 1_9, 112 (1951).

[2] H. E. Beckett, J. Inst, of Heat and Vent. Eng. 3, 79 (1935-6).

[3] W. H. McAdams, Heat Transmission (McGraw-Hill Book Company), 62

(1954).

[4] A. W. Hendry and J. K. Page, "Thermal Movements and Stresses in

Concrete Slabs in Relation to Tropical Conditions", International

Symposium on Concrete and Reinforced Concrete in Hot Countries",

Haifa, Israel (RILEM, 12 rue Brancion, Paris XV°, France) (1960).

[5] W. C. Cullen and W. H. Appleton, Am. Roofer and Building Improvement Contractor _53, No. 3, 19 (1963).

[6] James McCawley, Am. Roofer and Building Improvement Contractor 52,

No. 10, 11 (1962).

[7] J. Marin, Mechanical Behavior of Engineering Materials (PrenticeHall, Inc.), 338 (1962).

The author acknowledges with thanks the assistance of William H.

Appleton who constructed and instrumented the built-up roof specimens

and prepared the specimens for and made the measurements of the co- efficients of thermal expansion of the roof membranes. The author also

acknowledges with appreciation the advice and encouragement given to him

by W. W. Walton, Chief of the Organic Building Materials Section,

National Bureau of Standards, and the members of the Tri-Service Committee,

This investigation was conducted under the Tri-Service Engineering

Investigations of Building Construction and Equipment, National Bureau

of Standards, sponsored by the Directorate of Engineering, U. S. Air

Force, Bureau of Yards and Docks, U. S. Navy, and the Office of the

Chief of Engineers, U. S. Army.

19
SURFACING (MINERAL OR SMOOTH)

4 PLY BUILT-UP MEMBRANE.

2 ONE INCH LAYERS EXPANDED

POLYURETHANE INSULATION.

DECK (2" CONCRETE OR 3/4" PLYWOOD),

I8"XI8"

COPPER-CONSTANTAN

THERMOCOUPLES.

SURFACING (MINERAL OR SMOOTH)

-—4 PLY BUILT-UP MEMBRANE

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'IGURE U SCHEMATIC DIAGRAM OF TYPICAL BUILT-UP ROOF SPECIMENS EXPOSED

OUTDOORS

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<r U.S. GOVERNMENT PRINTrNG OFFICE : 1964

O - 728-563

THE NATIONAL BUREAU OF STANDARDS

The scope of activities of the National Bureau of Standards at its major laboratories in Washington, D.C.,and

Boulder, Colorado, is suggested in the following listing of the divisions and sections engaged in technical work.

In general, each section carries out specialized research, development, and engineeringin the field indicated by

its title. A brief description of the activities, and of the resultant publications, appears on the inside of the

front cover.

WASHINGTON, D. C.

Electricity. Resistance and Reactance. Electrochemistry. Electrical Instruments. Magnetic Measurements.

Dielectrics. High Voltage. Absolute Electrical Measurements.

Metrology. Photometry and Colorimetry. Refractometry. Photographic Research. Length. Engineering Metrology.

Mass and Volume.

Heat. Temperature Physics. Heat Measurements. Cryogenic Physics. Equation of State. Statistical Physics.

Radiation Physics. X-ray. Radioactivity. Radiation Theory. High Energy Radiation. Radiological Equipment.

Nucleonic Instrumentation. Neutron Physics.

Analytical and Inorganic Chemistry. Pure Substances. Spectrochemistry. Solution Chemistry. Standard Refer- ence Materials. Applied Analytical Research. Crystal Chemistry.

Mechanics. Sound. Pressure and Vacuum. Fluid Mechanics. Engineering Mechanics. Rheology. Combustion

Controls. Polymers. Macromolecules: Synthesis and Structure. Polymer Chemistry. Polymer Physics. Polymer Characterization. Polymer Evaluation and Testing. Applied Polymer Standards and Research. Dental Research.

Metallurgy. Engineering Metallurgy. Metal Reactions. Metal Physics. Electrolysis and Metal Deposition.

Inorganic Solids. Engineering Ceramics. Glass. Solid State Chemistry. Crystal Growth. Physical Properties.

Crystallography.

Building Research. Structural Engineering. Fire Research. Mechanical Systems. Organic Building Materials. Codes and Safety Standards. Heat Transfer. Inorganic Building Materials. Metallic Building Materials. Applied Mathematics. Numerical Analysis. Computation. Statistical Engineering. Mathematical Physics. Operations Research. Data Processing Systems. Components and Techniques. Computer Technology. Measurements Automation.

Engineering Applications. Systems Analysis.

Atomic Physics. Spectroscopy. Infrared Spectroscopy. Far Ultraviolet Physics. Solid State Physics. Electron Physics. Atomic Physics. Plasma Spectroscopy.

Instrumentation. Engineering Electronics. Electron Devices. Electronic Instrumentation. Mechanical Instru- ments. Basic Instrumentation. Physical Chemistry. Thermochemistry. Surface Chemistry. Organic Chemistry. Molecular Spectroscopy. Ele- mentary Processes. Mass Spectrometry. Photochemistry and Radiation Chemistry.

Office of Weights and Measures. BOULDEK, COLO.

CRYOGENIC ENGINEERING LABORATORY

Cryogenic Processes. Cryogenic Properties of Solids. Cryogenic Technical Services. Properties of Cryogenic

Fluids. CENTRAL RADIO PROPAGATION LABORATORY

Ionosphere Research and Propagation. Low Frequency and Very Low Frequency Research. Ionosphere Research. Prediction Services. Sun- Earth Relationships. Field Engineering. Radio Warning Services. Vertical Soundings Research. Troposphere and Space Telecommunications. Data Reduction Instrumentation. Radio Noise. Tropospheric Measurements. Tropospheric Analysis. Spectrum Utilization Research. Radio-Meteorology. Lower Atmosphere

Physics.

Radio Systems. Applied Electromagnetic Theory. High Frequency and Very High Frequency Research. Fre- quency Utilization. Modulation Research. Antenna Research. Radiodetermination.

Upper Atmosphere and Space Physics. Upper Atmosphere and Plasma Physics. High Latitude Ionosphere

Physics. Ionosphere and Exosphere Scatter. Airglow and Aurora. Ionospheric Radio Astronomy.

RADIO STANDARDS LABORATORY

Radio Standards Physics. Frequency and Time Disseminations. Radio and Microwave Materials. Atomic Fre- quency and Time-Interval Standards. Radio Plasma. Microwave Physics.

Radio Standards Engineering. High Frequency Electrical Standards. High Frequency Calibration Services. High

Frequency Impedance Standards. Microwave Calibration Services. Microwave Circuit Standards. Low Frequency

Calibration Services.

Joint Institute for Laboratory Astrophysics- NBS Group (Univ. of Colo.).
NBS
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