Approved for public release by the

Director of the National Institute of

Standards and Technology (NIST)

on October 9, 2015.

NATIONAL BUREAU OF STANDARDS REPORT

7470

THE EFFECT OF INSULATION ON THE DURABILITY OF A

SMOOTH- SURFACED, BUILT-UP ROOF

by

William C. Cullen

and

William H. Appleton

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
THE NATIONAL BUREAU OF STANDARDS

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Not. fo

for re

Celt ion

NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT NBS REPORT

1004-12-10447 30 March 1962 7470

THE EFFECT OF INSULATION ON THE DURABILITY OF A

SMOOTH-SURFACED, BUILT-UP ROOF

by

William C. Cullen

and

William H. Appleton

Organic Building Materials Section

Building Research Division

Sponsored by

Office of the Chief of Engineers

Department of the Air Force

Bureau of Yards and Docks

IMPORTANT NOTICE

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however, by the Government agency for which the Report has been specifically prepared if that agency wishes

to reproduce additional copies for its own use,

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

THE EFFECT OF INSULATION ON THE DURABILITY OF A

SMOOTH- SURFACED, BUILT-UP ROOF

1 . INTRODUCTION

During the past 25 years or more, insulation in many forms has been used to

separate the roof membrane from the roof deck. This has been the accepted

practice for several reasons, but primarily for its thermal insulating

properties which ultimately contribute to the comfort of the building

occupants. As early as 1935, Rogers [1]—/ stated that all roofs should

be insulated and normally insulation should be placed above the roof

deck on a flat roof. He further indicated that above deck insulation,

thoroughly waterproofed at all points, prevents solar heat from reaching

the roof deck so that the mass will not continually radiate heat within

the structure during a protracted period of hot weather. The use of

insulation above deck also reduces the maximum temperature reached by the

deck and this in turn reduces the chances of structural cracking due to

temperature changes in a concrete deck. In addition, insulation is often

used for economical reasons; less power is utilized to cool the interior

during the summer and less fuel is used to heat the structure in the

winter. Further, the use of insulation is mandatory in many cases to

provide a suitable surface for the application of a built-up roof.

There is no doubt that the accomplishment of these ends dictates the

necessity for insulating above a roof deck. However, how does the use

of insulation above a deck affect the durability of a built-up roof?

At the request of agencies of the Department of Defence, as a task under

Project 10447, Performance of Roofings, Tri-Service Engineering Investigations

of Building Construction and Equipment, NBS, a program was conducted to study

roof temperatures and their effect on the durability of a built-up roof.

Previous studies under this project indicated temperature differences

among various roofing systems on various types of decks and deleterious

effects of the higher temperatures on the durability of an insulated,

smooth-surfaced, built-up roof. In the continuation of these studies,

the following experimental work was undertaken.

1/

Figures in brackets refer to list of references at the end of this report.

2 . BACKGROUND

A number of investigations relating to the mechanisms of degradation of

roofing asphalts carried out at the National Bureau of Standards have

shown that a chemical reaction occurs within a bitumen during the

weathering process. Based on extensive laboratory tests, Greenfeld [2]

has established that when asphalts are exposed to light (ultraviolet)

,

heat, and water, numerous and complex changes take place. Other investigators [3,4,5] connected with these studies, describe in detail the

various chemical and photochemical reactions which occur during weathering.

The effect of rise in temperature on the speed of a chemical reaction is

rather astonishing. It is generally accepted that a rise of 10°C (18°F)

may double or even triple the velocity of a chemical reaction [6].

If these statements are valid, the temperatures attained by a built-up

membrane, other factors being similar or equal, will have a definite

effect on the durability of the bitumen and felt comprising the membrane.

In addition to the chemical changes which occur within the bitumen,

another factor must be considered in the degradation of a built-up

roof, i.e-, the thermal stresses produced in the membrane and surfacing

bitumen during the temperature changes which occur. A combination of the

hardening effect on the surfacing asphalt due to oxidation and to the

thermal stresses due to temperature change result in an alligatoring and

cracking of this component and differential movements in the various

components of the roof system. These events may develop to such an extent that water eventually enters the waterproofing membrane. Therefore, two factors should be considered in relating temperature to the

degradation of a built-up roof as follows: (1) the acceleration of the

rate of degradation by increasing the temperature, and (2) the effect of

thermal stresses and movements due to rapid temperature changes.

Little information regarding the temperatures attained by an insulated

built-up roof membrane has been published. In 1957, Ballantyne and

Spencer [7] reported the results of a study of temperatures attained by

bituminous roofs coated with black, white, and aluminum surfaces. Their

results indicated that appreciable (15°F or more) reductions in temperatures

were obtained by treating a black, bituminous surface with a suitable white

reflective coating, and to a lesser extent with an aluminum paint. In

comparing the placement of the insulation above the deck versus that placed

below the deck, Stephenson [8] reported that the maximum surface temperature

and the rates of change of temperature are higher with the insulation

placed between the roof deck and the waterproof layer. Martin [9]

recognized the effects of the high temperatures built up in an insulated

3

roofing membrane and stated that it was essential to provide a reflective

surface in such cases. He further reported the coefficients of solar

reflectivity of different surfaces and the approximate values are listed

in Table 1

.

TABLE 1 . COEFFICIENT OF SOLAR REFLECTIVITY*

Material

Coefficient of Solar

Reflectivity

Asphalt and Bituminous Compounds 0.07

Bituminous Roofing Felt 0.10

Asbestos Cement, old and dirty 0.17

Asbestos Cement, new 0.39

Galvanized Iron, old and dirty 0.10

Galvanized Iron, new 0.35

Slate, blue-grey 0.15

Marble, white 0.56

Sand, fine and white 0.59

Paint, light grey 0.25

Paint, red 0.26

Paint, aluminum 0.46

Paint, light green 0.50

Paint, light cream 0.65

Paint, white 0.75

White wash 0.80

Aluminum Foil, new 0.87

*Coefficient of reflectivity is defined as the numerical ratio of

radiant energy reflected to the radiant energy impinging on a surface.

In a paper on the thermal movements and stresses in concrete slabs in

relation to tropical conditions, Hendry and Page [10] enumerated eight

factors on which the rate of temperature use will depend and discussed

their significance. They are as follows:

1. The intensity of short-wave radiation received from the sun and sky.

2. The rate of increase of outside air temperature.

3. The velocity of wind blowing over the surface.

4. The heat loss back to the atmosphere in the form of long- wave radiation.

5. The surface absorbtivity for short-wave solar radiation.

6 . The surface emissivity for long-wave radiation.

7. The thermal properties of the roof system.

8. The interior thermal conditions.

In discussing roof insulation, they further state that insulation on

the top of the reinforced concrete deck decreases the temperature of

the concrete, but raises the temperature of the built-up membrane, which

may lead to its accelerated deterioration.

3. EXPERIMENTAL RESULTS

In planning the experiments, it was decided to use asphalt-surfaced,

built-up membranes, since a black surface absorbs nearly all the radiant

energy falling upon it and consequently attains higher temperatures when

exposed to solar radiation than most surfaces of other kinds under the same conditions.

Identical specimens of a roof system were constructed using a four-ply,

asphalt, built-up roof membrane on a concrete deck. The membrane was separated from the concrete deck by two inches of cork insulation on one specimen, while the membrane was applied directly to the deck on the

other. A copper-constantan thermocouple was placed directly beneath

and in contact with the roof membrane. The construction details of the

specimens are shown in Figure 1

.

The specimens were exposed at a zero slope on the roof of the Industrial

Building, National Bureau of Standards, and the temperatures attained in

the respective membranes were recorded continuously on a Brown Recorder.

The results obtained during the exposures, expressed as time-temperature

curves as they were recorded on summer days in 1961, are illustrated in

Figures 2 and 3. The illustrated curves were selected from the many

obtained, since they represented the extremes in temperatures and

temperature changes which occurred.

In addition to the above results, the temperature determinations made

on clear, cool nights indicated significant differences varying from

5°F to 15°F below the ambient for the insulated membrane during the summer and winter, respectively, and from 2°F to 5°F below ambient for

the non- insulated specimen. On the other hand, no apparent temperature

difference was observed among the ambient, the insulated specimen, and

the non-insulated specimen when the sky was cloudy and overcast.

4 PLY MEMBRANE

THERMOCOUPLE

2" INSULATION

CONCRETE DECK

INSULATED SPECIMEN

o

t>« 0 o 0 0

0 0 0

o T ° o

Oj 0

n 0

o 0 o o Oo^ —

NGN-INSULATED SPECIMEN

4 PLY MEMBRANE

THERMOCOUPLE

CONCRETE DECK

FIGURE I. CONSTRUCTION DETAILS OF BUILT-UP ROOF SPECIMENS

o

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» do dN31

TIME

OF

DAY

FIG.

2

TIME

-

TEMP

CURVES

OF

INSULATED

AND

NON

-

INSULATED

BUILT-UP

ROOF

SPECIMENS

Jo awj±

FIG.

3

TIMETEMP

CURVES

OF

INSULATED

AND

NONINSULATED

BUILT-UP

ROOF

SPECIMENS

4. DISCUSSION OF RESULTS

A study of the time- temperature curves in Figure 2 shows that not only was

a temperature differential of 42°F (162°F vs, 120°F) observed between the

insulated and the non-insulated specimen, but also that greater temperature

changes occurred in the insulated specimen over short periods of time

due to varying weather conditions (clouds, winds, etc.).

The time-temperature curves presented in Figure 3 represented the conditions

as recorded on a day which included sunshine, clouds, and heavy rains. The curves illustrate the extremes in temperature evidenced by the insulated

specimen as opposed to those observed in the non-insulated specimen. The

morning was generally cloudy with intermittant sunshine and, it should be

noted, that these conditions were reflected to a much greater extent by the

insulated specimen. This specimen reached a maximum temperature of 142°F

at 11:30 A M when a heavy rain occurred The temperature of the insulated

specimen declined 62 degrees to 80°F in a period of only 1-1/2 hours, and

after the rain ceased, the temperature climbed 45 degrees to 125°F in a matter of 30 minutes. In contrast, the temperature of the non-insulated

specimen declined only 13 degrees and rose about 10 degrees in the same period of time.

It was observed that the temperatures of both the insulated and non-insulated

specimens were generally below the ambient on clear, cool nights, with the

insulated specimen showing the greatest declination. In this connection, it

is generally accepted that surfaces which radiate energy at long wave lengths

are coolest at night. However, since identical surfaces were employed in

these experiments, the deviation in the temperatures of the specimens

observed on clear, cool nights was attributed solely to the use of insulation.

The results which were observed in these series of experiments were not

entirely unexpected, since it was felt that a concrete deck would act as

a stabilizing influence to the built-up membrane. It was predicted that

the insulated membrane would not only exhibit the higher maximums and

lower minimums in temperature, but also an increased rate of change since

it is divorced from the concrete deck by an insulating medium. The

insulation, of course, prevents the heat absorbed by solar energy from

being dissipated by the roof deck during the day and, conversely, allows

the energy to be readily emitted to the sky on a clear, cool night.

It is further expected that the temperature differential between an insulated and non-insulated membrane will depend on the density of the

roof deck., For example, this differential will probably be somewhat

less on a wood deck than on a concrete deck.

- 6 -

The data which were obtained in these studies indicated that the insulated

membrane will not only degrade chemically more rapidly due to the higher

temperatures attained, but will also deteriorate physically at a faster

rate. The greater temperature changes in short periods of time result

in large thermal stresses being set up. In the laboratory specimens, the

differential weathering of the surfacing asphalt on an insulated membrane

versus that protecting a non- insulated membrane was readily apparent as illustrated in Figure 4. The specimens were photographed after 2 years

exposure side by side.

The performances of insulated and non-insulated built-up roofs have also

been observed during a number of field surveys carried out under this

project. Generally, the non-insulated roofs gave the more trouble-free

performance and longer durability. For example, Figure 5 illustrates

the weathering of an insulated section (left) and an uninsulated section

(right) of an aluminum painted, asphalt-surfaced, asbestos felt, built-up

roof exposed on Guam Island after about 2 years. It was readily apparent

that the insulated section had undergone the greater degree of weathering

as evidenced by the alligafcoring, cracking, and general deterioration.

Temperature and rate of change of temperature will, no doubt, have an affect on other changes which occur in built-up roofs such as blistering,

wrinkling or buckling, cracking, membrane ruptures, etc. Each of these

conditions is caused or accelerated by thermal movements or stresses and

changing air and/or vapor pressure which are, of course, directly related

to temperature and temperature changes.

5. CONCLUSIONS

The following conclusions may be drawn based on this study;

1. Smooth surfaced built-up roofs insulated from the roof

deck attained appreciably higher temperatures under solar

radiation than their non-insulated counterparts.

2. The average temperature of an insulated membrane was re- corded at about 15°F below the ambient and about 10°F

below the temperature of its non-insulated counterpart

when exposed to a clear, cold, night sky.

3. The insulated specimen registered the greater fluctuations

in temperature when exposed to varying weather conditions

when compared to the non-insulated specimen.

4. Evidence of greater deterioration of the bitumen protecting

the insulated membranes was observed both in laboratory

experiments and in field exposures.

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FIGURE

5.

DIFFERENTIAL

WEATHERING

OF

AN

INSULATED

VERSUS

NON

-INSULATED

SECTION

OF

A

BUILT-UP

ROOF.

7 5.

The greater deterioration of the surface protecting the

insulated membranes apparently resulted from both the

higher sustained temperatures and the thermal stresses

produced by rapid temperature changes.

6

.

RECOMMENDATIONS

Because of the wide variety of roofing systems, insulations and surface

treatments available for use in the construction and maintenance of

built-up roofs, it is recommended that additional studies be conducted

to gather information about how these factors influence the performance

of a roof over both insulated and non-insulated decks.

7

.

NOTE

This paper deals with the effect of temperature on the performance and

degradation of a built-up membrane and not with the performance of the

entire roof system; therefore, it should not be construed to imply that

insulation should, in every case, be eliminated between the roof deck

and the membrane. This is especially true where there is a risk of

failure in some other component of the roof, such as the deck, due to

thermal stresses and movements.

8

8 . REFERENCES

[ l] Rogers, T. S., Roofing Materials, American Architect Reference Data,

No. 16, February 1935.

[ 2] Greenfeld, S. H,, Chemical Changes Occurring During the Weathering

of Two Coating Grade Asphalts, J. Research, NBS, 64C, 287-297,

October-December 1960.

[ 3] Schreinsheim, Alan and S. H. Greenfeld, Water Soluble Degradation

Products of Asphalt, ASTM Bulletin 220, 43 (1956).

[ 4] Kleinschmidt , L. R. and H, R. Snoke, Effect of Light and Water on

the Degradation of Asphalt, J. Research, NBS, 63C, 31 (1959).

[ 5] Beitchman, B. D., Infrared Spectra of Asphalts, J. Research, NBS,

63A, October 1959.

[ 6] MacDougall, F. H., Physical Chemistry, The Macmillan Company, 1936.

[ 7] Ballantyne, E. R. and J. W. Spencer, Temperatures of Bituminous

Roof Surfaces, ASTM Bulletin 69-73, July 1957.

[ 8] Stephenson, D. G., Contribution to discussion of reference 7.

[ 9] Martin, K. G., Thermal Insulation of Flat Roofs, Building Materials

Magazine, Vol. 2, No. 5, May- June 1961.

[10] Hendry, A. W. 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,

r.i.l.e.m. 1960. Haifa, Israel.

USCOMM-NBS-DC

II. S. DEPARTMENT OF COMMERCE

Luther II. Hodges, Secretary

NATIONAL BUREAU OF STANDARDS

A. V. Antin, Director

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