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BUILDING SCIENCE SERIES 37

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U.S.

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National

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The Effect of Moisture on

the Heat Transfer

Performance of Insulated

Flat-Roof Constructions
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EC 2 1971 ' t^e-^ . a(C UNITED STATES DEPARTMENT OF COMMERCE • Maurice H. Stans, Secretary ^ 3$" NATIONAL BUREAU OF STANDARDS • Lewis M. Branscomb, Director

The Effect of Moisture on the Heat Transfer Performance

of Insulated Flat-Roof Constructions

Frank J. Powell and Henry E. Robinson

Building Research Division

Institute for Applied Technology

National Bureau of Standards

Washington, D.C. 20234

Building Science Series 37

Nat. Bur. Stand. (U.S.), Bldg. Sci. Ser. 37, 81 pages (Oct. 1971) CODEN: BSSNB

Issued October 1971

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402 (Order by SD Catalog No. C13.29/2:37) , Price 75 cents STOCK NUMBER 0303-0857
Library of Congress Catalog Card Number: 76—611463
Contents

Part I. Concrete Decks

Page

1. Introduction to Part 1 2

2. Experimental Exposure Conditions and Method of Test 2

3. Specimens and Procedures 3

4. Results 5

5. Discussion of Results g

5.1. Effect of Moisture on Insulating Value, and the Influence of Exposure Conditions on this Effect. . . 5

5.2. Effects of Vapor Barriers and of Dense Concrete Decks 10

5.3. Behavior of Specimens Having No Dense Concrete Deck or Vapor Barrier 11

5.4. Pressure Relieving Vents 12

5.5. Changes of Indoor Air Temperature and Relative Humidity 13

5.6. Roof Insulating Materials 13

5.7. Comment 14

6. Summary, Part 1 16

7. References, Part I, Part II 18 Figures 1 through 13 19

Part II. Self-Drying Designs

8. Introduction to Pai't II 25

9. Experimental Exposure Conditions and Method of Test 25

10. Specimens and Procedures 26

11. Results 32

12. Discussion of Results 33

12.1. Effect of Moisture on Insulating Value 33

12.2. Summer Self-Drying, Initial Construction Moisture 37

12.3. Winter Gain of Moisture 38

12.4. Self-Drying From Roof Leaks 39

12.5. Arrangement and Physical Properties of Components 40

12.6. Design Criteria 42

12.6.1. Scope 43

12.6.2. Thermal Insulating Value 43

12.6.3. Self-Drying Time for Initial Construction Moisture and Equilibrium Moisture Content ... 44

12.6.4. Equilibrium Moisture Content 44

12.6.5. Regain of Moisture—Winter Exposure Conditions 45

12.6.6. Summer Self-Drying Time for Winter Regain Moisture 45

12.6.7. Summer Self-Drying Time for a Simulated Roofing Leak 45

12.6.8. Water Vapor Permeance and Hygroscopic Moisture Capacity 46

12.6.9. Exposure Conditions 46

13. Summary, Part II 46

Table of Conversion Factors to Metric (S.I. Units) 47 Figures 14 through 45 48

in

The Effect of Moisture on the Heat Transfer Performance

of Insulated Flat-Roof Constructions

Frank J. Powell and Henry E. Robinson

A solution to the problem of unwanted moisture in the thermal insulation of flat roofs was found during a

recently completed laboratory mvestigation of the effects of moisture on heat transfer through these constructions The objective of the research was to ascertam how much the insulating performance of conventional constructions having insulation over concrete decks, was affected by moisture. This is presented as Part I. Also, the objective was

to investigate properties of materials, their arrangement and dimensions which would yield a construction having an adequate degree of self-drying ability, combined with low winter moisture regain rate. This is presented as Part

II. The results show that the best insulating and moisture performance was obtained by utilizing the heat of the summer sun on the roof to vaporize and transfer to the room beneath any free moisture contained within the construction. Roof specimens made from moderately vapor-permeable materials without conventional vapor barriers were, in winter, able to accommodate the small quantity of slowly accumulated condensation without dripping or se- vere loss of insulating value. During the nme-year investigation, the performance characteristics of 73 insulated roof deck specimens were obtained and two new methods of measurement were developed. Criteria for the design of self- drying insulated flat roofs were developed and limits of the parameters containing the main variables that affect performance were suggested.

This paper presents complete results of the research which was sponsored jointly by the National Bureau of Standards, the Army, the Navy, and the Air Force. Key words: Flat roofs; heat transfer; moisture transfer; thermal insulation.

Part I. Concrete Decks

1 . Introduction

The primary functions of a roof are to provide permanent protection to the structure from

the elements, to maintain the comfort and privacy of the occupants, and to serve as protection for valuable property vv^ithin the building.

Insulated flat roofs are v^^idely used on large area buildings, such as Government buildings,

shopping centers, industrial plants and multistoried office buildings. Flat roofs are selected

primarily because they are functional and eco- nomical, are relatively easy to build and maintain, and they are aesthetically pleasing.

Many designs of insulated flat roofs are available. Most designs consist of a structural support or deck, insulation, and built-up roofing

for protection from the vi^eather. A typical

much-used design consists of a steel-reinforced

structural concrete deck covered with a factorymade board-type of insulation or an insulating

fill material placed on the job. A water-vapor

barrier, when used, is located between the insulation and the interior of the building. The exterior built-up roofing is usually covered with

gravel or slag. This type of construction is the

subject of Part I of this paper.

A literature search reveals that flat roofs cov- ered with built-up roofing have performed sat- isfactorily more often than not. However, some

expensive failures have occurred that have re- quired replacement of built-up roofing, wet in- sulations, and other components of the construction [1, 2, 3, 4].i Hutcheon [5] stated in

1958 that by far the most costly building moisture problem is that encountered in roofs consisting of impermeable coverings laid over in-

^ Figures in brackets indicate the literature references on p. 18.

sulation on flat decks or slabs. The reasons for

failure generally have been attributed to premature deterioration of the built-up roofing al- lowing water leaks; installation of wet materials whose trapped moisture may contribute to pressure buildup when the roof is sun-heated;

and transmission of water vapor from the in- terior of the building with subsequent condensation in the insulation beneath the roofing.

Failure of the built-up roofing manifests itself in the forms of loss of adhesion, open laps, alli- gatoring, blistering, hummocking, or cracking.

Principal reasons for built-up roofing failure are the presence of moisture in the construction, poor workmanship, and excessive damaging traffic on the flat roof. The processes of

failures have been explained in detail [2, 3, 4,

5, 6, 7, 8]. Generally, the forces that produced

failure were generated by changes in temperature and pressure in the presence of moisture,

causing stress, dimensional changes, and fracture of materials.

Designers of roof constructions usually compute the heat transfer coefficient of the assembly on the basis of published values for the

thermal conductivity or thermal resistance of

the drrj component materials [9] . The thick- nesses of the various materials used are selected

to provide adequate structural strength and to yield the desired insulating value. In service, some moisture is usually present. Moisture contents of materials in service are variable and

unpredictable. Useful information on the insulating value of materials in service with moisture present in them is scarce. This is because

of the difficulty in measuring heat transfer in

moist materials and because the effect on heat

transfer of a given amount of moisture is influ- enced by its distribution within the materials.

1
which in turn is controlled by the properties of

the materials, their arrangement in the construction, and variations of temperature imposed upon them by climatic exposure. Also,

changes in insulating value are apt to be gradual requiring long term exposures to observe

them, such as a full summer or winter season. In general, the effect of moisture in permeable roof insulation is to reduce seriously its insulating value, and to cause deterioration of

the roofing or other components of the roof construction. Failure of a roof to produce its insulating value causes increased expense for heating and cooling the building, and may cause the

heating and cooling systems to be inadequate if their capacity has been selected on the basis of

dry insulation in the roof construction.

In 1954, an experimental investigation was

undertaken at the National Bureau of Standards under the joint sponsorship of the Office of

the Chief of Engineers, U.S. Army; the Bureau

of Yards and Docks, U.S. Navy; and the Office

of Civil Engineering, U.S. Air Force. The initial objective was to obtain laboratory data as

to the impairment of insulating value of insulated flat roof constructions as a result of contained moisture. Part I of this paper presents

the experimental exposure conditions selected

for laboratory tests, a brief description and

reference for the method of test devised, a tabulation of specimens and materials, the test procedures, results in tabular and graphic

form, a discussion of meaningful results, and

a summary. Exposure tests for Part I, Concrete

Decks, spanned a total of three years and 46

insulated specimens were used, each 18 inches^

square. Specimens were subjected to successive

periods of simulated summer and winter out- door surface temperature conditions, each with

simulated daily solar heating of the roofing surface. The tests included exposures of specimens

initially at an "as-received" moisture content,

initially at an oven-dry condition, and initially

oven-dry with later addition of known amounts

of water.

2. Experimental Exposure Conditions

and Method of Test

2.1. Experimental Temperature Exposure

Conditions

If water is present in a roof construction, the

distribution of moisture and the migration of

vapor are subject to the saturated vapor pres- sures of the water at the various temperatures

existing within the construction where there is liquid water, as well as to such properties of

the materials as moisture sorbency, capillarity,

and vapor and air permeability. The thickness

of materials, their capacity for storing heat,

and their hygroscopic nature must also be con2 See metric conversion table on page 47.

sidered. The movement of moisture as vapor

and the effect of moisture present on insulating

value are affected very substantially by temperature level, since the vapor pressure of water increases more than linearly with the absolute temperature. It was necessary in this work,

therefore, to select experimentally feasible temperature conditions representative of those ex- perienced by roofs in service.

For summer conditions, it was assumed in

1954 that the temperature of the outdoor sur- face of built-up roll roofing of a flat insulated

roof would be 75 °F at night, on the average.

During the day, when the roof was subject to solar heating, its temperature was assumed to

rise to about 150 °F. For the condition designated as winter, it was assumed that the aver- age temperature of the roofing at night would

be about 38 °F and that, due to solar heating,

it would rise during the day to about 75 °F. For

the condition designated as severe winter, the

roofing temperature was assumed to be about

22 °F at night, and 53 ''F when heated by the

sun. Other investigators [2,7,10,11,12] have

recently shown that exposed roof surface temperatures vary widely and depend chiefly on

the angle of incident solar radiation, the color

of the surface, wind velocity, and whether insulation is present immediately beneath the

surface. Their observations and calculated esti- mates indicate that roof surface temperatures

may rise as much as 95 °F above ambient in

summer, and about 30 °F in winter. The temperatures selected for experimental use in 1954,

in general, show good agreement with the ref- erences cited and were applied with slight

modifications until 1964 when the laboratory

work given here was finished. Exposure temperatures originally selected were considered

to be auspicious.

The roofing surface was heated or cooled by

air regulated in temperature in accordance with

the daily cycle shown in figure 1.^ In the tests,

the steady-state temperatures of the roofing o'f well-insulated specimens approached the air

temperature quite closely, and thus approximated those postulated. Only slight differences

in specimen insulating value were found between the nighttime temperature of 38 °F and

the below-freezing temperature of 22 °F. Accordingly, the so-called severe winter exposure

condition was abandoned early in the program.

The relative humidity of the outdoor air was

not controlled, since the roofing constituted an

almost perfect vapor barrier between the air

and the insulation of a specimen.

The indoor air condition beneath the specimens was 90 °F and 70 percent relative humidity, or a dewpoint of 79 °F and a vapor pres- sure of 1.00 inch of Hg. These conditions were

maintained closely throughout all winter test

= Figures 1 through 13 follow page 18. Figures 14 through 45 fol- low page 47.

2
exposures, but during summer exposures, the

j indoor air tended to rise in response to heat

I transmitted downward through the test speci1 mens during the simulated solar heating period,

j as shown in figure 1 by dashed lines. The temperature of 90 °F was selected in order to maintain a more easily measurable temperature difference across the specimens from indoors to outdoors under summer exposure conditions

when the outdoor temperature was 75 °F in the

I early morning hours. The dewpoint tempera-

, ture of the indoor air rose above 79 °F during

I periods when the indoor air temperature rose

I above 90 °F because the humidifier control was

set to maintain a relative humidity of 70 percent. The temperature and relative humidity se- lected for the indoor air were higher than might

be expected in the majority of buildings, in the

winter at least, but they were adopted to speed

the rate of moisture accumulation in specimens

under winter exposures. The dewpoint temperature of 79 °F, maintained under the specimens, was always higher than the roofing temperature during the winter conditions and the

' night hours of the summer conditions. Therefore, during these times, the water vapor presI sure differences were such as to cause vapor

flow into the specimens toward the outdoors.

: However, it was intended that the vapor presj sure difference would be directed toward the

I indoors during the solar-heating hours of the

j summer exposure. In view of the much greater

) vapor pressures that would exist in a wet speci-

' men at the relatively high temperatures preI vailing in the insulation during the solar-heating period, it was estimated that the somewhat

jj high indoor water vapor pressure would cause

J only a small percentage reduction of the vapor

pressure difference directed toward the indoors,

j Near the end of the program for Part I the

indoor air conditions were changed to observe

effects on the insulating values of the speci- mens. The changes made are given in table 1. Table 1. Changes in indoor air conditions

Test conditions changed

From To To

Indoor air temperature, °F. . . . Relative humidity, per cent , . . Water vapor pressure, in. of mercury

9070

1.0 79

80

50

0.5260

88

39

0.52 Dewpoint temperature, °F . . . . 60

2.2 Method of Test

A primary need in this investigation was to be able to measure the effective thermal resist- ance of the insulation of a roof deck specimen,

and to record its changes as the specimen was

subjected to repeated daily cycles of temperature change simulating those to which a roof is subject, under both summer and winter climatic

conditions. To simulate the daily and seasonal

temperature conditions in a controlled and

repetitive way 15 specimens, each 18 inches

square, were mounted flat in 3 rows of 5 each on a steel grid-support mounted in the lower

half of a large horizontally divided box. See

figures 2 and 3. The desired temperature conditions were achieved by varying the temperature of the air in the upper well-insulated box

in accordance with the patterns shown in figure

1, while maintaining the air in the lower box at constant conditions. The heat flux at the upper

surface of each specimen was measured by means of a 4-inch square heat flow meter

bonded to the center of the built-up roofing that

covered each specimen. See figure 4. If steady-temperature conditions prevail, the

observed heat flow meter reading and the temperature drop across the insulation enable cal- culation of the effective thermal resistance of

the layer of insulation. As figure 1 shows, the

air temperatures in the chambers of the box were substantially constant from 2 a.m. to 10

a.m. Sample temperatures and heat flow patterns observed at 75-minute intervals throughout typical 24-hour cycles of the imposed sum- mer and winter exposure conditions are shown

in figure 5 (summer) and 6 (winter). The two

insulations, labeled (a) and (b) in each figure are representative of the extremes of insulation

thickness and heat capacity. These figures show

that the temperatures in the specimens became

nearly constant from 5 a.m., or earlier, to 10

a.m. The heat flow meter readings also became

substantially constant in this period. Thus, with

substantially steady conditions prevailing for

more than five hours prior to 10 a.m., observations of heat flow and temperature difference across the insulation, made between 8:30 and

10 a.m., should yield reasonably comparable

values of the effective thermal resistance of the

insulation at its existing moisture content and

distribution. It was expected that the thermal

resistance so obtained would be influenced by,

and serve as a measure of, the combined effect of both the amount of moisture in the insulation, and its average distribution as governed

by the imposed temperature cycle. Further details of the method of measurement and the test apparatus are given in reference [6].

3. Specimens and Procedures

3.1. Description of Specimens

The fabrication and installation details of a

typical specimen are shown in figure 4. The

heat flow meter was bonded to the surface of

the built-up roofing membrane and thermally

lagged with a i/>-inch thick steel plate to damp

out temperature fluctuations when recording

3
observations. All temperatures were measured

by means of No. 30-gage copper-constantan

thermocouples with a manually operated potentiometer readable to the nearest microvolt,

corresponding to a temperature difference of

about 0.05 deg F. Each specimen was moisturesealed except for its bottom face and insulation was packed between adjacent specimens and

against exposed sides of specimens to reduce

transverse flow of heat and edge effects in each

specimen. The roofing was held in place on the

specimen periphery by aluminum angles re- tained by rods screwed into the steel frame that

supported all specimens.

The construction of each of the 46 specimens

tested is given by the first three columns in

table 2. Each 18-inch square specimen was designed to provide an overall calculated thermal

transmittance, or U-value, of about 0.12 Btu/

hr ft- deg F on the basis of published thermal

conductivity values for the dry materials. The

thicknesses of the various insulations used were

selected accordingly. Some specimens were as- sembled omitting the concrete deck, others con- tained phenolic plastic tubes inserted in the

dense concrete deck to allow indication of pres- sure within the specimen, and some contained

a 0.004-inch thick polyethylene plastic film between the insulation and the concrete deck to act as a vapor barrier. In the last phase of the

investigation selected specimens were equipped

with pressure-relieving vents through the roofing made from 3/16-inch inside diameter phenolic plastic tubes that projected for a distance

of 1 inch above the roofing. The vents were also

used to admit water to the insulation at selected

times during the exposure periods as given in

the footnotes to table 2. The materials for speci- mens were selected to provide as wide a range

as possible of physical properties of thermal

conductivity, water vapor permeability, density,

hygroscopicity, capillarity, and water absorptiveness.

3.2. Testing Procedures

The specimens were subjected to simulated

daily periodic solar heating as shown in figure

1. Daily observations of heat flow and temperatures were made just prior to 10 a.m. when the

specimens most closely approached steady-state

conditions at the existing moisture content and

distribution. Effective thermal resistances were

calculated by dividing the appropriate temperature difference by the heat flow meter heat flux

per unit area. Daily and weekly changes of

effective thermal resistance were considered to indicate the effect of moisture on heat transfer. Each specimen, and its components, was

weighed at the start and end of its exposure.

Gain or loss of moisture during exposure was

determined as the accumulated effect of the

succession of imposed test conditions, with an

allowance made for the weight of water added.

if any. Final moisture content was determined

by oven-drying the specimen to constant weight

and, if possible, its separate components.

The investigation was conducted in three

phases prosecuted in sequence. Each phase consisted of alternating seasonal periods of exposure that simulated winter or summer tempera- i ture conditions. Each of the seasonal periods : was of several weeks duration, varying in : length as necessary to observe apparent trends ' in the insulating value of the majority of the 15

simultaneously exposed specimens.

The three phases were aimed at somewhat

different objectives, reflecting the changing emphasis of the investigation as it progressed.

Summarized briefly, the chief objectives and i the types of specimens employed in each phase (; were as follows :

f Phase 1. "Investigation of the insulating li value of roof constructions employing boardtype or light-weight concrete insulations

placed on a 3-inch dense concrete deck, with

or without a vapor barrier".

The specimens were constructed from materials with "as-received" moisture contents,

that is, factory-made materials were taken

from cartons stored a few months in the laboratory; dense concrete decks were air-dried

approximately 4 months in the laboratory;

insulating concretes were air-dried in the laboratory from 3 to 7 weeks.

Phase 2. "Investigation of the effects _ of

known amounts of moisture on the insulating

value of various insulated roof constructions

under simulated seasonal exposure conditions."

This was accomplished with specimens, ovendried prior to installation, into which water was introduced during later exposure periods in incremental amounts of 1, 2, and 7

percent to give a total of 10 percent by volume of the insulation. Also, because of the

apparent high vapor resistance of the dense

concrete decks observed in Phase 1, the effect

of omitting these decks on the average effec- tive resistance of the insulating materials was investigated in Phase 2. Selected specimens were continued in Phase 2 unchanged

from Phase 1 for information on the effects of a prolongation of exposure.

Phase 3. "A more thorough investigation of

the apparent self-drying characteristics observed in Phase 2 of some constructions not

made with dense concrete decks."

Observations were made of the performance

of several types of roof constructions without dense concrete decks, and also of the ef- fects of opening the pressure-relieving vents

through the roofing, and of changes of the

indoor air temperature and reduction of the

water vapor pressure of the indoor air from

the original value of 1 inch of mercury to about 1/2 inch.

4
! 4. Results

j The tabular results for all three phases of the

I investigation are given in table 2, columns 4

through 17. Sample results shovv^ing temperatures and heat fluxes for one day of a summer

and a v^^inter exposure period for two specimens

are given on figures 5 and 6. These are typical

of the temperature and heat flux results obtained for all 46 specimens in all seasonal ex- posure periods used. These two specimens were

selected as samples because they represented

the extremes of thermal conductivity, thickness, and heat capacity. In the interest of sav- ing space typical results for the remaining 44

specimens are not given here. Results that illus-

' trate conclusions are presented in figures 7 to

13, where each plotted value of R/Ro is the

average of the daily values obtained during

j each week, usually five in number.

Table 2 gives for each specimen the type of

! insulation used, its thickness, its dry density

and thermal conductivity, the weight of water

I added to the insulation during exposure (if

I any) , and gain or loss of weight from the time

I of installation to the completion of testing for

I each phase. The heat transfer results are given

in terms of a ratio of thermal resistance for the

i insulations, R/Rq. R is the weekly average ef- fective thermal resistance. Rq is the value of R

obtained under winter exposure conditions

when the insulation was in an oven-dry condiI tion, or, if the specimen had not been ovendried, when its observed resistance was a maxij mum. The observed resistance under winter

conditions of an insulation containing only

„ moderate amounts of moisture approached

' closely that of an oven-dry insulation. The

values of R/Rq given in the table generally apply to the last week of each exposure period

shown, Si, Wi, and so forth, since at these

times the trend of the effect of moisture on the

insulating value had been established. The total

number of weeks of exposure at each winter or summer exposure condition is also given in the

table for each phase of the investigation. In

sequence, the order of tests during Phase 1 was

Si, Wi, We, and So. The severe winter condition,

I Wo, was used only once, in Phase 1. The footnotes to table 2 identify for each specimen the

time during exposure when water was added to

I the insulation and when the vent through the

I roofing was opened during Phase 3.

I Plotted data in figures 7 through 13 cover all weeks of exposure given in table 2 for these

specimens and each individual specimen can be

further identified by means of symbols in the

figures.

5. Discussion of Results

Daily and weekly changes of thermal resist- ance were considered to be a measure of the

effect of moisture on heat transfer through the

insulations of flat roof constructions. Observations made just prior to 10 a.m., figures 5 and

6, yielded thermal resistance data for a condition approaching as nearly to an ideal steadystate condition as was possible within the

restrictions of the imposed 24-hour periodic

temperature cycle. Values of heat flux and temperature during the simulated solar-heating

portion of the cycle, 10 a.m. to 4 p.m., and the

cooling portion, 4 p.m. to about 10 p.m., were

clearly transient and would not have yielded a meaningful insulation resistance. Use of the

heat-flow meter and presentation of results as a ratio R/Ro has several advantages: (a) proportionate values of thermal resistance can be

given on a scale from 0 to 1. For example, an R/Ro of 0.5 indicates that the effective thermal

resistance or insulating value is one-half that

of the winter value for substantially dry in- sulation; (b) relative changes of thermal re- sistance of different constructions are readily

compared; and (c) the absolute value of the

calibration constant of the heat-flow meter is not involved, provided only that it remains

substantially constant.

5.1. Effect of Moisture on Insulating Value, and

the Influence of Exposure Conditions on this

Effect

5.1.1. Insulating Concrete Type Specimens

Appreciable amounts of moisture in permeable roof insulations seriously reduced their

insulating value under both winter and summer

conditions. This effect is illustrated in figure 7 for the cellular concrete insulation of Specimen

14. During the initial 10-week summer exposure

period Si, the values of R/Ro ranged from 0.39

to 0.50. During subsequent winter exposure

periods Wj and Wc values of R/Ro became substantially constant at about 0.70 and 0.73, re- spectively, and in the final six-week summer

exposure period of Phase 1, S-j, the values of

R/Ro ranged from 0.43 to 0.50. These tests were made with the specimen materials at "as- received" moisture contents or moderately airdried. The quantity of moisture present was not

determined at the start of the test for Phase 1, but the specimen weight was recorded. When

the specimen was assembled, the top surface of

the cellular concrete insulation appeared dry

and a good bond between the built-up roofing

and the insulation was obtained. During the

exposure of Phase 1, Specimen 14 gained 1.1 pounds or about 0.9 percent by weight (see

table 2). At the conclusion of exposure period

So of Phase 1, Specimen 14 was oven-dried and

lost 10 pounds or 8.2 percent by weight. The

dried specimen was re-installed in tne apparatus for exposure in Phase 2 and re-numbei-ed

as Specimen 29. Figure 8 shows the performance of the cellu5
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7
lar concrete insulation of Specimen 29 during

Phase 2. The values of R/Ro for the first fourweek summer exposure period, Si, ranged from

0.83 to 0.88 and at the start of the five-week

winter exposure period, Wi, was about 1.0. After cumulative amounts of water, equal to 10 percent of the insulation volume (6.7 lb.),

had been introduced into the specimen during

periods Wi, S2, and W2, the value of R/Rf^ decreased sharply during the fourth week of the

eleven-week winter exposure period Wo when

an increment of water 7 percent by volume was

added. During the subsequent fifteen-week sum- mer exposure period, S3, R/Rf^ increased slowly

from 0.43 to 0.51. For the five-week winter

exposure period W3, the values of R/Ro in- creased from 0.68 to 0.71 and this range is comparable with winter exposure periods Wi

and Wp of Phase 1 on figure 7. At the completion of winter exposure period W?, of Phase 2

the specimen was oven-dried. Table 2 shows

that the cellular concrete insulation contained

15.5 percent moisture by weight at this time

and the dense structural concrete deck con- tained 2.3 percent; the moisture content for

the specimen as a whole was 6.0 percent.

Figure 9 shows the performance of the cellu- lar concrete insulation of Specimen 41 during

Phase 3. Specimen 41 was closely similar in

construction and physical properties to Specimens 14 and 29. The specimen was oven-dried

iust prior to installation and was found to contain 11.7 pounds of water or 10 percent of

specimen weight. This amount was subseouently added to the insulation of the specimen

during exposure in Phase 3. The value of R/R^

near the end of the first six-week summer exposure period. Si, was 0.93 and about 1.0 for the

first seven-week winter exposure period Wi,

indicating that the specimen was substantially

dry during these exposures and contained only

hygroscopic moisture. At the start of the sec- ond thirteen-week summer exposure period. So, 11.7 pounds of water was added to the specimen, causing the R/Rn ratio to decrease to 0.37

and it remained less than 0.41 over ten more

weeks of exposure. This behavior is similar to that observed during the third summer exposure period Ss of Phase 2 for Specimen 29 (fig- ure 8) when 6.7 pounds of water were added to the insulation, causing the R/Rn ratio to drop

to 0.43 and increase slowly to about 0.51 after

fourteen weeks of continuous exposure. Similarly, the R/R(^ ratio under winter exposure

conditions for Specimens 41 and 29 ranged

from about 0.5 to 0.57 and 0.6 to 0.72, respectively, in Phases 2 and 3. Valore [13] reports on literature sources that

indicate for cellular concrete types of materials

at ordinary temperatures an increase in effec- tive thermal conductivity of from 2 to 4 percent

for each percent increase in density due to moisture. The data shown in figures 8 and 9 indicate

an average increase of 67 to 100 percent for

winter exposure conditions versus 41.8 and 69.2

; percent as would be predicted by the concept of | a 2 to 4 percent increase of conductivity for il each percent increase in density. For summer il exposure conditions a similar comparison shows y 122 to 170 percent versus 83.6 and 137.4 per-

| cent. The concept of a proportional increase of a effective conductivity with density due to mois- |' ture is desired because of its simplicity. Use of

y this concept for predicting heat transfer in in- n sulation of flat roof decks in service does not I appear to be valid because of seasonal differ-

' ences observed when the boundary conditions

of temperature simulating solar exposure for

winter and summer conditions were applied

exnerimentally. Pratt [14] concludes that the

relationship between conductivity and moisture,

however reliably known, is of little value in

| matters of practical insulation without knowl-

| edge of the moisture content of the material in \ actual installations.

Sereda and Hutcheon [15] state that understanding of the phenomena of moisture migra- t tion is as yet only partial and has not pro- \

gressed to the point of development of an

adequate set of equations describing it. Also,

they state that it seems necessary to suppose

from what is already known that some variables that enter into the micro-distribution of

both thermal and moisture potentials will al- ways have to be accounted for by coefficients

obtained by empirical methods.

Exposure conditions definitely influenced the

insulating value when the same quantity of

moisture was present in a specimen. Sumnier

exposure conditions yielded lower insulating |

values than winter conditions. In figure 7, the

moisture content of Specimen 14 was little changed from week 10, summer period Si, to week 12, winter period Wi. However, the values

of R/R(, for these two conditions were quite i different ; 0.50 and 0.67 for summer and winter,

'

respectively. An explanation for these results

is the different effect of the exposure conditions on the distribution of moisture in the insulation. In the summer exposure condition, the

temperature gradient in the insulation reversed

direction daily. See figure 5. Moisture present

in the insulation would tend to migrate back

and forth in response to the temperature gradient reversals and thus to be distributed and

available for latent heat transfer. In the winter

exposure condition, there was no daily reversal

of temperature gradient (except possibly in the

upper portion of the thickness of insulation)

and the larger part of the thickness of insulation was relatively dry, and therefore the overall effect was a higher R/R(^ ratio as compared

to the summer exposure conditions.

The discussion of results thus far has been

limited to the cellular concrete insulation of

Specimens 14, 29, and 41. The results and com8
, parisons obtained for other types of insulating

concrete specimens placed over structural concrete decks are similar (Specimen groups 13,

1 28, 44, 45; 15, 39; 11, 26, 38; 12, 27, 40) hence

I will not be presented in detail. Table 2 allows

i comparison of results amongst specimens of the

j same type of material and similar specimens

containing different insulations. However, in

the interest of brevity, in table 2 only the values

of the ratio R/Ro for the last week of each ex- posure period are given, since these values are

considered to be indicative of the long-term performance of most specimens.

5.1.2. Board Type Insulation Specimens

J The R/Ro ratio for board types of insulation

ij in Specimens 1 through 10 was higher in Phase

1, as compared to the insulating concretes,

Specimens 11 through 15, because they coni tained only hygroscopic moisture when in-

' stalled, although their concrete decks contained

moisture estimated at 3 to 4 percent by weight.

During exposure in Phase 1, moisture from the

concrete decks plus water vapor from the in- door air, was transferred into permeable insulations during the consecutive thirteen-week

winter period, Wi, and the eight-week severe winter period, Wc, of those specimens that did

not contain a vapor barrier between the board

insulation and concrete deck. Table 2 shows that

Specimens 2, 5, and 7 gained 1.0, 1.1, and 1.0

, pound, respectively, from the start to the end

of exposure during Phase 1. The effect of the

moisture transferred into the permeable insulations on the insulating value is shown by comparing in table 2 the respective R/R^ ratios at

' the ends of Phase 1 summer exposure periods

Si and So for Specimens 2, 5, and 7. Results for

Specimens 1, 4, and 8 were similar in Phase 1, since these specimens were identical in construction to their counterpart Specimens 2, 5, and 7 respectively. The decline of the R/R

' tio was attributed to the moisture added to the

insulations during the winter exposures. After

, exposure period So of Phase 1, Specimens 2 and

i 5 were oven-dried, but not disassembled, in

j preparation for Phase 2 exposures of the

I specimens. They each released 4.1 pounds of

moisture.

Inspection of table 2 will show that in Phase

I 1 the practically vapor-impermeable board-type

insulations of Specimens 9 and 10 show little

! change in R/R^, values under summer and winter exposure conditions, even though the dense

, concrete deck, and vapor permeation through

I the deck during winter exposures, would have

I been sources of moisture to cause an effect during the second summer exposure period. So, if the insulations had been permeable. Table 2

shows that these specimens did show a net gain

of weight during exposure in Phase 1. This gain was attributed to a gain in hygroscopic moisture content of the dense concrete since the

insulations did not contain evaporable moisture

upon oven-drying at the conclusion of Phase 1. These specimens were eliminated from the program after Phase 1.

5.1.3. Comparison of Insulation Types

Figure 10 shows a comparison of three speci- mens whose insulations cover a span of values

for such physical properties as thermal conductivity, water vapor permeance, hygroscopicity,

and water absorptiveness. Each specimen was

oven-dried prior to installation in the apparatus and had water added to the insulations in

increments as shown. In winter exposure period

Wo the R/R(i value for the cellular concrete,

Specimen 29, dropped rapidly and remained

low ; but the wood-fiber, Specimen 20, and glassfiber, Specimen 17, insulation boards regained

their insulating value after an initial drop be- cause they allowed moisture to migrate more

freely to the cold side. During the subsequent

summer exposure period, S3, the R/R^ ratio for

the hygroscopic and absorptive materials of

wood-fiber and cellular concrete was about 0.5 and showed only a slight tendency to increase

with prolonged exposure, whereas, the R/R^ for

glass-fiber was initially low but showed a tendency to increase with time of exposure. From

the start of Phase 2 to its end. Specimens 17,

20, and 29 showed a gain in weight of 1.3, 2.2,

and 5.9 pounds, respectively. The respective

weights of water purposely added were 2.0,

2.6, and 6.7 pounds, so that the specimens had

net losses of 0.7, 0.4 and 0.8 pounds, respectively, over the whole of Phase 2. Apparently,

each specimen allowed some transfer of moisture through its dense concrete deck since it is not impermeable and for the case of Specimen

17 the amount was sufficient to allow an in- crease in the R/Ro ratio upon extended summer

exposure in period S3, but the quantity of moisture transferred was not enough to significantly

affect the R/Ro ratios of Specimens 20 and 29

because these specimens contained more water

in the insulations and concrete decks than

Specimen 17 as is shown by their final moisture

contents in table 2. Later work presented in

Part II of this paper will show that when a larger quantity of water—about 5 pounds—was

added to the same thickness of glass-fiber insulation placed over a concrete deck, its R Ro ratio remained lowered for over ten weeks and

did not show a tendency to increase.

The performances of all board types of insulation placed over dense concrete decks in

Phases 2 and 3 can be similarly deduced and

explained using the values given in table 2. It will be noted that all board types of insulation

that were dry or contained only small amounts

of moisture yielded good insulating value under

both summer and winter exposure conditions.

9
During Phase 3, Specimen 31 was removed

from the apparatus after its eleven-week winter exposure period W2. In its place, Specimen

32 was installed in an oven-dry condition. Fig- ure 9 shows the performance results obtained

for Specimen 32. As shown, the R/Rq ratios for

summer and winter were high but declined

when water was added to the specimen.

5.2. Effects of Vapor Barriers and of Dense

Concrete Decks

Dry insulations protected by a vapor barrier

and placed over a structural concrete deck

tended to remain dry and provide good insulating value. Omission of the vapor barrier

allowed moisture to migrate from, and to some

extent through, the green concrete decks to the

insulations. When the insulations were wetted,

as in simulations of a roofing leak, they tended

to remain moist and showed poor insulating

values for long periods of time because of the

impedance of the vapor barrier or concrete deck

to movement of water vapor.

In Phase 1, Specimens 3, 6, and 15 had a

sheet vapor barrier under the insulation, others

did not. Table 2 shows that at the end of sum- mer exposure S2 the values of R/R^ were about

the same as they were for the initial summer

exposure Si, although they had been subjected

for a total of 21 weeks to winter exposure periods Wi and Wo, during which moisture from

the dense concrete or the air below would ordinarily have tended to migrate into the insulations. Similarly constructed Specimens 1 and 2, 4 and 5, 13 and 14, respectively, which had no

sheet vapor barriers, had average values of

R/Ro in Period So appreciably lower than those

in Si, indicating increase in insulation moisture

content due to winter exposures Wi and We.

Similar behavior was recorded for the insulations of Specimens 7 and 8, which were initially

fairly dry and were moderately vapor-permeable, but not for the insulations of Specimens 9

and 10, which had very low water vapor permeance. It should be noted that only the specimens

having vapor barriers—3, 6, and 15—showed

a loss in weight during exposures in Phase 1 as shown in table 2. This presumably was the re- sult of drying of the dense concrete deck, the

moisture of which could not enter the insulation during winter exposures Wi and Wc. Why

a similar loss of weight did not occur for Specimens 9 and 10, which had quite vapor-resistant

insulations, although no vapor barriers, has

not been accounted for. It is speculated that

the hygroscopic moisture equilibrium of these

specimens was initially lower than that which

would be obtained upon exposure to 90 °F and

70 percent relative humidity indoor air. Also,

the precise free moisture content of the concrete deck for each specimen was not determined before Phase 1 exposure and it would

be expected to find absolute differences among

the 15 slabs used. The weight data given in

table 2 confirm that moisture did migrate into

the insulation of the specimens without vapor

barriers. From the start to the end of Phase 1, Specimen 2 without a vapor barrier gained 1 pound, while Specimen 3 with a vapor barrier

lost 2.4 pounds. Upon oven-drying Specimen 2

lost 4.1 pounds and Specimen 3 lost 2.9 pounds.

Before oven-drying at the end of exposure period S2, the insulation of Specimen 3 contained no measurable moisture and its concrete deck

contained 3.2 percent by weight. Since Specimen 2 was carried over intact to Phase 2, as Specimen 17, the moisture content of its materials was not determined after Phase 1. Similar comparisons may be made of the data in

table 2 for Specimens 6 and 5, and Specimens

15 and 14. Further data on the eifect of a vapor barrier can be seen in table 2 for Specimens 39 and 44.

Both were oven-dry at the start of exposure

Si in Phase 3; Specimen 39 had a sheet vapor

barrier, 44 had only a dense concrete deck.

Throughout all exposure periods of Phase 3,

these insulations had R/Ro ratios only moderately less than 1.0 ; although for both the ratios

show a tendency to decrease slowly in successive similar exposure conditions. For Specimen

39 the decrease of R/R^ was relatively less than

for 44, indicating the effect of the vapor barrier. It will be noticed, however, that the dense

concrete deck of Specimen 44 evidently had

substantial vapor resistance, and also that the

deck was initially oven-dry and therefore unable to contribute moisture of its own to the

oven-dried insulation. Similar behavior of other

dry insulations was recorded for Specimens 31

and 33.

On the other hand, R/R^ ratios for moist insulations used on concrete decks with or without a sheet vapor barrier remained low for long

periods of exposure. This is evident in table 2

for Specimens 17, 20, 26, 27, and 29 in Phase 2

and Specimens 32, 35, 38, 40, 41, and 45 in

Phase 3 by comparing the R/R^ ratios of these

specimens during exposure periods when they were dry and when they contained moisture.

For example, table 2 for Specimen 29 shows

R/Ro values of 0.88 and 0.98 for the cellular

insulation at the ends of exposure periods Si

and Wi. After water was added the R/Rq ratios

declined to 0.6 and 0.51 for W- and S3, respectively, and remained low for many weeks during exposure S3 as shown in figure 8. Figure 9

shows similar characteristics for Specimens 32

and 41, and figure 10 for Specimens 17, 20, and

29. Specimens 1, 4, 8, and 13 in Phase 1 and

Specimens 16, 19, 23, and 28 in Phase 2 did not

have water added to them. However they did

contain some moisture and the differences between summer and winter R/Rq values for the

several exposures are evident in table 2.

10
5.3. Behavior of Specimens Having No Dense

Concrete Deck or Vapor Barrier

Specimens consisting of roofing on a layer of

insulation, without a vapor barrier or dense

concrete deck beneath, showed, when once wet,

I a marked tendency to regain insulating value,

I presumably due to drying under the simulated

I summer exposure conditions. Under winter conditions, a slight decrease in insulating value

I occurred with continued exposure, presumably

j due to moisture gain from the indoor air, when

I the moisture absorptive capacity was large.

I However, for a vapor permeable construction

of low moisture absorptive capacity, the insulating value was seriously impaired on con- tinued winter exposure.

In Phase 2, three board type insulations

(Specimens 18, 21, and 22) and three fill-type

insulating concrete materials (Specimens 24,

25, and 30) were exposed without dense concrete decks or vapor barriers underneath. Specimen 18 was highly permeable to vapor, but of

very low moisture absorptiveness ; Specimens

21 and 22 were moderately vapor permeable

and of substantial absorptiveness; Specimens

24, 25, and 30 were moderately vapor permeable and highly absorptive. All specimens were

oven-dried before installation, but some may

have gained moisture hygroscopically before

i the test exposures started. Only Specimen 30

had water added to it during exposure. Table

2 gives R/R() ratio data for all specimens and

figure 11 shows Phase 2 performance for Speci- mens 18, 21, and 30. Specimen 18 showed, to an extreme degree,

rapid decreases in its R/Rq ratio during winter

exposures, and rapid increases during summer

exposures, in all periods of Phase 2. This behavior would be expected for a permeable material exposed to a high relative humidity and

during the first winter exposure period, Wi, a

quantity of water dripped from the undersurface of the specimen. Evidently this insulation was too permeable to vapor, and too little absorptive of moisture, to endure long winter

exposure without accumulation of excessive

free water, when used without a vapor resist- ance underneath. At the end of the five-week

winter exposure W3 the average moisture content of the insulation of Specimen 18 was 54.7

percent by weight as shown in table 2. For Specimen 21, R/Rq values changed moderately with different exposure conditions, and

tended to decrease slightly during winter exposures. For the exposure conditions imposed, the

insulation appeared to reach repetitive R/Ro

values for alternating summer and winter ex- posure conditions, because of the ability of the

material to hold sorbed moisture and dissipate

it as vapor to the indoors under summer exposure conditions. The performance of Specimen

22 was similar to Specimen 21 except that its

R/R^) ratios were higher as shown in table 2. However, it is believed that for longer winter

exposures than those shown in figure 11, .some vapor protection underneath Specimens 21 and

22 would be necessary to avoid the extremes of

the type evidenced by Specimen 18. Specimen 30 showed decreases in its R/Ra

ratio under winter conditions especially when

water was added in periods W, and of Phase

2. However, when Specimen 30 was given a long-term summer exposure, S.-t in figure 11,

the R/R(^ ratio showed a definite increasing

trend, indicating that the specimen was drying

from its undersurface and recovering its insulating value. This observation was considered

important because it showed experimentally

the feasibility of self-drying insulated roof constructions that have become wetted from roofing leaks.

At the end of winter exposure period W3 of

Phase 2, figure 11, the percentages of moisture

by volume were about the same (approximately

10 percent) in Specimen 18, made from glassfiber board, and Specimen 30, madie from cellu- lar concrete, but their R/Rq ratios were very

different, as shown. The difference appears to be due to the greater thickness, volume, and

much greater moisture absorptive capacity of

the insulating concrete specimen, and possibly

also to its lower vapor permeability and higher

thermal conductivity (which reduces temperature gradients in the insulation). Thus, al- though an exposed somewhat vapor-permeable

insulation may gain moisture from indoors during winter exposures, such gains do not have an

excessive depressive effect on the R/Ro ratio

if the moisture absorbing capacity of the construction is adequate.

The self-drying trend observed in Phase 2 was further investigated in Phase 3. Figure 12

shows a comparison of results for Specimens 37

and 40. Similar results were observed for Specimens 34 and 38, and 46 and 41, as can be seen

in table 2. Specimens 37 and 40 were similar,

except that 37 had no dense concrete deck. Both were initially oven-dried and both had water

added to them at the start of summer exposure

period So, in amounts equal to that removed

when they were oven-dried (18.6 and 23.0

pounds, respectively) . The R/Ro ratios for both

specimens were approximately the same during

exposure periods Si and W]. Addition of water

at the start of exposure period So caused an

immediate sharp decrease in R/Rq ratio for

both specimens, but during the remainder of

S2, the R/Ro for Specimen 37 increased considerably while that for Specimen 40 showed only

a minor change. In subsequent winter exposure

period Wo, values of R/Ro were markedly different. In summer exposure period S3, the R R^

ratio for Specimen 37 increased to 0.90, while

the R/Rq ratio for Specimen 40 increased to only 0.28. In winter exposure period W3, and

11
summer exposure period S4, the R/Ro ratio for

Specimen 37 reproduced approximately the

values observed in exposures Wi and Si when

it was substantially oven-dry. But for Specimen

40, the R/Ro ratio in exposures W3 and S4 had

values only a little larger than those in exposures Wo and S3 when the specimen contained

much moisture. Table 2 shows that Specimen 37

had an exposure loss of weight of 15.2 pounds,

and Specimen 40 had a loss of 3.3 pounds. The

final moisture content of the insulation of Specimen 37 was 7.7 percent by weight as compared

with the insulation of Specimen 40, which was

50.4 percent by weight. The conclusion that the

increase of R/Ro ratio of Specimen 37 was due

to drying during summer exposure period S3 is supported by the change of its R/Ro ratio from

winter exposure period Wo to winter period W3,

while in the case of Specimen 40 this change was negligible.

The factors that influence the ability of

wetted roof insulation materials to dry to the

room beneath under summer exposure conditions, and of withstanding winter exposure conditions without serious loss of insulating value

due to moisture gain from the indoor air are

believed to be the moisture-absorbing capacity

of the construction, and probably also the thickness, thermal conductivity, water vapor permeance, and arrangement of the components of

the construction. The question of the deteriorating effects of freezing and thawing of moisture

in the construction also requires consideration,

but the importance of this problem is probably

greater for a roof that does not dry in service

than for one that does.

The insulating value of some roof constructions moderately permeable underneath after

being wetted tended to reach from below a

stable or plateau value that was not greatly

different from that reached from above by similar initially dry constructions when both were

exposed to the same alternating seasonal exposure conditions of summer and winter.

This observation is shown in figure 13. Specimens 36 and 46 had no dense concrete deck or vapor barrier and both were initially oven-dried

before Phase 3 exposure. Their R/Ro ratios were almost identical during exposure periods

Si and Wi. At the start of exposure period So

water was added to Specimen 46 in an amount

equal to its original moisture content, 20.8 percent by weight. Subsequently the R/Ro values

for Specimen 46 increased during summer ex- posure periods S2 and S3, while R/Ro values for

Specimen 36, which had no water added to it, decreased slightly during exposure periods fol- lowing Wi. At the end of exposure period

S4 the R/Ro values for Specimens 36 and 46 were 0.78 and 0.69, respectively, and their insulation moisture contents were 6.7 and 9.3

percent, respectively. In short, without dense

concrete decks beneath them, a specimen of

insulating concrete initially oven-dried, and

one brought to 20.8 percent moisture content,

tended toward the same operating values of

thermal resistance and moisture content after a period of about 1 year exposure to the test conditions. Although there were no other pairs

of specimens for making similar comparisons,

it is evident that Specimen 37, figure 12, to which water was added, rapidly approached,

during simulated summer exposures, R/Rq

values and a moisture content (7.7 percent

after period S4, table 2) typical of fairly dry

insulation.

5.4. Pressure-Relieving Vents

The use of small pressure-relieving vents

through the roofing of specimens having moist

permeable insulations had no appreciable effect in reducing the effective thermal resistance of

the insulation. The vents used were sufficient to relieve internal pressures in pervious insulations, but were not ventilating channels

through which outdoor air might move as a

result of wind forces.

In all specimens in Phase 1, a tubular connection was made between the interior of the insulation and either a manometer or limp polyethylene balloon of about 0.1 cubic foot volume

when distended. Observed changes of volume of

the balloon as a result of air volume changes were on the order of 75 to 100 cubic inches, during summer exposure Si, for a cellular concrete

specimen which had a relatively large void vol- ume. The mercury manometer readings indicated daily changes of internal pressure in

specimens for the same period equal to not more

than 3 pounds per square foot. Since the specimens to which manometers were attached did

not have sheet vapor barriers, it is believed that

pressure relief occurred through the dense concrete slab, since otherwise pressure differences as large as 100 to 200 pounds per square foot

would be expected for the temperature changes

experienced by the insulations.

In view of the breathing action observed with

the limp balloons, which was approximately of

the magnitude to be expected, an effort was

made in Phase 3 to determine if breather-type

vents through the roofing would assist in in- creasing the R/Ro values obtained with moist

insulations. Accordingly, such vents were in- stalled in 11 specimens as shown in table 2. These specimens, which were installed in ovendried condition, had restored to them during

summer exposure period So the amount of moisture removed by the oven-drying process. Fifteen weeks later, during winter exposure W2,

the vents were opened to communicate with the

air over the roofing.

The criterion used for evaluating the effect

of such vents was that if they had a substantial

effect, it would be indicated by a change in the

12
i trend of the R/Rq ratios in period Wo and subsequent periods. For the specimens shown in

\ figures 9, 12, and 13, and all other specimens,

^1 the trends that were established in periods S2

j and Wo before the vents were opened were not

materially changed after the vents were opened.

' A calculation of the probable drying rate of a cellular concrete specimen as a result of

' breathing action, based on daily venting of air

I saturated at the higher mean temperature of

the insulation and its replacement by dry air

1 as the specimen cooled during a summer exposure indicated that years would be required to

I effect a change in moisture content of only a few percent. The experimental evidence was in

accordance with the calculated estimate.

5.5 Changes of Indoor Air Temperature and

Relative Humidity

Changing the indoor air to a lower tempera-

;! ture and relative humidity decreased the insulating value during summer exposure of speci- mens containing much moisture but had much

less effect on specimens of similar materials

' containing little moisture.

' An example of the statement above is shown

in figure 12. In Phase 3 during summer exposure Sn, Specimen 40 shows a decrease in R/Ro

value in the time period when the indoor conditions were changed while the drying trend of

Specimen 37, which contained much less moisture, was less affected. The observed decrease was probably caused by a redistribution of

moisture in the specimen and possibly by a

I decrease in sensitivity of the apparatus when

operated with small temperature differences of

about 5 °F across the specimens prevailing during these weeks. The change of indoor air temperature from 80 to 88 °F, but with no change

in vapor pressure, resulted in R/Rq values

! which continued, approximately, the trend of

' R/Ro values existing prior to the change, when

the same indoor air vapor pressure was mainI tained. However, Specimens 37, 36, and 46, all

I without dense concrete decks, showed an immej diate increase at week number 54 when the

temperature was increased from 80 to 88 °F,

as shown in figures 12 and 13, respectively.

This was attributed to a redistribution of the

moisture in the specimens, which for these

{ specimens was already much lower in quantity

as compared with similar wetted specimens

which contained a dense concrete deck.

It is important to note that the indoor dewpoint temperature for all exposures from week

number 50 on was 60 °F, whereas before that

time it was 79 °F. For lowered dewpoints,

which would be probable for all ordinary buildings in winter, the plateau-values of R/Ro for

constructions having self-drying characteristics

probably would be greater and their moisture

contents probably would be somewhat lower,

than the values obtained experimentally.

5.6. Roof Insulating Materials

The roof insulating materials used in this

investigation can be separated into groupings

that have common characteristics important in

connection with moisture problems and the ef- fect on insulating value of moisture present in

the roof construction. There are differences

between insulation materials in each group, but

such differences are less important than those

between groups. A general description of the

materials in each group and, based on data obtained in this investigation, the common general

characteristics of each group are summarized

below.

Group 1. This group comprises insulations that are

practically impermeable to vapor or moisture:

Specimen 9, cellular glass; Specimen 10, ex- panded polystyrene. The effective thermal re- sistances were substantially equal to those of

dry materials, and within the precision of the

measurements were approximately the same for

both summer and winter exposure conditions.

Their resistances did not change appreciably

with exposure time, under conditions where

moisture was available from the dense concrete

deck underneath.

Group 2. This group comprises vapor permeable insulations which are delivered and installed in a

substantially-dry condition: Specimens 1, 2, 3, 16, 17, 18, 31, and 42, glass-fiber; Specimens 4,

5, 6, 19, 20, 21, 33, and 35, vegetable fiber insulation board; Specimens 7, 8, 22, and 23, corkboard; Specimen 32, a perlite-asphalt mixture

fill insulation; and Specimen 43, a perlite-mineral wool insulation board.

These materials when installed at air-dry or oven-dry moisture contents, over a good vapor

barrier, maintained effective thermal resist- ances approximately corresponding to those of

the dry materials, throughout the duration of

the tests. However, effective thermal resist- ances were perceptibly lower under summer ex- posure conditions than under winter exposure

conditions and there was some evidence that

thermal resistance tended to decrease slightly

with time, presumably as a result of slow moisture gain through the vapor barrier.

Installed over "air-dry" dense concrete decks

without a vapor barrier, the insulations apparently gained enough moisture from the decks

in a relatively short time to cause perceptible

decreases in effective thermal resistance during

summer exposure conditions, but not during

13
winter conditions. Effective thermal resistances

decreased slowly with exposure time, the de- creases becoming observable during both sum- mer and winter conditions.

Upon addition of appreciable water to these

insulations, effective thermal resistance values

decreased markedly, and were substantially

less during summer exposure conditions than

during winter conditions. Thermal resistance

values with water added in the amount of 10

percent of the insulation volume were from 50

to 67 percent of values expected for dry materials under summer exposure conditions, and

from 83 to 100 percent under winter conditions.

The specimens so treated had no vapor barriers; if there had been vapor barriers to impede transfer of some of the added water to the concrete deck, the effective thermal resist- ances would almost certainly have been still lower.

Installed without a dense deck, or a vapor

barrier, so that the insulation was exposed to the air underneath, these insulations were subject to apparent gains of moisture during the

winter exposure conditions and losses during

summer exposure conditions, as indicated by

changes in effective thermal resistance values.

These changes were apparently dependent on

the permeability and moisture absorptive ca- pacity of the insulation. See discussion under

section 5.3. The essential conclusions concerning permeable insulations of Group 2 when used over dense concrete decks are: a) that when dry,

and kept dry by a vapor barrier, they yield approximately their dry-value insulating effect; b) that with a vapor barrier, and more so when

placed directly on a dense concrete deck without a vapor barrier, they tend to decrease

slowly in insulating value, especially for sum- mer conditions; c) that if they are wetted, as by a leak through the roofing, or are installed

with excessive moisture, their insulating effect

is very seriously reduced, especially for summer

conditions, and is thereafter kept for long periods of time at reduced values as a result of

the moisture transfer resistance of a dense concrete deck and/or of the vapor barrier usually

placed under these insulations. Except for in- sulations of this kind of moderately low vapor

permeance and high moisture absorptiveness,

they should not be used alone directly exposed

to the air underneath.

Group 3. This group comprised permeable insulations

mixed with water in the process of application,

and any of the second group which for any

reason contained moisture, when installed, in excess of their normal hygroscopic moisture

content or were made wet by adding water:

Specimens 13, 14, 15, 28, 29, 30, 36, 41, 44, 45,

and 46, cellular concrete; Specimens 11, 24, 26,

34, and 38, perlite aggregate concrete; Specimens 12, 25, 27, 37, and 40, vermiculite aggre- .j gate concrete. The water-mixed insulations of ; this group had free moisture contents ranging \ from about 20 to 60 percent by weight after '\ several days of air drying. When used as speci-

| mens at these air-dry moisture contents, their

insulating effects were from about one-half to t one-quarter of their observed values when ovendry, depending upon whether they were being i subjected to winter or summer exposure con- f ditions, respectively. When insulations were i placed on dense concrete decks, the impaired j insulating values continued almost v/ithout sig- !i nificant improvement, throughout the period of » observation. The lack of improvement is con- ( sidered due to the effective resistance or im- » pedance of the dense concrete decks to moisture i escape from the undersurface of the insulation, l The same insulations, exposed to the air be- \ low without a dense concrete deck underneath,

increased rapidly in insulating effect during i exposure to a few alternations of the imposed s summer and winter exposure conditions. In one ' case, the insulation attained, during the period > of observation, an apparent plateau-value of |l insulating effect scarcely different from that of

the substantially dry material, although its moisture content had been increased to about

63 percent by weight at the start. Similarlytreated specimens of other insulating concretes i without decks showed an approach to the same I behavior, but at a less rapid rate. Specimens of these insulations, without decks,

installed in an oven-dried condition, decreased i perceptibly in insulating value when subjected

to alternations of the test summer and winter

exposures, and tended to approach, from a dry

initial condition, the approximate plateau-value

of insulating effect being reached from the j other extreme by similar insulations to which

water had been added.

5.7. Comment

5.7,1. Test Conditions

Surprisingly small values of effective thermal

resistance were obtained in this investigation

for moist insulations during the imposed sum- mer exposure condition. In many cases, they were so much smaller than the values obtained

for the same materials when dry that it appears

reasonable to attribute a large part of the de- crease to latent heat transfer involving evaporation of moisture at one place, its migration

as vapor to a cooler region, and its condensation

there.

In a noncapillary permeable insulating material subjected to a steady temperature gradient

in one direction, this distillation process usually

results in a concentration of the moisture in the

cooler parts of the insulation. Thus, most of

the insulation contains only the m.oisture cor14
I responding to its hygroscopicity at humidities

lower than 100 percent, and the over-all effec- tive resistance is only moderately decreased

by the loss of thermal resistance of the wetter

portion. This was, in fact, approximately the

result observed during the winter conditions

of this investigation for insulations containing

moderate amounts of moisture.

Under the imposed summer conditions of this

investigation, however, the daily simulated solar

heating caused daily reversals of temperature

gradient in the insulation and thus, by distilla- tion back and forth in each day, kept probably

the whole thickness of the insulation at a condition of sufficient free water content to allow

substantial latent heat transfer. This process

I affords reason to believe that the simulated

solar heating period in the imposed summer

exposure condition was the essential cause for

the small effective resistances observed with

moist insulations. In this connection it should

! be noted that for substantially dry insulations,

I effective resistances were approximately the

I same during the imposed summer and winter

' conditions ; that is, when there were, and when

I there were not, daily temperature gradient re-

! versals, respectively, and therefore that heat

capacity effects at the times when resistances

j were determined were not the cause. Whether the very low effective resistances

! indicated for moist permeable insulations in

I this investigation actually occur in roofs in

j service, and whether this finding is important

i practically, depends therefore upon whether

1 daily temperature gradient reversals do occur

; in roof insulations in service, during summer

j or other seasons. There is little doubt that in

I summer, at least, in most areas of this country,

the top surface of an insulated roof is considI erably hotter than its undersurface during the

sun-hours of the day, and that at night the un-

' dersurface of the roof is warmer than its top

surface, for buildings not air-conditioned and

i possibly also for air-conditioned buildings, since

' roof surfaces are subject to comparatively rapid

cooling by the night air and by radiation to a

clear sky. Accordingly, one must conclude that

free moisture in excess of the hygroscopic ca- pacity of an insulation very seriously impairs

the insulating effect of a permeable roof insulation in service under summer exposure conditions. It is felt, in fact, that if the investigation had been conducted without a simulated

daily solar heating period causing temperature

gradient reversals, this practically-important

finding might easily have been missed.

It must be pointed out that in this investigation, effective thermal resistances were obtained

during the substantially steady temperature

conditions existing just before the onset of the

solar heating period. However, during the solar

heating period, and subsequently while the

insulation was regaining the steady temperature conditions mentioned above, considerably

steeper temperature gradients must have existed within the insulation. Consequently, latent

heat transfer must have been occurring at greater rates at these times than at the time

measurements were made. Because of heat ca- pacity effects, it is not feasible to estimate the

over-all effective resistance of the insulation

during the times of changing temperatures.

Nevertheless, it seems reasonable that at such

times over-all effective resistances of moist in- sulations were less than the values at the time

observations were made. In other words, the

effective resistances given are probably greater

than the average values over the 24-hour daily

cycle.

One further point should be made concerning

the exposure conditions used. The roofing sur- face temperatures were probably reasonably

typical for roofs in service, but the 79 °F dewpoint temperature maintained under the specimens throughout most of the tests undoubtedly was higher than typical for ordinary buildings.

More typical maximum dewpoints would probably not exceed 65 °F in summer and 40 °F in

winter. The effect of the 79 °F dewpoint in

these tests was in the direction of greater gain,

and smaller loss, of moisture by a specimen

through its undersurface, than would occur at lower under-roof dewpoints. The effect was

therefore to decrease to some extent the

"plateau-values" of effective resistance discerned in these results. The effect on values

obtained for specimens containing considerable

free moisture was probably small, but the inferred "drying-rates" observed would probably

have been greater with lower under-roof dewpoints. Some slight evidence of increased drying-rates was obtained for Specimens 34, 37,

and possibly 45, during Phase 3 when the bottom air dewpoint was lowered to 60 °F.

5.7.2. The Moisture Problem

As mentioned in the introduction, there are

several avenues by which excessive moisture can enter an insulated roof construction in

service. The degree to which precipitation or

roofing faults or punctures during construction may contribute moisture is an imponderable that is peculiar to each application and its history. Possibly in the same category is the

moisture content of a dense concrete roof deck on which an insulation may be placed without

a vapor barrier—but almost certainly, such a deck contains some excess moisture.

Insulations of appreciable hygroscopicity

very probably contain moderate amounts of

moisture when installed, depending on the ambient relative humidity. If the average moisture

content is not well below the maximum hygroscopic capacity of the insulation, it is likely

that its concentration by a temperature gradi15
ent would yield "free moisture" in parts of it, and that latent heat transfer effects would be

appreciable with reversing temperature gradients. It must be added that even hygroscopic

moisture redistributes under a temperature

gradient, and that for permeable insulations

of high hygroscopicity some latent heat transfer will occur, at least while temperatures are changing.

Water-mixed insulations must be considered

as containing excess moisture when roofed

over. In view of the serious effects of this moisture, the only practical recourse appears to be

to arrange matters so that these initially-moist

insulations will dry out in a comparatively

short time in service. As discussed below, this

appears to be a possibility.

The most difficult problem is that of inleakage of water through leaks in the roofing.

The possible quantity is an imponderable, ex- cept that usually a leak is not suspected until

water has appeared at the undersurface of the

roof. Apart from accidental leaks in new roofs,

it must be regarded as a practical certainty

that eventually leaks will occur as the roofing

ages. In such event, maintenance of satisfactory

insulating performance would require either

that the insulation be removed and dried or

replaced, or that the roof construction be capable of adequate self-drying in service. In this

respect, insulations wetted by leaks and the

water-mixed insulations are allied, and a com- mon solution is required.

5.7.3. Self-Drying Roofs

By a self-drying roof is meant an insulated

roof construction which under its in-service

exposure conditions will expel excessive moisture from its insulation in a relatively short

time; that is, in one summer or one year, and

which subsequently will yield an insulating

effect approximating that to be expected with

the insulation dry. It is assumed that the moisture expulsion will take place through the undersurface of the roof, rather than through a system of ventilating channels or ports in the

roof, which may constitute an effective means

of drying a wet roof construction if suitable

means are provided to attach and preserve the

built-up roofing.

In this investigation, and for the exposure

conditions used, it was found that wet insulations over vapor barriers, or on dense concrete

decks, dried very slowly, if at all. On the other

hand, roof specimens of thick monolithic insulating concrete exposed to the under-roof air

dried quite substantially in 20 or 30 weeks of

the imposed summer conditions. In one case, the

insulation attained, after being wetted, the performance it had when installed oven-dried.

It is desirable to explain clearly what has

been shown—namely, and merely, that a dense

concrete deck under an insulating concrete impedes its necessary drying, and that without | this deck satisfactory drying occurs. Apart j from the important practical questions of the ] strength, and feasibility, of a roof of mono- j lithic insulating concrete, one must consider 5 that a form-board or decking is necessary to j pour the roof. The question enlarges therefore ' to a consideration of practical permanent deckings other than dense concrete, and what properties these must have so as not to impede dry-

{ ing, or allow excessive moisture gain under [ winter conditions. It was found desirable in

Part II to investigate self-drying designs for

a wider selection of insulations than only the

water-mixed insulating concretes, in view of

the ubiquitous problem of roof leaks.

At present, understanding of the mechanisms t of movement of moisture as water or vapor in | materials is quite incomplete. When a problem 1 involves differences in temperatures, and chang- 5 ing differences, as in the case of roofs, present , knowledge does not allow a rigorous analytical t approach with the complexities involved, al- t though it does indicate possible fruitful lines . of experimental attack. For instance, the im- 1 pedance of a dense concrete deck for outward

moisture passage has been thought here to be ; possibly a result of the relatively small temperature gradient in it because of its high conductivity. If this should be so, a deck material

of low conductivity and probably of moderate

permeability might be much more satisfactory

for the construction of self-drying roofs. Because of the complexity of the total problem,

and the need for temperature regimes simulating those of a roof construction in service, however, it was believed necessary to examine the

various possibilities by means of laboratory or

field roof exposure tests.

6. Summary

Appreciable amounts of moisture in permeable roof insulations seriously reduced their

insulating value under both simulated winter

and summer exposure conditions. For approximately the same moisture content, the effective

thermal resistance of a permeable insulation was considerably less under summer conditions

than under winter conditions, both of which

included simulated solar heating of the roof for

part of the daily cycle of temperature exposure.

For some insulations containing considerable

but probably not untypical amounts of moisture, such as insulating concrete, the effective

thermal resistances observed were as little as one-quarter of the values observed for the dry

insulation, under the same summer exposure

conditions, and about one-half the dry values,

under winter exposure conditions. For nearlydry insulations, the decreases and seasonal differences were much smaller.

16
A possible explanation for the smaller insulating effect of a given amount of moisture under

j summer exposure conditions as compared with

I winter conditions is that for the simulated

' summer exposure condition used, the temperature gradients in the insulation reversed direction daily. The reversal would tend to cause moisture to migrate back and forth each day,

i thus keeping it distributed throughout the insulation during the summer condition and available for latent heat transfer. Under the simuI lated winter condition used, there was no daily

reversal of direction of the temperature gradi-

, ent, except possibly in the upper portion of the

insulation for a relatively short time, and the

j moisture would tend to migrate to and concentrate in the colder upper part of the insulation

I leaving the remainder of the insulation thick-

' ness relatively dry. Thus, the method of meas- urement used revealed that cyclic temperature

conditions were essential to properly evaluate

1 the effect of moisture on heat transfer in roof

jl insulations.

The degree of impairment of insulating value

with moisture content varied with the properties and the arrangement of the component

materials of the specimens. In general ,the rej duction of insulating value was not proporI tional to moisture content, since it was also a function of the temperature conditions imposed

and the resulting distribution of the moisture

in the specimen.

For roof insulations that are dry, a vapor

barrier and a dry dense concrete deck are advantageous, since they help keep the insulation

dry. But for roof insulations which initially

contain appreciable moisture, or those made

wet from a leak in the roofing during service,

the moisture transfer resistance of a vapor

barrier and/or of the dense concrete deck impedes drying of the insulation through its undersurface, and keeps the insulation in a moisture condition at which its insulating value

may be seriously reduced.

A vapor barrier between an initially-dry

roof insulation and a concrete deck that is not

dry prevents dampening of the insulation in

winter by moisture from the deck, which otherwise would occur because air-dried concrete

contained approximately 1.3 pounds of water

per square foot of deck.

Changes in the effective thermal resistance

of permeable roof insulations with time for the same seasonal exposure condition were almost

inappreciable when a sheet vapor barrier separated the insulation and the dense concrete

deck. In most cases, changes were also materially retarded when the insulation was placed

directly on the deck without the sheet vapor

barrier, apparently because of the effective re- sistance to moisture transfer of the dense concrete deck.

Apparent self-drying tendencies were observed for roof specimens consisting of roofing

and a layer of insulating material installed

without a dense concrete deck or sheet vapor

barrier underneath. For insulations of moderate permeability to water vapor and relatively

high absorptive capacity installed in this way,

it was found that the originally low effective

thermal resistance of the initially moist insulation tended to increase rapidly during the

simulated summer exposures, and to approach

a value not much less than that of the dry insulation, presumably as a consequence of drying

through the undersurface. When the insulation was initially dry relatively small and slow de- creases in thermal resistance were observed

because of a slow gain in hygroscopically held

moisture. The results indicated that for some

insulations the effective thermal resistance,

whether the insulation was dry or moist initially, tended to reach an approximately stable

level value, or plateau, not greatly different

from that of the substantially dry material during corresponding seasonal conditions.

The same type of behavior was observed for

all specimens without dense concrete decks un- derneath and such constructions were considered to be generally satisfactory for practical use except for those specimens containing insulations of high permeability to water vapor and

of low absorptive capacity. Changes in effective

thermal resistance of the latter with changes in

seasonal exposure were extreme, and the level of resistance observed for winter was considered unsuitable for practical use. Specimens of some insulations used without

dense concrete decks, initially at appreciable

moisture contents, attained high values of

effective thermal resistance in one or two alternations of the imposed summer and winter ex- posure periods. In contrast, for specimens of

similar insulations, also initially appreciably

moist, placed on dense concrete decks with or without a sheet vapor barrier, the effective

thermal resistance remained at substantially

their original low values throughout the same

exposure periods.

The fact that some constructions had the

ability to self-dry through permeable undersurfaces under simulated summer exposure has

major practical significance. Even the best

built-up roofing will ultimately leak and should

a roofing leak develop it would easily be detected because no vapor barrier was used. Once

repaired, the roof would tend to recover its original insulating value due to self-drying and

expensive replacement of large areas of insulation and roofing would be eliminated. The

performance of suitable self-drying constructions is the subject of Part 11. The use of small pressure-relieving vents

through the roofing of specimens having moist

permeable insulations had no appreciable effect in reducing the effective thermal resistance of

17
the insulation. The vents used were sufficient to relieve internal pressures in pervious insulations, but were not ventilation channels

through which outdoor air might move as a

result of wind forces.

Changes of the indoor air to a lower temperature and relative humidity decreased the effec- tive insulating value during summer exposure

of specimens containing much moisture but had

much less effect on specimens of similar materials containing little moisture.

7. References

[1] Prestrud, Kristian K., Compact roofs (in Norwegian with English summary), Royal Norwegian

Council for Scientific and Industrial Research,

Building Research Department, Report No. 4, 69.024.3:693.55, (1951).

[2] Lund, C. E., and Granum, R. M., Principles affect- ing insulated built-up roofs. Engineering Experiment Station Bulletin No. 34, University of Minnesota Institute of Technology, (May 1952).

[3] Roof decks and built-up roofing. Federal Construction Council Technical Report No. 16, File

C, Building Research Advisory Board, National

Academy of Sciences—National Research Council

(August 1955).

[4] Gumpertz, Werner H., Insulation for flat roofs. Progressive Architecture, (December 1964).

[5] Hutcheon, N. B., Vapor problems in thermal insulation, American Society of Heating and Air Conditioning Engineers Journal Section, Heating,

Piping and Air Conditioning, (August 1958).

[6] Powell, F. J., and Robinson, H. E., Measurement

of effect of moisture on heat-transfer through in- sulated flat-roof constructions, Special Technical

Publication No. 312, Symposium On Methods of Testing Building Constructions, American Society

For Testing and Materials, (1962).

[7] Topping, C. H., Myhre, A. M., Warren, W. B., and

Allen, J. R., A study to improve bituminous roofs. Monograph No. 1, Building Research Institute,

National Academy of Sciences—National Research

Council, (1960).

[8] Joy, F. A., Premature failure of built-up roofing, Better Building Report No. 5, Engineering Experiment Department, Building Research, Pennsylvania State University, (1963).

[9] American Society of Heating, Refrigerating and

Air-Conditioning Engineers Guide and Data

Book, Fundamentals, (1967).

[10] Ballantyne, E. R., and Spencer, J. W., Temperatures of bituminous roof surfaces. Bulletin, American Society for Testing and Materials, No. 223

(July 1957, TP 135).

[11] Ballantyne, E. R., and Martin, K. G., Bituminous

roofs. Building Study No. 1, Division of Building

Research, Commonwealtli Scientific and Industrial Research Organization, Australia (1960).

[12] Cullen, W. C., Solar heating, radiative cooling,

and thermal movement—their effects on built-up

roofing, NBS Technical Note 231 (1963),

[13] Valore, R. C, Jr., Cellular concretes. Parts 1 and

2, Journal of the American Concrete Institute,

(May and June, 1954).

[14] Pratt, A. W., Heat transfer in porous material. Research (May 1962).

[15] Sereda, P. J., and Hutcheon, N. B., Moisture equilibrium and migration in building materials,

American Society for Testing and Materials Symposium on Material Behavior During a Fire Test, (1964).

[16] Whippo, H. M., and Arnberg, B. T., Survey and

analysis of the vapor transmission properties of building materials. Final Report No. 1, Engineering Experiment Station, University of Colorado,

(January 1955), also available as PB 131219,

from the National Technical Information Service, U.S. Department of Commerce, Springfield, Virginia 22151.

[17] Johansson, C. H., and Persson, G., Moisture ab- sorption curves for building materials, Byggmastaren, 17:311-314, Stockholm, Sweden (1946),

available as Technical Translation 747, by H. A.

G. Nathan, National Research Council of Canada,

Ottawa, (1958).

[18] Hansen, David, The process of drying, American

Society for Testing and Materials Symposium on Material Behavior During a Fire Test (1964).

[19] Standard Method of Test for Thermal Conductivity of Materials By Means of the Guarded HotPlate: ASTM Designation CI 77. [20] Standard Methods of Test For Water Vapor

Transmission of Materials in Sheet Form: ASTM

Designation E96-66T, Method A.

[21] Standard Methods of Test for Water-Vapor

Transmission of Thick Materials: ASTM Designation C355-64 (Desiccant Method).

18
TIME OF DAY

Figure 1. Daily air temperature variations used to simulate winter and summer exposure conditions, including a

solar heating period, for insulated flat-roof specimens.

LEGEND

C 4 8 12 SCALE H»LKKkJ INCHES

• THERMOCOUPLE

X TEMPERATURE CONTROL D HUMIDITY CONTROL ^ HEAT FLOW METER

Figure 2. Apparatus for measuring the effect of moisture on heat transfer using the heat-flow-meter method.

19
Figure 3. Apparatus for heat-flow-meter method with

the top chamber in a raised position. Fifteen specimens are visible with their attached heat-flow meters.

1. INSULATION SPECIMEN 8. VAPOR BARRIER

2. HEAT FLOW METER 9. STEEL FRAME SUPPORT

3. 5-PLY BUILT-UP MEMBRANE 10. DENSE CONCRETE , 145 pcf

4, MASTIC II. EDGE INSULATION

5. ALUMINUM ANGLE ABOUT PERIPHERY 12. THERMOCOUPLE

6. PLASTIC FILM WRAPPING 13 TUBING INSERT

7. ASPHALT EDGE SEAL 14. STEEL BLOCK

15. VENT TUBE

Figure 4. Construction and installation details of a typical specimen used with the heat-flow-meter method.

20
H 1 1 1 1 1 1 1 1 1 1 1 1 1 h

H 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 h

TIME OF DAY

Figure 5. Temperatures and top surface heat flux results for one day of a S2immer

exposure. , , ...

(a) Specimen No. 3-DTB was made from a 3-inch-thick structural concrete deck over which was

placed a vapor barrier and 1%-inch-thick glass-fiber roof insulation.

(b) Specimen No. 14-DT was made from a 3-inch-thick structural concrete deck over which was

placed a 5%-inch-thick cellular concrete insulation without a vapor barrier at the interface of the materials.
TIME OF DAY

Figure 6. Temperature and top surface heat flux results for one day of a winter exposure.

(a) Specimen No. 3-DTB was made from a 3-inch-thick structural concrete deck over which was

placed a vapor barrier and 1%-inch-thick glass-fiber roof insulation.

(b) Specimen No. 14-DT was made from a 3-inch-thick structural concrete deck over which was

placed a 5-11/16-inch-thick cellular concrete insulation without a vapor barrier at the interface

of the materials.

22
Figure 7.

TIME, WEEKS

Effect of moisture and exposure conditions on the effective thermal resistance of the

cellular concrete insulation of Specimen No.

during Phase 1.

"1

I

1 1 1 r

t t 1% 2% 7%

Water Added, Percent By Volume

J L

Sp«. 29 - DTOW

J I I I 10 IS —s,-HFlGURE 8.

TIME, WEEKS

Effect of moisture and exposure conditions on the effective thermal resistance of the

cellular concrete insulation of Specimen

29—DTOW during Phase 2.

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 I—S| -t— W| -I- S2 4- Sj 1—Wj—t-S^^

TIME, WEEKS

Figure 9. Effect of moisture and exposure conditions on the effective thermal resistance of the cellular concrete insulation of Specimen Ul —DOWV and the perlite-asphalt insulation of Specimen 32-DOWV during Phase 3.

Water Added, Percent By Volume

0.2 -

0.1-

n l I I I I I I I I L

^0 5 10 15 20 25 30 35 40 45

S|— W| -

I- S2 — Wz S3 ^^Wj-H

TIME, WEEKS

Figure 10. The effect of moisture and exposure conditions on the effective thermal resistance of the glass-fiber insulation of Specimen 17-

DTOW, and the wood-fiber insulation of Specimen 20-DTOW, during Phase 2.

23
1% 2% 7% e -Spec. 18

0. I — Woter Added, Percent By Volume Spec. 30 —

q

I I

\ \ \ [ \ \ \ \ L

5 10 15 20 25 30 35 40 45 S|^|^W|— Sj -I- W2 S3 J^VI^-^

TIME, WEEKS

Figure 11. The effect of moisture and exposure conditions during

Phase 2 on the effective thermal resistance, and the wetting

and self-drying effects on specimens made without dense

concrete decks. Specimen 18-0 contained glass-fiber insulation, Speci- men 21—O wood-fiber insulation, and Specimen 30—OW cellular concrete

insulation.

Figure 12. Comparison of the performance of vermiculite concrete in- sulation used with a dense concrete deck, Specimen UO— DOWV; and without a dense concrete deck, Specimen 37—

owv.

Figure 13. Comparison of the performance of cellular concrete insulation without a dense concrete deck, specimen 36-0 had no water

added to the initially oven-dried insulation, while Specimen 46-OWV did.
Part II. Self-Drying Designs

8. Introduction

In Part I it was found that roof specimens

whose undersurfaces were of a moderately

vapor-permeable nature tended to dry out significantly during simulated summer exposure

conditions, and to recover nearly all of the insulating value expected of them in a dry condition, even if they had been excessively moist.

Also, the moisture accumulated during winter

exposure was reasonably low. On the basis of

these findings, it was reasoned that the most

practical and economical solution to the problems of moisture in insulated flat-roof constructions was to provide a design that would have

in-service self-drying characteristics, should it become wetted for any reason. Since it is virtually impossible to keep moisture out of building constructions, the basic assumption was

j made that a construction will be wetted at

I some time during its service life; for example,

I from initial construction moisture or from a

I roofing leak. If the roofing leaks and the con-

\ struction possesses self-drying characteristics,

j all that would seem necessary would be simple

patching of the roofing, as compared to some

designs that have required costly replacement

of roofing and insulation with no assurance of

I non-recurrence in the future. Should the roofI ing remain water-tight for relatively long peri1 ods of time, as is probable, the construction

' should also be capable of absorbing and retaining without physical damage moisture that

slowly accumulates over a winter. In the spring

and summer the winter-gained moisture should

be expelled by utilizing the heat of the sun on

the roof. Thus, the insulating integrity of the

roof construction could be maintained yearround.

In order to obtain information on the heat

transfer and self-drying characteristics of se- lected types of insulated flat-roof constructions an experimental investigation was undertaken.

The work was organized to provide a systematic method and procedure for experimental

study of the factors that influence the in-service

self-drying performance of particular types of

insulated roof constructions. Designs that utilize formboards, precast channels, slabs, or

tiles were selected because they allow considerable control in connection with insulating

values and moisture passage through their undersurfaces. The factors that appeared to influ- ence the wetting and drying rates and insulating value of an insulated roof construction were

the exposure conditions of temperature and

relative humidity, the arrangement and dimensions of the components of the construction,

!i and physical properties such as thermal conductivity, vapor permeability and moisture absorbency. Materials for specimens were selected

to provide as broad a range of each physical

property as was available from commercially

obtainable products. The designs of most specimens were chosen so that they could easily be

adapted for use on buildings.

The method of measurement and apparatus

used in Part I of this paper was considered in- adequate for Part II because it did not allow

weight measurements during the heat transfer

tests. A new apparatus was designed and built

that would allow simultaneous weight and heat

transfer measurements on individual specimens

during their exposures. The new equipment also

provided better control, and increased flexibil- ity and convenience of operations. A new calorimetric technique for measuring heat flux over

the whole of the specimen area was devised and

incorporated in the new apparatus.

Part II gives: the experimental exposure

conditions used; a brief description and a ref- erence for the method of measurement and

apparatus ; a tabulation of the specimens, materials, and their physical properties; the test procedures ; results in graphical form ; a discussion of meaningful results; proposed criteria

for evaluating insulated self-drying designs for

flat-roof constructions; and a summary. Exposure tests spanned a total of six years and in

all 27 specimens were used, each 22 x 23 inches

in plan. Specimens were subjected to successive

periods of simulated summer and winter out- door surface temperature conditions, each with

simulated daily solar heating of the roof surface. In some cases simulated spring and fall temperature conditions were used. The tests in- cluded exposures of specimens initially at a moisture content commensurate with that of a

roof when roofing is normally applied and when

wetted in simulation of a roofing leak.

9. Experimental Exposure Conditions

and Method of Test

In Part I it was shown that use of temperatures that simulate in-service exposure is nec- essary for proper evaluation of insulating value,

especially under summer conditions. In fact,

many of the findings would not have been determined had steady-state temperature exposures been used. In the method of measurement

used in Part II, a water-cooled metal calorimeter plate was used in place of roofing on a specimen. Steady water temperatures in the

plate were adjusted to approximately the val- ues observed at the underside of the roofing in

25
the tests using the method in Part I. The calorimeter plate was heated or cooled by water

whose temperature was regulated in accord- ance with the daily cycles shown in figure 14. Temperatures of 75 and 138 °F were used for

simulated summer night and day conditions, re- spectively, 56 and 106 °F for simulated spring

and fall season, and for simulated winter conditions temperatures of 38 and 75 °F were

used. Also indicated in figure 14 is the indoor

air temperature used, 90 °F. The indoor air

condition beneath the specimens was maintained at 90 °F and 30 percent relative humidity (0.43 in. Hg water vapor pressure and 54.5

°F dewpoint), which corresponds in respect to water vapor pressure to an air-conditioned en- vironment of about 75 °F and 49 percent relative humidity, which is considered comfortable

and typical for buildings of average occupancy.

During the course of the program, the periodic

time-temperature wave-forms, figure 14, were

temporarily changed for several weeks to constant temperatures of 138 °F for summer and

38 °F for winter, for observing specimen performance under steady-state conditions.

For the calorimeter method of measurement,

8 specimens 22 by 23 inches in plan were placed

in an insulated room, whose temperature and

relative humidity were controlled. See figure 15. Each specimen was mounted horizontally on a weighing scale, sensitive to 0.01 pound, for determination of changes in moisture content as

the tests progressed. When desired during a

test of a specimen, a measured amount of water

could be admitted to the specimen through a

small hole and tube in the center of each calorimeter plate. Heat gain to the calorimeter plate was determined by measuring the temperature

change of the water as it passed through the

plate and the mass flow rate. For this purpose

a 10-fold differential thermocouple was used at the calorimeter plate inlet and outlet, and at the same time the water discharged from the

plate was collected for one minute. The top and

sides of each specimen and calorimeter plate

assembly were moisture-sealed and surrounded

with guarding insulation. The heat gain of the

plate, during the substantially steady-state condition preceding the simulated solar heating

part of the daily cycle, included both the heat

flow through the specimen and that through the

guarding insulation. To determine the latter

and enable a correction for it to be made to the

calorimetric measurement, a coefficient was dedetermined by separate experiments to express

the heat transfer to the plate through the

guarding insulation per unit of temperature

difference between the plate and the top surface

of the guarding insulation. The details and results of the two calibration procedures used are given in reference [6], of Part I. The effective

thermal resistances of components of each specimen were obtained by dividing the appropriate

observed temperature differences, as indicated

by the thermocouples within the specimen, by

the average specimen heat flux determined from

the calorimetric measurement. Details of the

apparatus are given in reference [6].

On two specimens heat flow meters were

bonded to the underside of the specimen to determine the heat flux at these surfaces. Also,

the electrical resistance across layers of component materials of several specimens was

measured by means of a bridge connected to the copper leads of the thermocouples to indi- cate wetting or drying in the layers as the

specimens were subjected to the various sea- sonal exposure conditions.

Upon completion of performance tests, samples of component materials from all specimens were used for physical property determinations.

The dry thermal conductivity was determined

using ASTM Method C177 [19], and the water vapor permeability was determined using

ASTM Method E96, [20]. Hygroscopic properties of some materials were determined by

exposing the material to an environment of 75

°F and 90 percent relative humidity until the

specimens achieved constant weight. The specimens were then oven-dried and weighed, en- abling calculations of the percent hygroscopic

moisture content by weight.

10. Specimens and Procedures

A schematic drawing of a typical specimen

is shown in figure 16. Materials used in each

specimen are listed in table 3. Each specimen

was 22 by 23 inches in plan. Most specimens were designed to provide a calculated thermal

transmittance (U-value) of about 0.12 Btu/hr

ft- deg F on the basis of published values of

the thermal conductivity or thermal resistance

of the dry materials. Thermocouples were ce- mented or cast in place, as shown in figure 16. The calorimeter plate was placed on the top of

the specimen and edges were sealed with 1/32-

inch-thick neoprene rubber sheet bonded in

place with a rubber adhesive. A thin, soft blanket of glass fiber insulation was used as a gasket between sections of the board-type expanded

polystyrene guard insulation. The guard insulation was held in place by steel band strapping

tensioned by wedges around the periphery of

the specimen. All joints were sealed with mastic, covered with tape, and painted with a rubber-base paint paste. The water-feed hole

through the center of the calorimeter plate was

connected to plastic tubing that penetrated the

top piece of guard insulation. Before assembly, a 1-inch square hole i/g-inch deep was made in

the center of the top surface of the specimen as a well for receiving water admitted through

the drill hole. In table 3, specimens numbered

26
16 and 22 contained three layers of an absorbent cotton muslin scrim placed between component materials and directly beneath the calorimeter to act as a water-distributing wick.

Differential thermocouple wells were made from

brass pipe nipples and tees and were insulated

and vapor sealed. The assembled specimens were supported by slotted phenolic plastic tubes on the platform scales, and water connections

to the calorimeter were made using flexible rubber tubing.

Specimens were designed for the most part

to represent practical constructions thought

likely to possess in-service self-drying characteristics when installed containing initial construction moisture, or when wetted during their

service life. In general, most specimens were

selected to be compatible with the construction

technique of permanent formboards between

steel subpurlins (bulb tees). The formboard or deck materials can be grouped as precast channel elements, slabs or tiles, and formboards.

The materials listed in table 3 include two

types of concrete made with portland cement:

structural concrete (Specimens 1, 2 and 3) ; and

insulating concrete (Specimens 4 through 10,

14 through 17, and 19 through 24) . Other materials were: gypsum concrete (Specimens 11, 12,

18, and 25 through 27) ; formboards made from

gypsum (Specimen 4), cement-asbestos (Specimen 6), wood-fiber (Specimens 8 through 13),

glass-fiber (Specimens 14 through 18) ; and in- sulations made from mineral wool (Specimens

1, 19, 22, 25 and 26), wood fiber (Specimens

3, 12, 13, 20 and 27), and expanded polystyrene

I plastic (Specimens 7 and 21). The expanded

shale concrete slabs were cured in molds for one day, in a damp room for three days, and

allowed to dry in the laboratory out of their

molds for about two weeks before fabrication

into specimens. These one-inch-thick specimens were intended to simulate the web portion of a

prefabricated concrete channel roof deck. All

insulating concretes were cast in a separate

mold or directly on top of formboards in a mold and were allowed to cure for one day in

the mold, three days in a damp room, and to dry in laboratory air for three days out of the

mold before fabrication into a specimen and

installation in the apparatus, except for Speci- mens 16 and 22, which were air-dried for several months before installation. Gypsum concrete for Specimens 25, 26, and 27 was obtained

from a local contractor during the installation

of this material in an insulated roof deck of a new building. The gypsum concrete was al- lowed to cure in a form for one day and was

laboratory-air-dried for one day before installation in the apparatus. Other gypsum concretes were hand-mixed in the laboratory and cured

in the same manner as the field-obtained material. Strength tests were made on cylinders or cubes of each concrete batch to assure that the

specimens were representative of materials that

would meet specifications for field use. All other

materials and insulations used were taken di- rectly from their cartons after storage in the

laboratory for several weeks.

Specimens were installed with materials in a moisture condition similar to what might be

expected shortly after roofing had been applied

in the field. The investigation was conducted by

subjecting specimens to alternations of periods

of exposure simulating winter or summer temperature conditions as indicated on figures 17

through 43. In some cases exposure conditions

of spring and fall were also used. Each period

consisted of repeated 24-hour cycles of the appropriate daily temperatures to simulate nighttime and solar heating conditions, as indicated on figure 14. Heat-flow, temperature, and weight

measurements of each specimen were recorded on working days within the two-hour period

prior to the start of the simulated solar heating

part of the daily cycle. At selected times data were taken for a full 24-hour period to observe

the patterns of heat flow and temperature

change and also several calorimeter calibration

check tests were made using the specimens un- der test. The seasonal periods were of several

weeks' duration each, varying in length as nec- essary to observe apparent trends in the insulating value and weight change of a majority

of the simultaneously exposed specimens.

The exposure periods were aimed at different

objectives. An initial summer exposure period was used to observe the insulating value and

the ability of a specimen to self-dry when it contained construction moisture. A first winter

exposure period was used to observe the change

in heat transfer and the rate at which a specimen gained weight. Spring and fall exposure

periods were used to determine whether drying

or wetting would occur in a specimen during

these seasons. A second summer exposure was

used to determine how rapidly the specimen

would expel winter-gained moisture and after

simulating a roofing leak, for observation of

subsequent drying rates. At selected times in- door surfaces were spray painted to determine

whether painting affected the rates of drying

or wetting. Some thin formboards were re- moved during exposure to determine whether

the drying or wetting rates would be significantly affected. The electrical resistance across

layers of the component materials in ten of

the specimens was measured daily as a part of

each test to indicate the migration of moisture

resulting from wetting or drying of the components.

At the conclusion of its exposure, each specimen was dismantled and the moisture content

of its component materials was determined by

drying to constant weight. Samples of exposed

materials, 8 inches square and 1 inch thick were used for determining the thermal con27
(11) Final moisture content %

Above

dry

weight

14.8 3.0 3.9 3.217.2 2.3

26.0 24.8 15.5 29.1 56.9 0

31.8 13.4 5.1 5.432.5 13.4

34. 8

(comb.)

(OT) Specimen drythermal resistance

Btu/hr

IV

Rd 6.96 8.29 7.37 8.17 4.98 4.73 7.27 8.02 7.48 5.89 8.08

Dry

thermal

resistance

Btu/hr

ft'

6.73 0.23 8.06 0.23 7.14 0.23 7.69 0.48 4.50 0.48 4.67 0.06 3.70 3.57 3.85 4.17 3.57 3.91 2.04 3.85 1.03 7.05

00 Specimen

permeance Grains

U) X

0.81 0.80 0.78 1.74 0.90 0.83 0.96 3.0(2.77) 3.07(3.47) 1.59(1.58) 5.03

Water

vapor

permeance Grains

to X

4->U

25.3

0.84 19.20.84

11.5

0.84 1.85 29.40.93

27.0

0.57 0.8 4.9 1.2 3

97

12.31(9.17)

4.87

14.91(12.03) 1.813.87(13.0) 13.0

8.22

Water

vapor

permeability Grains

g

to

tEl

44.3

0.84 48.00.84

28.8

0.84

11.1 14.7 6.3

13.5 4.0 0.214.7 1.2

11.9 20.0(14.9) 14.6 23,3(18.8) 5.4

20.8(19.5) 26.0 22.6

Properties

—

Oven

Dry

Thermal

conductivity Btu 0.26 4.26 0.31 4.26 0.35 4.26 0.78 1.04 1.50 1.04 1.50 4.13 0.81 0.28 0.78 0.39 0.84 0.40 1.47 0.39 1.95 0.39

Densitylb/ft3 P 12.6 98.6 16.3 98

.6

14.2 98.6 31.6 50.4 40.1 50.4 40.1

109.8 34.4 2.0

31.6 18.9 33.4 19.5 41.1 18.9 56.7 18.1

TO Thickness inches \*\^\^

1 1 ^ "^rt \-"j^^ c-N? «w cc ^V^^ 1 «c»^ n, <n " •HC^N ^ ^ 0^

Materials

Glass-fiber

board

Expanded

shale

concrete

Mineral

wool

/binder

Expanded

shale

concrete

Wood-fiber

board

Expanded

shale

concrete

Perlite

concrete

Gypsum

formboard

Vermiculite

concrete

Gypsum

formboard

Vermiculite

concrete

Cement-asbestos

board

Perlite

concrete

Expanded

polystyrene

Perlite

concrete

Wood-fiber

formboard

Perlite

concrete

Wood-fiber

formboard

Vermiculite

concrete

Wood-fiber

formboard

Gypsum

concrete

Wood-fiber

board

Specimen number

28
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ductivity in an oven-dry condition, using the

guarded hot-plate method (ASTM C177), and

additional samples, 414 inches in diameter and

1/2 inch thick, were used for measurement of

water vapor permeability, using the dry cup

method of ASTM C 355.

1 1 . Results

Results of the investigation are presented in

Figures 17 through 43 and in tables 3 and 4. Several sets of curves showing the behavior

of each specimen during selected 24-hour periods were obtained. These were similar in shape

and magnitude to figures 4 and 6 (Part I) and

a sample is shown as figure 44.

Figures 17 through 43 show graphically the

heat transfer, moisture content, and, for ten

ST3ecimens, electrical resistance results. The

heat transfer data are presented as thermal

resistance values (reciprocal of thermal con- ductance) for the components of the specimen

and the specimen as a whole. In some cases the

thermal resistance of the air film from the in- door surface to the air beneath a specimen is also plotted. Thermal resistance is plotted

against time in weeks for the duration of the

tests. Also plotted against the same time scale are: the moisture content at the end of each

week in terms of pounds per square foot of roof area and also as percent of specimen dry weight

;

and the electrical resistance of each layer for

ten of the specimens. In the tests when the

electrical resistance in ohms exceeded 10'^, this

value was plotted in the figures, since only

values lower than this became meaningful in

the investigation. Each value of thermal resist- ance plotted is the average of the daily values

obtained during each week of the tests, usually

five in number. In each of these figures, the

thermal resistance of each specimen in a dry

condition is indicated by a horizontal dashed

line labeled "Design-R-Dry." This value of re- sistance was calculated using thermal conductivities determined by guarded-hot-plate tests on dry samples of the materials of each specimen obtained after their exposure in the apparatus. Thus, the figures show a comparison

between the thermal resistance of the dry

specimen and that of the specimen at various

moisture contents observed experimentally during the several seasonal exposure periods. The

electrical resistance indicates the location of

the bulk of the moisture in the specimen and

the downward or upward trend of the curve

indicates whether the specimen was wetting or drying, respectively, as a result of its exposure.

Table 3 lists the specimens and their component materials and their thicknesses (x) and

the results of density (p) , thermal conductivity

(k), and water vapor permeability (/jl) determinations on each component material of the

specimens. From these determinations dry

values of thermal resistance for each component material (r) and each specimen (Ra) and

the water vapor permeance for each component

(p) and each specimen (P) were calculated

using the relationships r = x/k, Ra = Ti + rs, p = fi/x, and 1/P = 1/pi + I/P2. Also listed in

table 3 is the moisture content of each material

as determined at the conclusion of its final ex- posure period by oven-drying the materials. In

table 3 values of water vapor permeability and

permeance given in parentheses for Specimens

8, 9, 10, and 20 are values obtained for samples

of the materials that contained an asphaltic

binder for water resistance which were not

used in the exposed specimens.

From the results obtained, tentative criteria were developed for evaluating the insulating

and self-drying abilities of an insulated flat- roof design. Table 4 lists for each specimen

tested the results of the application of the criteria. Figure 45 illustrates in a general way

typical performance characteristics of a self- drying design and includes the items listed as column headings in table 4. In table 4, Rt is the average thermal resistance of the specimen

over the total test period obtained by averaging

weekly test values of tliermal resistance over a

period totalling 37 weeks. Ra is the dry thermal

resistance as calculated on the basis of test re- sults for the thermal conductivity of the dry

components as given in table 3. The fifth column

in table 4 lists the ratio Rt/Rd- The sixth col- umn lists the drying time, in weeks, that was

required to expel initial construction moisture

from the specimen by self-drying under sum- mer exposure conditions while the seventh col- umn states whether the specimen reached a

defined moisture content equilibrium within 16

weeks. The eighth column gives the hygroscopic

moisture content at 75 °F and 90 percent relative humidity in percent by weight, as taken

from the literature [16,17] or estimated from

tests on these materials at the National Bureau

of Standards. Column nine gives the calculated

quantity of hygroscopic moisture present, Mc,

for each specimen in pounds per square foot.

Column ten gives the quantity of moisture present, Mt, when the specimen was at a moisture

content equilibrium during the tests. The ratio

Mt/Mc is given in column 11. Column 12 gives

the summer drying time in weeks to expel moisture accumulated during the specimen's previ- ous winter exposure. Column 13 gives the sum- mer drying time for expelling moisture after

the specimen had been wetted in simulation of a roofing leak. Column 14 gives the ratio of the

specimen permeance, P, from table 3, to the

specimen hygroscopic moisture content, Mc.

Column 15 gives the average rate of moisture

gain of the specimen during exposure under

winter test conditions in lbs/ft- week. A discussion of the criteria is given in section 12.6.

32
12. Discussion of Results

In this section the principal findings are given separately as subsections and the performance of those specimens that illustrate the

findings will be discussed, using the figures and

tables. Other specimens that also illustrate the

point will be mentioned but not discussed in

detail. As in Part I, daily determinations of thermal

resistance of the specimens and their components were made just prior to the start of simulated solar heating. Several times during the

investigation data were taken to show the heat

flux and temperature history over a 24-hour

period. The results of these several tests are

similar to the curves shown in figure 44, and in

figures 5 and 6 of Part I, and hence will not be

presented in detail here. The curves show that

all temperatures and the heat flux through the

upper surface of the specimens were nearly

constant for a period of two hours before the

start of the simulated solar heating. Therefore,

observations made during this time period

yielded thermal resistance data for a condition

approaching as nearly to an ideal steady-state

condition as was possible within the restrictions of the imposed 24-hour periodic temperature cycle. Values of heat flux and temperature

during the solar heating and the cooling portion

of the daily cycle were clearly transient and

would not have yielded a meaningful thermal

resistance. The daily and weekly changes of

thermal resistance were considered to be a measure of the effect of moisture on heat transfer through the insulated flat-roof specimens.

In order to maintain reasonably high thermal insulating values in summer and winter,

the solution to the problems arising from the

presence of moisture in insulated flat-roof construction, as proposed in the introduction, was

to use a deck or formboard material that would

allow passage of water vapor through it and

thus foster self-drying of the wetted construction to the indoors as promoted by natural solar

heating of the outdoor surface in summer. In

winter the deck or formboard material must

also inhibit the accumulation of moisture in the

construction resulting from condensation of

water vapor transferred from indoors. The re- sults obtained indicate that the above solution can be considered feasible for several designs

of practical insulated flat-roof decks provided

the indoor vapor pressure is not excessive and

is of the order of that found in normal-occupancy buildings (75 °F, 50% humidity).

12.1. Effect of Moisture on Insulating Value

Appreciable amounts of moisture in insulated

flat-roof constructions reduced their thermal in- sulating value under both winter and summer

exposure conditions. The magnitude of the effect of moisture on insulating value was influ- enced by the quantity of moisture present in the construction, its location, and the arrangement and volume of the component materials in

the construction, as well as their properties of

moisture hygroscopicity and absorbency, thermal conductivity and water-vapor permeability.

In figure 17 the thermal resistance during

winter and summer exposure periods of Specimen No. 1, when it contained about 0.1 lb per sq

ft of moisture, was about equal to that of a dry

specimen. The thermal resistance of the specimen and its insulation decreased markedly, to about 31 percent of their dry values, when water was added to the insulation, during week

number 33, in simulation of a roofing leak.

Resistances remained low as this summer ex- posure period continued. With continued ex- posure, the moisture content of the specimen

gradually decreased from about 1.15 lb per sq

ft to an equilibrium moisture content of about

0.5 lb per sq ft without a significant change in

the specimen's insulating value. Also, during

weeks 42 to 45 of this exposure period, the calorimeter plate was operated continuously at 138

°F, causing a steady temperature gradient from

the top to the bottom of the specimen. Such

operation during this time period did not significantly affect the rate of drying of the specimen but did cause an increase in its thermal

resistance to a value about 60 percent of that

of a dry specimen. During the following winter

exposure period, the thermal resistance of the

specimen, when it contained about 0.5 lb per

sq ft of moisture, was about 91 percent of that

of a dry specimen. The specimen gained little weight during its winter exposure periods, even when the calorimeter plate was operated con- tinuously at 38 °F for 9 weeks (weeks 52 to 61), demonstrating the resistance to watervapor transfer of the expanded shale aggregate

concrete deck used for this test whose water

vapor permeability was measured after the

test and is given in table 3 as 0.84. Water vapor

permeance is expressed in a unit (perm) defined as the number of grains (7000 per pound)

of water vapor transferred per hour over an

area of one square foot with a water vapor

pressure difference of one inch of mercury

across the specimen.

The insulating performance of the insulation

of Specimen No. 1, figure 17, was very similar

to that of specimens numbered 1-DT, 2-DT,

16-DT, and 17-DTOW, in table 2, Part I. The

constructions of these specimens differed only

in the type and thickness of their concrete

decks. Also, the insulating performance of

Specimens 2 and 3 was similar to that of Specimen 1, where each specimen had in common a

one-inch-thick expanded shale aggregate concrete deck but had different types and thick- nesses of insulations over the deck. Further

examination of the figures and table 3 shows

33
that for most moist specimens whose overall

permeance to water vapor was less than 2

perms a poor insulating value was evident for

long periods of time. See figures 17 through 23,

and 39. For Specimens 1, 2, 3, 6, and 7 this can

readily be attributed to the low permeance of

the deck material. However, Specimens 4 and

5 contained a highly permeable deck material

but poor insulating value resulted because of

the high moisture content of the thick concrete over the deck, as was the case for Specimen 23

that had no deck at all, although a trend of

slowly increasing insulating value with time

is evident. It is interesting to note that for

moist specimens containing a low permeance

deck material, and a permeable low conductivity insulation over it. Specimens 1, 2, and 3,

there were relatively large changes between the

summer and winter values of specimen thermal

resistance while those specimens that had comparatively high moisture contents. Specimens

4, 5, 6, and 23 showed relatively little change

between summer and v/inter thermal resistance

regardless of the permeance of the deck material.The long-term impairment of insulating

value of the above specimens, when wet, was

attributed to the high resistance to moisture

passage of the over-all specimens and especially

their decks. During summer exposure, moisture

in the specimens and especially in the permeable insulations was free to migrate in response

to the twice-daily reversal of temperature gradient and thus was kept distributed and available for latent heat transfer. In the winter

exposure condition there was no daily reversal

of temperature gradient and highly permeable

insulating components allowed moisture to migrate to the cooler regions of the insulation,

which resulted in improved specimen insulating value. For hygroscopic and moisture absorbing materials the migration was not as complete resulting in a thicker layer of wetted

material that allowed the specimen insulating

value to remain relatively low. When a nonreversing temperature gradient was applied

(steady-state) during both winter and summer

exposure conditions, moisture tended to migrate and accumulate in the cooler regions of

the insulation, resulting in a higher over-all

specimen thermal resistance and a greater in- sulating effect. The construction of Specimen 19, figure 35, was such that the deck material, perlite aggregate concrete, was moderately permeable to water vapor, 4.5 perm, as compared to the

denser concrete decks of Specimens 1, 2 and 3,

0.84 perm, and the 3-inch-thick structural concrete decks used in Part I. The perlite concrete

constituted about one-third of the specimen's

dry thermal resistance. The top-layer material,

glass-fiber insulating board, was relatively high

in permeance and thermal resistance but low in

hygroscopicity and moisture absorbency and of

lesser thickness than the deck material. The

initial moisture contents of Specimen 1, figure

17, and Specimen 19, figure 35, were approximately the same, about 0.5 Ib/sq ft, but the

moisture content of Specimen 19 was greater

after water had been added in simulation of a

roofing leak, about 1.15 and 1.8 Ib/sq ft, respectively.

The thermal resistances of both Specimens 1 and 19 were about equal to the dry values for

all exposure periods when their moisture contents were less than about 0.5 lb per sq ft. After

water had been admitted to both specimens in

simulation of a roofing leak their thermal re- sistances decreased considerably. Specimen 19

also showed a rapid decrease in moisture content even though it was first subjected to spring

season roof-side temperatures of only 106 °F

before its summer exposure, and its thermal

resistance gradually increased until insulating

value was fully recovered, while the resistance

of Specimen 1 exposed at steady roof-side temperatures of 138 °F remained low and did not

show a trend toward recovery. This finding is considered to have major practical value even though the more rapid expulsion of moisture

from Specimen 19 would constitute a considerable short-term latent heat load to the room

beneath the roof construction which would have

to be dissipated by ventilation or an air-conditioning system. The recovery of insulating value

of Specimen 19 was attributed to the increased

permeance of the perlite concrete deck, which

allowed drying of the wetted specimen.

Results similar to those of Specimen 19 were

obtained for Specimen 20, figure 36; Specimen

21, figure 37 ; Specimen 22, figure 38 ; Specimen

23, figure 39; and Specimen 24, figure 40; all containing a perlite concrete deck. The insulation of Specimen 20, wood-fiber board, was

hygroscopic and showed a lesser depression of

thermal resistance with approximately the same

time to recover its insulating value. The insulation of Specimen 21 was nonabsorptive to water

and the large amount of water added was con- tained chiefly in the perlite concrete. The construction of Specimen 22 was the same as for

Specimen 19 except that a cotton muslin scrim was used between the materials and beneath

the calorimeter plate in Specimen 22 to assure uniform lateral distribution of added water.

The rate at which the insulation recovered its insulating value was more rapid and uniform

initially for Specimen 19 and was almost sudden for Specimen 22 after several weeks at low

insulating value while wet. The reason for delay in recovering insulating value in Specimen

22 was attributed to the wetted scrim on each

surface of the glass-fiber insulation which

caused a smaller temperature difference across the insulation as water was evaporating from

the scrims. When the scrims were dried suffi34
ciently, especially the scrim at the interface,

the insulating- value of the glass-fiber insulation rapidly rose to dry values.

Specimens 23 and 24 were made from perlite

concrete only. Note that Specimen 23 was a

six-inch-thick layer of perlite concrete that had

a high moisture content, about 3.3 Ib/sq ft, when its test began. The distribution of the

moisture in the specimen is indicated by the

electrical resistance curves for each 2-inch-thick

layer of its total thickness as shown at the top

of figure 39. The bottom layer of the specimen

had less moisture than the middle or top layer

but as drying proceeded during the first spring

and summer exposure periods the electrical re- sistance of the top layer increased, indicating

that it too was relatively much more dry when

compared with the middle layer. In fact, the

electrical resistance of the middle layer did not

increase substantially during all exposure periods, indicating that the middle layer of the

specimen remained moist regardless of exposure. Evidence of a moisture band in the middle is further indicated because its thermal resist- ance values are lower as compared with the

values for the top and bottom layers of the

specimen. In figure 39 during the second winter

exDOSure period, week numbers 28 through 34,

moisture was migrating from the middle layer

to the top layer as indicated by a decrease in

electrical resistance for the top layer as shown.

When water was added during the second

spring exposure period the electrical resistance

decreased to a low value for the top and middle

layers but decreased only slightly, and temporarily, in the bottom layer. These changes were

accompanied by similar decreases in thermal

resistance for each respective layer. The specimen gradually recovered its thermal resistance

and showed a considerable reduction in moisture content during the second spring and sum- mer exposure periods. The value of the thermal

resistance of the specimen when it was most dry

during the tests was about 4 °F/(Btu/hr ft^).

A similar history of exposure was obtained

for Specimen 24, figure 40, but note that a central wet band was not as apparent when compared with the performance of Specimen 23.

The thermal resistance of the three-inch-thick

Specimen 24 at its lowest moisture content was

about 2.6 °F/(Btu/hr ft^). Doubling the thick- ness of insulation apparently did not double

the thermal resistance for this material be- cause of the considerable additional amount of

moisture present in the thick specimen. Specimen 23 achieved as a maximum approximately

73 percent of its dry insulating value while

Specimen 24 achieved a maximum of about 90

percent.

The construction of Specimen 9, figure 25, was such that the deck material, wood-fiber

insulating board, was permeable to water vapor, 14.91 perm, and constituted about half of

the specimen's thermal resistance when dry.

The top layer of the specimen was perlite aggregate concrete. The permeance of the insulating concrete layer was lower, 4.87 perm,

than that of the wood-fiber formboard but its thermal conductivity, thickness, and waterholding capacity were higher. Thermal resist- ances, as plotted in figure 25, changed relatively

little with changes of exposure conditions from

winter to summer or with substantial changes

in specimen moisture content, when compared

with other specimens tested. In view of the

greatly reduced insulating values observed over

long periods of time for specimens tested under

summer conditions containing wetted insulation over relatively impermeable decks, the stability

of the insulating performance of Specimen 9 was a definite practical improvement.

Figure 25 shows that Specimen 9 was drying

and at the same time providing a reasonably

fair insulating value, although not as much as expected for dry materials. The insulating val- ues shown in figure 25 were lower than dry

values because the specimen contained some

moisture throughout the tests. A sudden in- crease in moisture content, when water was

added at week number 36 to simulate a roof

leak, caused a considerable percentage decrease,

about 60 percent, in the insulating value of the

perlite concrete but little percentage change,

about 3 percent, in the thermal resistance of

the wood-fiber formboard. The performance of

the wetted specimen, from week 57 onward,

appeared substantially similar to its performance during weeks 6 to 15, when its moisture

content was about the same. The electrical re- sistance plots indicate that most of the moisture present in the specimen was contained in

the perlite insulating concrete. Similar results were obtained for Specimen

8, figure 24, and Specimen 10, figure 26. For

Specimen 11 the wood-fiber insulation was 2%

inches thick and constituted about 87 percent

of the specimen's insulating value. Figure 27

shows that the insulating board was more sensitive to change when wetted during exposure as compared to Specimens 8, 9, and 10, but it too was remarkably stable when subjected to changes of seasonal exposure conditions. The

performances of Specimen 12, figure 28 and

Specimen 13, figure 29, were, in general, similar to that of others that employed a wood-fiber

insulation as a formboard.

The construction of Specimen 15, figure 31, was such that the deck material was very permeable to water vapor as compared to the decks

of other specimens, and the deck constituted the

major portion of the thermal resistance. The

top layer of material, perlite aggregate concrete, was of moderate permeance to water

vapor, and had a higher thermal conductivity

and moisture absorptancy, and a greater thick35
ness, than the deck material. Specimen thermal

resistance decreased but not seriously during

the first spring-summer-fall exposure conditions

when compared with the first winter exposure,

with initial distributed moisture present. The

thermal resistance decreased rapidly when the

specimen was wetted in simulation of a roofing

leak during the second spring exposure period,

but the loss of insulating value was regained

rapidly. When wetted the specimen thermal re- sistance decreased from a value of about 7 °F/

(Btu/hr ft^) to about 3.2 and then increased

to about 6 within a week's time and remained

stationary while the specimen was gradually

drying. The value of the thermal resistance

again reached 7 °F/ (Btu/hr ft^) when exposure to the second summer condition commenced. The moisture content and electrical

resistance performance curves also indicated

rapid drying during these time periods. The

drying curve shows large changes in specimen

moisture content from about 4 to about 1.75

lb/ft- with relatively little change in specimen

thermal resistance during the spring exposure condition when roofing-side temperatures

reached only 106 °F during the simulated solar

heating portion of the daily temperature cycle.

An explanation for this desirable result is that

the glass-fiber formboard was quite permeable

to water vapor and of low conductivity.

Due to its comparatively low thermal resist- ance, the entire volume of the insulating concrete top layer during exposure to simulated

solar heating was raised substantially in temperature. The concomitantly increased water

vapor pressure in the moist permeable concrete

promoted vapor escape through the permeable

deck material, and thus effected rapid drying of

the specimen. During the early morning hours,

when the near steady-state heat flow and temperature measurements were made, the moisture in the concrete was distributed in the

specimen as a result of the daily reversals of

temperature gradient. The temperature at the

interface of the materials was initially near the

temperature of the calorimeter (75 or 56 °F)

because of the lowered thermal resistance of

the moist concrete. Thus, initially the concrete

had a small temperature difference across it, while the glass-fiber formboard had a higher

temperature difference across it. Heat flowed

from the undersurface of the specimen through

the formboard to the underside of the insulating concrete, which at that stage was relatively

moist, and there a part of it went to evaporate

moisture, which passed freely from the specimen through the permeable formboard to the

indoor air. Only a part of the heat reaching the

insulating concrete remained to be conducted

through it to the calorimeter plate and measured. As the vaporization and drying progressed, the temperature at the interface of

the materials increased, yielding a gradual increase m the temperature difference and thermal resistance of the concrete. The thermal

resistance of the glass-fiber decreased slightly,

because its temperature difference decreased as the interface temperature increased. The total

temperature difference across the specimen re- mained relatively unchanged, as was the heat

flow at the calorimeter. Thus, the specimen was able to maintain its insulating integrity

while expelling a considerable amount of moisture. Similar results and behavior are shown

in figures 30, 32, 33, and 34 for glass-fiber

formboard of different thicknesses and for

other insulating-type concretes. During summer exposure periods, especially

when water was added in simulation of a roofing leak and the specimen was drying rapidly

(notably Specimens 17 and 18, figures 33 and

34), the observed temperature drop across the

insulating concrete approached and became

substantially zero, and the whole temperature

drop at the time of observation occurred across

the formboard. At the same time the heat flow

to the calorimeter through the specimen decreased to practically zero. Thus, the concrete

appeared to have an indeterminate thermal

resistance (calculated a,s R = M/q), and the

formboard an apparent large resistance approaching infinity. This type of result is indicated in figure 34 by a broken line symbol and

is attributed to the latent heat transfer effect. That is, most or all of the heat entering the

bottom face of the specimen was used to evaporate moisture from the concrete. The apparent

thermal resistance of the glass-fiber formboard

decreased from a near-infinite value to a measurable value as the heat flow reaching the calorimeter increased. The total temperature difference across the specimen from the room to the calorimeter remained relatively unchanged.

The latent heat effect is also shown in figure 33

but is much less pronounced as compared with

figure 34.

The constructions of Specimens 25, 26, and

27 consisted of a 2-inch-thick gypsum concrete

deck which was permeable to water vapor, and

much higher in thermal conductivity as compared to the three types of insulation used

over the deck. When Specimens 25, 26, and 27,

figures 41, 42, and 43, respectively, were subjected to their initial summer exposure period,

each rapidly lost weight and reached a moisture

content equilibrium within a two-week time

period and thereafter during this exposure period remained at a constant moisture content, as shown. The thermal resistance of Specimen

25 was near its dry value, since during this time

it contained little moisture, but the thermal

resistances of Specimens 26 and 27 were about

89 and 75 percent of their dry values when they

contained 0.2 and 0.5 lb/ft- moisture, respectively. The lowered thermal resistances of the

latter two specimens were attributed to the

36
moisture held in equilibrium within them. The

insulations of these specimens were of greater

thickness and density, and less porous, than

that of Specimen 25, and thus were able to retain moisture in equilibrium, especially Specimen 27, whose insulation is hygroscopic. The

retained moisture would tend to migrate back

and forth within these specimens in response

to the twice-daily reversal of temperature gradient, thus remaining distributed and available

for latent heat transfer.

Specimens 25, 26, and 27 gained weight and

increased in moisture content with relatively

little change in insulating values during their

second winter exposure period after each had

been dried to an equilibrium moisture content

during its first summer exposure period. The

moisture gained was easily expelled from the

specimens within the first two weeks of the fol- lowing second summer exposure period. Their

performances during subsequent summer exposure periods show that the maximum time re- quired for expulsion of added moisture was

seven weeks for Specimen 27 when a maximum

of about 1.58 lb/ft- of moisture was expelled.

During the second summer exposure period,

water amounting to 10 percent of specimen volume was added to Specimens 25-27 in simulation of a roofing leak. The quantity of water

added to Specimen 25 was restricted to a total

of 7.4 percent of specimen volume, because the

indoor face of the gypsum concrete became wet

during the water addition process and droplets

of water began forming on it. Further addition

of water to this specimen would have resulted

in free water dripping from the indoor face,

which would cause a weight loss not attributable to self-drying of the specimen. Water added

to Specimens 25, 26, and 27 penetrated to the

indoor face of the gypsum concrete, visibly

wetting a circular area around the center of

the specimens. This sudden increase in moisture

content of all three specimens was accompanied

by a sharp decrease in thermal resistance. The

thermal resistance decreased because the temperature at the interfaces of the materials and

at the indoor surface decreased and approached

the calorimeter temperature, resulting in a much reduced temperature difference across the

insulating materials. Also, the heat flux at the wet indoor face of the gypsum concrete,

at the time the measurements were made, was

largely used to evaporate moisture from the

gypsum to the room, causing a reduced flow of

heat into the calorimeter. As the exposure continued, the specimens dried and their thermal

resistance increased. Specimens 25 and 26 were

easily wetted and upon continued exposure

dried rapidly, quickly recovering their insulating value. Specimen 27 did not wet or dry as

easily or as quickly and its loss of insulating

value was considerably less because it was able

to maintain a greater temperature difference

and heat flow across the materials during its longer evaporative process. It should be noted

that rapid self-drying of these specimens and

others would not occur if a vapor barrier were

used at the interface of the materials.

12.2. Summer Self-Drying, Initial Construction

Moisture

The quantity of moisture present in a roof

construction when initially placed determines

the immediate insulating effectiveness of the

construction. Factory-made board types of in- sulation when installed contain only hygroscopic moisture if protected from the weather

during installation. Water-mixed materials contain a considerable quantity of free water even when their surfaces are dry enough to apply

roll-roofing. Normally contractors apply roofing as soon as practicable to protect the construction from rain or snow. The specimens used in

this investigation, as shown in table 3, range

from those having materials practically dry

when installed to constructions that contain

considerable free water distributed throughout

the specimen. For example. Specimen 1, figure

17, when installed had about 0.4 lb/ft- of free

moisture, most of which was contained in the

1-inch-thick expanded shale concrete deck. This

deck was intended to simulate the web-portion

of a prefabricated channel-shaped slab. In contrast. Specimen No. 23, figure 39, contained

about 3.3 Ib/ft^ of moisture, all of which was

contained in the 6-inch-thick perlite aggregate

insulating concrete. Obviously some means

must be provided to dissipate the large quantity of moisture in the latter specimen after

the roofing has been applied. The self-drying

concept emphasizes the omission of a vapor

barrier and the importance of the vapor-transfer properties of the deck material and of

the specimen itself. It becomes important

that the design of the roof construction permit

the escape of initial construction moisture. To

illustrate: Specimens 7 and 21 were alike in

construction except for arrangement. In Specimen 7 the expanded polystyrene was used as

a formboard deck with insulating concrete over

it, while Specimen 21 had the expanded polystyrene insulation on top of the insulating

concrete. Examination of figures 23 and 37

show that when exposed to the first spring,

summer, and fall exposure periods. Specimen 21

dried to a moisture content equilibrium of

about 0.4 lb/ft-, while Specimen 7 showed comparatively little tendency to dry and had a moisture content of about 1.4 lb/ft- after being ex- posed for the same time periods.

The different performances of these two

specimens clearly indicate the importance of

arrangement of the components when one of

them is resistive to vapor transfer. Specimen

37
21 represents a good self-drying construction;

Specimen 7 does not. Most of the specimens tested that contained

initial construction moisture were able to dry

to an equilibrium moisture content and yield

good insulating values in a suitable time, provided the specimen and deck were reasonably

permeable to water vapor. However, some specimens even with permeable undersurfaces dried

comparatively slowly when exposed to summer

exposure conditions and insulating values were

low for long periods. In the latter cases the

quantity of moisture that had to be evaporated

and the thickness of the specimen was large.

Examples are Specimens 4, 5, 6, figures 20, 21,

and 22, respectively. Note in figures 20 and 22

that the average drying rate under summer

conditions was not materially improved when

the 1/2-inch-thick gypsum formboard (29.4

perm) was removed while the specimen was

being tested, but removal of the 14, -inch-thick

cement asbestos formboard (0.8 perm) of

Specimen 6 produced an immediate change of

drying rate. Also, Specimen 24, figure 40, whose

thickness was 3 inches, did dry to a moisture

content equilibrium while Specimen 23, figure

39, whose thickness of the same type of material was 6 inches, did not dry to a moisture content equilibrium over the identical time of

exposure.

The rate and time of drying appeared to be

controlled by the quantity of free moisture

present, the severity of the exposure conditions,

the thickness or volume of the specimen, the

water vapor permeance of the specimen and its deck material, the hygroscopic moisture capacity, the water absorbency, the thermal conductivity, and the arrangement of the materials

within a specimen. An attempt was made to correlate these variables and formulate simple,

practical engineering expressions that would be

useful in predicting thermal insulating and drying performance on the basis of geometry and

physical properties. Such a formulation was not

feasible and it appears necessary to rely upon

laboratory or field tests of the performance of

proposed constructions. Constructions consisting of deck materials of moderate to high pernieance to water vapor and also of moderate to high thermal resistance covered with materials

of high moisture absorbency and moderate per- meance appear to possess promising self-drying

and thermal insulating characteristics even if accidentally wetted in service.

12o3. Winter Gain of Moisture

It has been shown that initial construction

moisture can be dissipated through a permeable

deck material by utilizing solar heat on the

roofing. However, under winter exposure conditions the saturated water vapor pressure just

beneath the cold roofing at night is lower than

that of indoor air. This causes vapor to transfer into and through the materials to condense on the undersurface of the roofing because the

temperature of the roofing is below the dewpoint of the indoor air. Therefore, in winter it becomes desirable to restrict the flow of vapor

into the roof or provide sufficient moisture absorption capacity beneath the roofing to accom- modate reasonable amounts of condensate that

may accumulate over the winter.

Inspection of the figures for all the specimens

reveals that except for Specimen 13 the rate of

gain of moisture and the total weight of accu- mulation during winter exposure periods were

small and the moisture capacity was more than

suflScient to accommodate accumulated moisture. Generally, the figures show that higher

values of specimen permeance, especially that

of the deck, allowed faster rates of moisture

gain. Insulating values were not significantly

lowered unless a very considerable degree of

wetting of the specimen thickness occurred. In

winter under cyclic temperature exposure conditions the rate of moisture gain decreased as the exposure time and quantity of moisture

accumulated increased. For example, Specimen

17, figure 33, shows similar rates of moisture

gain during the second and third winter exposure periods. When the temperature of the calorimeter was held constant during the third winter and not cycled, the rate of moisture gain

increased; but when changed again to a cyclic

temperature procedure, the rate of moisture

gain decreased considerably and was less than

it was at the start of the third winter period

when its moisture content was lower. See the

winter condition moisture gains for Specimen

25, figure 41. Also, it can be shown, for moist,

thick, permeable specimens, that the daily

night-time gain of moisture under cyclic temperature conditions was offset by a daily daytime loss of moisture, resulting in little net

change of specimen weight, hence a nearly-zero

rate of moisture gain. See Specimens 4, 5, 8, 10,

11, 12, figures 20, 21, 24, 26, 27, 28, respectively.

In most specimens of comparatively high per- meance winter-gained moisture was easily and

rapidly dissipated through the deck to the room

beneath by subsequent exposure to a summer

condition.

Whether a particular design will dry, wet, or remain static under winter exposure conditions

appears to depend upon the quantity of moisture present in the specimen, the degree of

daytime solar heating on the roof, the temperature of the roof and indoor space, and the in- door water vapor pressure. Some information

is available for analysis from this investigation

but it is felt that further research in regard to these points is needed.

Also, the question of the deteriorating ef- fects of freezing and thawing of moisture in

the construction, and concomitant dimensional

38
changes, requires consideration in cold climates.

This problem is probably greater for a roof

that does not dry in service than for one that

does.

12.4. Self-Drying From Roof Leaks

Roofing leaks are probable sooner or later. Even newly-laid roofing may accidentally be

made to leak by operations such as installation

of airconditioning or other equipment on the

roof. Eventually most roofing materials will degrade, causing leaks. One can design against

entry of water to the occupied space from a

roofing leak by using a waterproof deck such

as monolithic structural concrete. The disadvantages of this are that the insulation or the

crack-space between slabs of insulation can be- come wetted often for a long time without the

building occupant knowing it and a serious loss

of insulating value results and eventually the

insulation and roofing must be replaced, at a very considerable expenditure. An alternative

is to use a moderately vapor-permeable and

water-absorptive construction. Should a leak in

the roofing occur with this type of construction, a wet spot or dripping of water from the

ceiling immediately affords detection of the

roofing leak. Simple patching of the roofing

corrects the leak and, if the construction possesses good self-drying characteristics, it will

dry to the indoors. Redecorating of the affected

interior area is all that would be necessary, re- sulting in a large saving of maintenance costs

as compared with replacing the roofing and

wet insulation over a large area of the roof.

Generally the performance results show that

the higher the permeance of the specimen and

especially of the deck the faster the roof will

dry after being subjected to a simulated roof

leak. It appears advantageous to provide some

moisture absorption capacity to prevent inundation of the occupied space should the roofing

leak during a hard rain storm. Also, some moisture absorption capacity appears advantageous

to retain the much smaller quantity of vapor

transferred and condensed during the winter.

The permeances of the decks of Specimens 1 through 7, and 23, or the overall permeances

of these specimens, apparently were too low to allow sufficient drying to cause recovery of in- sulating value in a reasonably short time period

such as a calendar summer. Given more time or

several summer seasons, these specimens probably eventually would dry, since their decks

and materials are not impermeable and because

their resistance to winter gain of moisture is high. All other specimens dried reasonably rapidly and maintained or recovered their insulating value after being subjected to a simulated

roof leak. In some cases, to simulate a roofing

leak, a fixed amount of water was added, such

as 10 percent of specimen volume; in other

cases the amount of water added was determined by the ability of the specimen to absorb

water to the point where droplets formed on

the undersurface.

Pertinent observations noted during the water-addition operation for the quantities of

water added as given in the figures include: no wet spots visible on the undersurface of Specimens 1 through 4, 6 through 10, and 20 through

23 ; circular-shaped wet spot not covering full area of interior surface. Specimens 11, 14

through 17, 26 and 27 ; circular-shaped wet

spots to the point of water dripping. Specimens

12, 18, 19, 24, and 25; and water flowed through

specimen with little resistance. Specimen 13. Specimen 5 had no water added to it during its exposure. During wetting it was observed that

wood-fiber board tended to expand when wet

and shrink as it dried.

Most specimens when removed from the apparatus showed little change in appearance

except for water-stain markings. However,

specimens containing gypsum concrete revealed

that leaching or cavitation had occurred in the

upper surfaces of the gypsum concrete, especially in areas immediately under the waterfeed hole in the calorimeter through which a water leak was simulated. It is probable that

cavities formed as the result of the solution of

gypsum and its deposition elsewhere in the pore

space of the specimen as drying progressed. The

summer cycle of the specimens probably carried

the temperature of the gypsum above the point

at which gypsum is in its stable phase, and

under these conditions it could be expected to

dissolve and re-precipitate as anhydrite, with

a higher density and hence a smaller specific

volume. From the practical point of view, cavitation as a result of a roofing leak was not con- sidered serious because it would tend to reveal a roof leak and repairing gypsum concrete is a

relatively simple chore.

It was mentioned earlier that wetting of materials beneath roofing as a result of a roofing

leak is an imponderable. Questions as to how

far laterally the materials will become wetted

or whether water will soak uniformly through

the thickness of the construction depend largely on the quantity of water available to pass

through the leak in the roofing and the ability

of the materials themselves to absorb water.

Some materials are very difficult to wet and

are designed to be so, while others by their very

nature are highly absorptive. The heat transfer

performance becomes very much dependent

upon how the moisture is distributed within

the materials and the time for drying is dependent upon the quantity of water to be vaporized. The criterion used in this investigation was to add a fixed quantity of water {10% by

volume) to the specimens and for later additions of water the quantity used was determined by the absorptive capacity of the speci39
men. In an actual roof the amount of water

absorbed will depend upon the rainfall or puddling: on the roof and the absorptive capacity

of the materials.

Specimens 15, figure 31, and 16. figure 32, were made the same way except that Specimen

16 had a cotton scrim on each surface of its nerlite concrete. About the same Quantity of

water was added in simulation of a roofing

leak. For each specimen the thermal resistance

dropped sharply but quickly increased as the

moisture became distributed in the concrete. Specimen 16, with the scrim, took longer to dry,

probably because most of its moisture was con- tained more uniformly in its concrete and scrim

where Specimen 15 probably had more of a concentration of moisture in its central area. Specimen 19, figure 35, and Specimen 22. figure

38, were made the same way, except that Specimen 22 had a cotton scrim on each surface of

its glass-fiber board. Approximately the same

nuantity of water was added to each specimen

in simulation of a roofing leak. The thermal

resistance of Specimen 19 decreased rapidly

and gradually increased while drying: but for

Specimen 22. the thermal resistance decreased

and remained depressed for several weeks before suddenly increasing as it dried and it took

this specimen a little longer to dry from its roofing leak. Latent heat effects kept the thermal resistance depressed in Specimen 22 and

it was concluded that the use of a scrim to provide for more uniform distribution of moisture

during testing was not indicative of what would

occur on an actual roof where the lateral distribution of moisture would be controlled by the

materials of the construction and other factors

such as local roof slope.

It would be desirable to be able to accurately

predict the long-term simultaneous insulating

and self-drying or wetting performance of any

combination of materials used in constructing

flat roofs from a knowledge of the geometric

and physical properties of the materials and

their exposure temperatures and relative humidities. However, such factors as variable

moisture contents of materials when installed

in a roof, the degree of wetting from a possible

roofing leak, temperatures of the roofing as influenced by the weather, the stacking arrangements of several different materials and other

imponderables, preclude accurate prediction of

field performance. Also, little theoretical information is available to support formulation of a useful mathematical model. A qualitative picture of the processes can be deduced from the

results of pertinent researches and empiric

equations can be written to describe laboratory

results. A mathematical model for quantitative

prediction of simultaneous heat and mass transfer performance in porous materials under

transient temperature conditions was attempted

in connection with this work but a correlation

of variables suitable for quantitative predic- ' tion was not obtained. This type of problem is very complex and Hansen [18] states that

j preparation of a quantitative analytical analy- ! sis which would permit accurate a priori prediction of drying behavior from a knowledge

of characteristics of the solid, the liquid, and

external conditions is not yet possible. Sereda ; and Hutcheon [15] state that there is a serious \ limit to the prediction of moisture migration l» and there is a similar restriction on the prediction of heat flow in moist materials because the

two flow mechanisms of heat and moisture are

so thoroughly interrelated that one cannot deal

with one without the other.

As an example of possible field imponderables, consider the following situation: A roof

construction consisted of poured gypsum concrete over glass-fiber formboard insulation on

bar joists with built-up roofing and aggregate ; surfacing installed on the gypsum concrete. Laboratory results as given in this paper for

this type of construction show rapid self-drying

characteristics. However, in the spring of the ^ year the insulation of the subject construction

had a layer of water near its lower surface. It was learned that the gypsum concrete was i| placed during the previous winter and also that

portable combustion-type heaters were used in

the weather-closed building while the interior

finishing operations were completed. Apparently the initial moisture plus moisture generated in the indoor space at a high rate under

conditions where an indoor-to-outdoor vapor

pressure gradient existed served to retain an

accumulation of moisture in the construction

until sufficient solar heating in the spring

caused a migration to the lower surfaces of the

formboard. Areas containing unventilated hung

ceilings were similarly affected with water damage to the ceilings. Ventilation of the indoors

in combination with solar heating of the roof soon dissipated the accumulated moisture. Simple ventilation of the indoors in winter and

spring would have prevented moisture damage

in this case.

12.5. Arrangement and Physical Properties

of Components

Discussion, thus far, concerning the insulating

and moisture performance of roof construction

has identified materials properties of hygroscopicity, water absorbency, water vapor permeability, and thermal conductivity as properties of major importance. Other factors, such

as temperature and relative humidity exposure

conditions, the quantity and location of moisture

present in the materials, and the thicknesses of

the materials were also shown to exert considerable influence on performance. The arrangement of materials in a roof construction was

thought to have a bearing on its performance.

40
Certain pairs of specimens listed in table 3 were

sufficiently alike and were exposed simultaneously to enable comparisons in respect to ar- rangement of materials within a specimen.

These pairs of specimens were 7 and 21, 9 and

20, and 15 and 19. For Specimens 7 and 21, figures 23 and 37,

respectively, arrangement was of major importance. Here the location of the expanded

polystyrene insulation, whose water vapor per- meance was low compared with that of the perlite concrete, controlled the self-drying characteristics. The best performance was obtained

with the polystyrene insulation board located

above the perlite concrete. Specimen 21, figure

37. With the polystyrene board located beneath

moist perlite concrete, relatively little drying of

the perlite concrete to the room beneath oc- curred because of the high resistance of the

polystyrene to the flow of water vapor through

it, as shown for Specimen 7 in figure 23. The performance characteristics for Speci- mens 9 and 20, figures 25 and 36 respectively,

show that it made little difference whether the

wood-fiber insulating board was located above

or below the perlite concrete. Each specimen

provided about the same insulating value and was able to dry to an equilibrium moisture content when wetted. Their insulating values were

lower than dry values because both component

materials in the specimens are hygroscopic and

in use always contain some moisture.

The performance characteristics of Speci- mens 15 and 19, figures 31 and 35 respectively,

show that a higher average insulating value was obtained for Specimen 19 when the glassfiber insulation was located above the perlite

concrete but it took a longer period of time for

this specimen to recover insulating value when

wetted as compared with Specimen 15. From these comparisons it was concluded

that the performance of a specimen cannot be

reliably predicted on the basis of any one

physical property of the materials. To evaluate

the insulating and self-drying performance, one must consider the thermal and moisture properties of the materials simultaneously as well as the factor of their location within the construction. Also, a general rule stating that insulation

should be placed either above or below concrete

in a construction was not considered to be

feasible.

The thermal resistance values shown in figure

35 for Specimen 19 were approximately what

would be expected for the nearly dry component

materials of perlite concrete and glass-fiber insulation, but for the same component materials

as used in Specimen 15, figure 31, comparable

resistance values were considerably lower.

Lower resistances occurred in Specimen 15 be- cause of a higher rate of heat flow through the

specimen as compared with that of Specimen 19

when each had approximately the same specimen surface-to-surface temperature difference across it during exposure. During the second

winter exposure period, week No. 34, figure 31, an in-place calibration of the calorimeter over Specimen 15 was completed using Procedure A

in Reference [6]. Results of the calibration

showed that a slightly higher heat flow through

the guarding insulation was occurring as compared to the value of heat flow allowed on the

basis of the original calibration determined

using dry specimen materials. However, allow- ance of a slightly increased guard insulation

heat flow did not increase the specimen thermal

resistances to dry values. The presence of moisture in the perlite concrete when used over in- sulation was considered to be the primary cause

of the observed increase in heat flow in Specimen 15 because of absorption by the calorimeter

of latent heat from condensation occurring in

the perlite concrete in winter at the time of the

calorimeter measurement and the original cali- bration factor obtained using dry materials was

considered to be valid.

Additional calibrations were performed as indicated in figures 31 and 35 and later on similar Specimens 16 and 22, as indicated in figures

32 and 38, respectively. Results in general indicated that the heat flow in specimens with

glass-fiber insulation over perlite concrete was

comparable to that to be expected for dry materials, while that for concrete over insulation was consistently higher. Also, throughout the

exposure history of Specimens 16 and 22, extra

heat-flow data were obtained using heat flow

meters attached to the indoor surfaces of the

specimens. For Specimen 22, insulation over

concrete, reasonably good agreement was obtained between the heat flows determined using

the calorimeter and the heat flow meter, and

the calculated heat flow based on the thermal

conductivity of the dry component materials of

the specimen during the periods when the specimen was fairly dry (week 90 to 113) . For Specimen 16, insulation beneath concrete, reasonably

good agreement was obtained between the heatflow meter and the heat flow based on the

conductivity for dry materials. The heat flow as

indicated by the calorimeter was greater. A rea- sonably good agreement of the heat flows of

Specimens 16 and 22 was obtained when the

heat-flow meter and calculated data were compared. The reason for the higher heat flow of

Specimens 16 and 15 when using the calorimeter technique was attributed to latent heat ef- fects as mentioned above. It is postulated that a

heat-flow meter attached to the indoor surface measures the heat flux across the meter but does

not account for latent heat of condensation that

would be absorbed by the calorimeter, as used, on the outdoor surface of the specimen. It can

be noted from the performance characteristics

of all of the figures shown here that all specimens that are made with moist concrete above

41
insulation consistently show decreased thermal

resistance when compared with dry values of

thermal resistance as calculated from the thermal conductivity of the component materials.

During- the course of the test series investig-ating arrangement of the same materials in

test specimens, temperature and heat-flow data were taken during- twenty-four-hour periods at several times in the exposure periods of some

specimens as indicated in figures 23, 25, 31, 32,

35 to 37. and 38. Figure 44 shows typical sample

results for Specimens 19 and 15 during summer

and winter exposure periods when the specimens were near their equilibrium moisture content. Similar results were obtained for all specimens in the arrangement-test series. Figure 44

shows, for the summer exposure period, that the

average temperature of the 3-inch-thick perlite

concrete when placed over the li/4-inch-thick

glass-fiber formboard of Specimen 15 reached a maximum of about 125 °F about 6 hours after

the start of simulated solar heating. If liquid

water were present in the concrete at this temperature, its saturated water vapor pressure

would be about 4.0 inches of mercury and with

the room beneath held constant at a vapor pres- sure of about 0.43 in. of mercury a vapor iDres- sure difference of 3.57 in. of mercury would be

available for drying at this time. The vaporDressure-difference potential for drying would

increase and decrease as the suecimen heats and

cools with a net tendency towards drying to a

moisture content equilibrium as is indicated

by the moisture content curves of the performance characteristics. The same process would

aDDly in Specimen 19 if liquid water were

present in the glass-fiber formboard over perlite concrete. After the specimens reach a moisture

content equilibrium, presumably as much vapor

leaves the indoor face during the heating phase

as enters the specimen during the cooling phase

over a 24-hour period.

The average temperature of the perlite concrete, when located under the glass-fiber insulation in Specimen 19 reached a maximum of

98 °F when subjected to simulated summer

solar heating as shown in figure 44. As previously discussed, the near steady-state heat flow

observed just prior to simulated solar heating was greater in Specimen 15 than in Specimen

19. In summer, condensation of indoor water

vapor in the specimen was not probable, as in

winter, because temperatures in the specimen were all above the dewpoint of the indoor air. The reason for greater heat flow in Specimen

15 in summer has not been fully accounted for. In winter, the increased heat flow was probably

due to the latent heat of condensation in the per-

lite concrete. An estimate of the quantity of

heat given up by vapor condensation using the

rate of weight gain of the specimen indicated

that this most probably was the cause, es- pecially since the temperature within the perlite

concrete of Specimen 15 fell to values lower

than the dewpoint of the indoor air (54.5 °F)

for several hours of the night period of the winter exposure condition as shown in figure 44.

In Specimen 19, the perlite concrete did not

at any time reach temperatures during the winter exposure conditions as low as the indoor air

dewpoint. This difference between the two specimens explains why Specimen 15 gained weight

during its first and later winter exposures,

while Specimen 19 lost, or gained little weight.

Similarly, consideration of the average daily

temperature of the perlite concrete during the

summer exposure condition indicates that Specimen 15 should dry faster than Specimen 19, as was in fact the case. The results and discussion presented indicate

that a design of an insulated flat roof that omits

a vapor barrier, has some moisture absorption

capacity, and is not used over indoor environrnents of very high relative humidity, will provide reasonably good insulating valueyear-round

and will self-dry by sun heat if wetted from a

roofing leak once the roofing leak or source of

accidental moisture is corrected. However, re- sults obtained were observed under carefully

controlled laboratory conditions and the question arises as to whether a particular design

would perform similarly in an actual building

in the field. Such factors as variable moisture

contents of materials when installed in a roof,

difficulties in obtaining consistent high-quality

workmanship on the job, changing weather, and

the ever-present possibility of roofing leaks

being accidentally created, preclude prediction

of field performance. The authors recommend

that field tests of promising self-drying-type

designs be performed for comparison with the

laboratory results presented here.

12.6. Design Criteria

Design criteria were prepared to allow a designer of insulated flat-roof constructions

much freedom of choice as to commercially

available insulations and building materials and

to provide a means to judge and evaluate these

designs, particularly those that utilize newlydeveloped materials. The basic rules in designing self-drying insulated flat-roof constructions

as developed in this paper, are: omit conventional sheet or mop type vapor barrier; use at

least one material in the layer that has water

absorptive capacity ; if more than one material

is used, locate the material of lowest water

vapor permeance just under the rooflng, though

this may not be necessary if the permeance

values of the materials are reasonably close ; do

not use alone an insulation of high watervapor permeance; avoid thicknesses in excess

of 3 to 4 inches of highly absorptive materials

;

arrange materials so that water vapor is readily

transferred indoors from the material should

42
there be moisture present (e.g. as from a roofing leak) ; provide edge venting and/or roofdeck venting to avoid a rapid increase of pressure in any layer of the composite roof ; and do

not allow the dev^^point of the indoor air to ex- ceed 60 °F in winter.

Simple formulas for general design purposes

that would predict simultaneously the thermal

insulating and wetting or drying performances

were not feasible on the basis of only the physical properties of the materials. Instead, certain

criteria for performance, as determined by laboratory tests, were prepared and are listed as column headings in table 4, and are shown

schematically in figure 45. The tentative criteria

require that designs be tested in the laboratory

to determine if performance is within the suggested limits. Laboratory performance testing is considered to be imperative for new designs of

roof construction. Observation of field performance would be even better, considering our national climatic variations, but this is slow, might

be expensive, and is subject to difficulties of ef- fective feedback of crucial evaluation and utilization of information. Table 4 lists values cal- culated from the laboratory performance tests and physical property determinations for the

27 specimens used in this investigation. In the

table values underlined with a solid line do not

meet the suggested performance criteria. A discussion is given below of each limiting factor,

with suggestions for numerical values to assure good performance.

12.6.1. Scope

This discussion covers suggested insulating

and self-drying performance requirements of

insulated flat-roof constructions intended for

normal occupancy buildings. Normal occupancy

spaces in buildings are defined here as those in

which the dewpoint temperature does not ex- ceed 60 °F, which corresponds to a relative

humidity of 50 percent at 80 °F dry bulb. Laboratory tests are required for simultaneously

determining insulating value and moisture content. The testing procedure requires exposure

of a sample of the proposed construction to repeated daily cycles of simulated in-service en- vironmental conditions of temperature and rel- ative humidity, starting with the specimen con- taining normal construction moisture. The tests required cover the determination of: (a) the

time required for the specimen to expel initial construction moisture by a process of self-drying to an approximate moisture content equilibrium when exposed to a simulated summer

exposure condition; (b) rate of moisture gain

during an immediately following winter ex- posure condition ; (c) duration of a second sum- mer exposure condition required to expel the

winter-gained moisture; (d) self-drying performance of the specimen during a summer exposure condition after it was wetted with water

in simulation of a roof leak. The tests include

making daily measurements of insulating effect

during these exposures. Adjunct laboratory tests to determine particular physical properties of

the component materials are also included.

The discussion of each factor, given below,

suggests limits for the performance of an ac- ceptable construction. These are illustrated and

indicated in figure 45, and are also given as suggested at the bottom of Table 4.

12.6.2. Thermal Insulating Value

The criterion for insulating performance of

the construction is a thermal resistance ratio,

Rt/Rd- Rt is the average thermal resistance of

the specimen over the total test period, evaluated as described below. R,, is the dry thermal

resistance as calculated on the basis of test re- sults for the thermal conductivity of the dry

components. The suggested minimum acceptable

value of Rt/Rd is 0.6. In this ratio the thermal

resistance of the construction is reckoned from

its underside (indoor) surface to the top sur- face of the construction underneath the roofing.

The thermal resistance of the construction as

tested, Rt, is determined daily (usually 5 days

per week) by using the National Bureau of

Standards Calorimeter Plate Method of test described in reference [6]. The individual daily

measured values of thermal resistance of the

construction are averaged for each week. The

average of the weekly averages, over a period

totalling 37 weeks (the 29th week is excluded

from this average) constitutes the thermal re- sistance, Rt, of the construction as tested. The

total test time is 38 calendar weeks. See figure

45.The thermal conductivities of the dry materials used in the specimen are determined at 75 °F mean temperature using the method of

test ASTM C 177, reference [19]. The thermal

resistance, R„ of each component shall be calculated using the formula R,, = X„/kn, where X,,

is the thickness of the component in inches and

k„ is its thermal conductivity in Btu/hr ft- (deg

F/in). The thermal resistances of the tandem

components shall be summed (Ri + Ro . . .

, etc.) to determine the total calculated dry thermal resistance, Ra, of the construction.

For any component material the value of its thermal resistance, R,„ can be firmly fixed be- cause the thermal conductivity of a dry specimen is determined by a standardized method

ASTM C 177. The value of the test thermal

resistance of a construction or a component

varies with time, moisture content, arrangement of the components of the specimen, and

exposure conditions. The suggested requirement that the average test specimen resistance

achieve a minimum of 60 percent of its dry

value may at first appear to be easily accom43
plished. However, the procedure for calculating

Rt consists of numerically averaging values that are obtained by exposure under moist conditions during simulated summer and winter seasons. For some specimens that are wet a weekly

value of Rt may be as little as 25 percent of the

dry value. Also, limits of time of exposure are

suggested. Thus, the average value Rt includes

values obtained with the specimen both near- dry and moist, as well as values for two seasonal

exposures. From laboratory data obtained on

specimens that demonstrated good ability to dry

in place the value of 0.6 for the ratio Rt/Ra was

selected as a minimum to assure a reasonably

efficiently-insulated construction year round.

For many constructions that are installed with

only normal hygroscopic moisture in them, the

weekly value of the ratio Rt/Ra should be mostly

in the range from 0.8 to 1.0. Further, some constructions that have decks that are relatively

quite permeable to water vapor may indicate

high values of Rt when covered with a wet material of high water absorptive capacity. This

occurs because the heat entering the bottom face

of the specimen during measurement can largely be used as latent heat of evaporation and the

quantity of heat remaining to be absorbed by

the calorimeter is small. The apparent weekly

thermal resistance of the specimen thus indicated may exceed the dry value and in such

cases the maximum value that should be used

to obtain Rt is the dry value R^. Such exceptional results were obtained only with rapidlydrying specimens having quite permeable decks,

so the procedure of using the dry value for an

obviously wet construction occurs for only a

small fraction of the total time of averaging.

12.6.3. Self-Drying Time for Initial Construction

Moisture and Equilibrium Moisture Content The maximum testing time suggested under

the first summer exposure condition for the construction to expel initial construction moisture

by self-drying to a moisture content equilibrium

is 16 calendar weeks. Specimen weight loss is determined from daily weighing, using scales

sensitive to 0.01 lb, made simultaneously with

the daily thermal resistance measurements. The

average of the daily weight observations for

each week constitutes the average gross specimen weight, Gt, for that calendar week. The

gross weight, Gt, is the sum of the actual net

specimen weight Nt and a tare. Weight measurements and thermal exposure

and testing should begin immediately upon fabrication and installation of the test specimen.

The start is in no case to be later than one week

after cast-in-place components are poured.

When installed, component materials should be

in a moisture condition similar to that to be

expected when roll roofing would be applied over such a construction in the field; that is,

materials are to be not more than air-dry, and

cast-in-place components should not have been

subjected to air-drying for longer than one week

after placing. r The specimen is considered to be at an approximate moisture content equilibrium during ' exposure testing when its change of weight, ! AG,, does not exceed 0.05 Ib/ft^week) for 2

consecutive weeks. , The quantity of moisture present initially will

j vary widely with various types of constructions.

Manufactured insulation normally will contain

only hygroscopic moisture. Green concrete will

contain a relatively high percentage of free

moisture by weight.

Those constructions that are placed in the

field with all materials nearly dry will, under

laboratory exposure conditions for the first summer exposure condition, reach a moisture

content equilibrium rapidly and the values of

Rt should be high and stable. For these cases it may be permissible to reduce the time of testing

to less than the maximum of 16 weeks suggested. However, when averaging weekly values

of Rt over the aggregate of 37 weeks the appropriate values of Rt should be included for the

full time period of 16 weeks. il For those specimens containing relatively

much moisture, the maximum of 16 weeks for

the first summer exposure condition will probably be needed to establish a moisture content

equilibrium. It is important that the weight and

heat transfer measurements begin within one week after concrete is poured because prolonged [ laboratory air drying can reduce the moisture

content considerably and this would not be compatible with field installation where roofing is , usually applied as soon as possible to prevent I rain-wetting.

12.6.4. Equilibrium Moisture Content The value of the approximate equilibrium , moisture content of the construction achieved as a result of self-drying at the conclusion of the ( first summer exposure period is evaluated by ( means of the ratio Mt/Mc, where Mt is the aver- age specimen moisture content at the end of the

first 16-week summer exposure period. Mt is , evaluated as (1/A) {Gt - G,) + {Nf - Na)], -'i where A is the plan area of the specimen, Gt is defined above, Gf is the gross specimen weight

after week 38 and just before determining

(A^; —Na), which is the loss of total weight of

the net specimen on oven-drying its components

to constant weight. Mc is the calculated moisture content based on equilibrium hygroscopic

moisture content determinations of the component materials, and is defined below. The maxi- ( mum acceptable value for the ratio Mt/Mc is [ suggested to be 2.0. >. The equilibrium hygroscopic moisture absorption of each component material is deter44
mined by exposing a conveniently-sized sample

having a thickness of at least 1 inch and an

exposed surface area of about 1/2 ft- to a uniform temperature of75°F±2° and a relative

humidity of 90 percent ± 3 percent until the

sample shows no further change of weight. The

dry weight of the sample is then determined by

oven-drying it to constant weight at a temperature commensurate with the use limit of the

material. For drying most building materials an oven temperature of 215 °F can be used,

but for some materials the softening point temperature or other factors must be observed. The

equilibrium hygroscopic moisture content, Mc,

in lb/ft-, is calculated as follows

:

M. =

12

where p

Wo +

P2 X2/ W/.2 wd2 12 \ Wd2 + ...

X =

Wn =

dry density of each component

material, Ib/ft^

thickness of each component material, in. constant weight of each component of the specimen at 75 ° ±

2 ° and 90% ± S% relative

humidity

constant weight of each component of the specimen after oven- drying

Evaluation of the approximate equilibrium

moisture content at the conclusion of the first summer exposure condition is required to as- sure that the quantity of moisture present is not excessive.

Laboratory results on specimens have shown

that for two-component constructions, after

several weeks of the summer exposure condition, the evaporable moisture present in the

construction is widely distributed throughout

it. To set a limit on the amount of evaporable

moisture an allowable construction may contain after 16 weeks of summer-condition ex- posure, a maximum value of 2 for the ratio

Mt/Mc was suggested, where Mt is the average

evaporable moisture content of the specimen at that time, and Mc is the hygroscopic moisture

content of the total specimen if each of its components has the moisture content corresponding to its hygroscopic equilibrium at a condition of 75 °F and 90 percent relative humidity.

It has been found that for most constructions

the latter quantity of moisture does not markedly affect the insulating value under the

winter exposure conditions.

The value of differs considerably among

constructions of different kinds, depending both

on their weight and on the hygroscopicity of

their materials. Information on hygroscopicity

at this condition available in the literature is sparse, and what is available indicates considerable variation for ostensibly the same material, and also shows variation with specimen

size. Hence, it appears necessary to require test measurements to ascertain for a construction. The quantity of hygroscopic data obtained

as an adjunct to the NBS tests on roof constructions is small. It is felt that as more information

becomes available it may be possible in the

future in some cases to omit specific measure- ments to obtain M,.. It may also be desirable in

the future to change the present limiting value

of the ratio Mt/Mc.

12.6.5. Regain of Moisture—Winter Exposure

Conditions

The maximum permissible average rate of

moisture regain under winter test exposure conditions is suggested as 0.05 lb/ft2(week) . The

time of exposure to determine the average rate

is six calendar weeks immediately following the

first summer exposure period at the conclusion

of which an approximate moisture content

equilibrium was established.

12.6.6. Summer Self-Drying Time for Winter Regain

Moisture

The maximum permissible testing time under

the second summer exposure period for expulsion by self-drying of moisture gained under the

first winter exposure period (12.6.5 above) is suggested to be six calendar weeks. The second

summer exposure test immediately follows the

first winter exposure period. The specimen

should dry to a moisture content not greater

than that at the end of the first summer exposure period.

12.6.7. Summer Self-Drying Time for a Simulated

Roofing Leak

During the second summer exposure period

and following 12.6.6 above, a roof leak is simulated by adding water to the center of the upper

surface of the top component material of the

specimen in an amount equal to 10 percent of

the total specimen volume.

In order to facilitate water admission and its lateral distribution, a small well Vs-inch deep

and 1 inch in diameter may be made if neces- sary in the upper surface of the top component

of the specimen, directly beneath the water feed

tube through the calorimeter plate. For admission, the head of water over the calorimeter

plate should not be more than 6 inches. The

maximum time allowable for admission of water

is one week. If the full 10 percent by volume

cannot be introduced in this time, the amount

actually introduced should be noted. If dripping

of water from the undersurface of the specimen occurs, the rate of admission must be

slowed to prevent dripping, if possible. Thermal

resistance values obtained during this week are

45
not to be used when averaging to determine

the test thermal resistance, Rt. The maximum allowable exposure time for

expulsion of added water by self-drying during

the second summer exposure period to a moisture content equilibrium should not exceed 9

calendar weeks. During this time period (9

weeks) the insulating value of the specimen

should return to approximately the same value

as that observed before water was added.

After the test is completed the amount of

moisture in each component is determined by

oven-drying to constant weight.

12.6.8. Water Vapor Permeance and Hygroscopic

Moisture Capacity

The ability of the construction to self-dry in

service is also evaluated by the ratio Pt/Mc,

where pt is the permeance in perms (grains/hr

ft- in. Hg) of the room-side component of the

construction calculated from water vapor permeability determinations, and Mc is the calculated moisture content in lb/ft- of the total construction based on equilibrium hygroscopic

moisture content determinations of the component materials. The calculated value of the ratio

Pt/Mc numerically should be between 2 and 200.

The water vapor permeance, pt, is calculated

from the values of water vapor permeability

and thickness as follows

where fi is the water vapor permeability in

perm-inch units [grains/hr ft- (in. Hg/in.)]

determined by either ASTM E96, Procedure A,

or ASTM C355, Desiccant Method [20, 21].

The hygroscopic moisture capacity, M,-, is cal- culated and determined as described in 12.6.4,

Equilibrium Moisture Content.

The ratio Pt/Mc is included to assure that the

construction and especially its deck component

has a permeance in a range suitable for adequate self-drying without excessive moisture

accumulation in winter. Permeability measure- ments to determine Pt are sometimes variable on the same type of material. The value of the

ratio Pt/Mc is also subject to variations of Mc as discussed above. Therefore the suggested interval of 2 to 200 should also be subject to future

adjustment as more data become available.

12.6.9. Exposure Conditions

During exposure the atmosphere beneath the

specimen should be maintained constant at 90 °F and 30 percent relative humidity.

The temperature of the calorimeter covering

the upper surface of the top specimen component material should be programmed to repeat

daily the following (24-hour) cycle:

Calorimeter plate

temperature, °F

Winter Summer

10 a.m o o 00 10 10 a.m. to 12:30 p.m 38>75 75>138

12:30 p.m. to 4:30 p.m 75 138

4:30 p.m. to 7:30 p.m 75>38 138>75

7:30 p.m. to 10 a.m 38 75

Measurements of the heat flux, and the temperatures at the surfaces of all component materials, and of the specimen gross weight should

be made each working day between the cyclic

hours of 7 :00 and 10:00 a.m. For convenience

in testing, the 24-hour cycle of test time can be

shifted to enable observations at a preferred

time of day.

13. Summary

The simultaneous thermal insulating and self- drying performances of 27 specimens are given.

The specimens, 22 x 23 inches in plan, were

exposed in the laboratory to successive periods

of simulated summer and winter temperature

conditions on their outdoor surfaces, each with

simulated daily solar heating, and with the

moisture content of the materials ranging from

a near-dry to a wet condition that simulated the

results of a roofing leak. The dewpoint of the

indoor air was held constant at 54.5 °F.

Results for three of the specimens indicated

that a construction consisting of an air-dried

one-inch thick structural concrete deck under

each of three near-dry board types of thermal

insulation without a vapor barrier between materials allowed the insulation to remain dry in

winter and summer and resulted in good_ in- , sulating values. However, when the insulations were wetted in summer to simulate a roofing

leak, the concrete decks opposed the escape of

vapor from the moist insulations to the room

beneath and caused specimen insulating values

to decrease to as little as of that observed

for near-dry materials. These results were very

similar to results given in Part I for similarly

made specimens with concrete decks 3 inches

thick.

The thermal resistance of several specimens was high when they were made without vapor

" barriers using materials of moderately high

water-vapor permeance and were dried to a hygroscopic moisture content equilibrium.

These constructions when wetted were reduced

in insulating value but during simulated sum- mer conditions self-dried to the indoors within a few weeks time and recovered their insulating

value. The quantity of moisture gained by these

specimens during winter exposure conditions was not great enough to seriously reduce their

46
insulating value or prevent rapid expulsion of

winter-accumulated moisture downv^ard

through the deck as vapor during a subsequent

summer exposure, especially if the deck was

covered by a moisture-absorbent material.

Reductions in insulating value were not proportional to changes in moisture content. The

ability of an insulated flat-roof deck to yield

good year-round insulating value and self-dry

in service if wetted was dictated by the outdoor

surface temperature conditions, the indoor dewpoint temperature, the arrangement of materials

in the construction, the thickness of highlyabsorptive materials, the avoidance of a vapor

barrier on or between materials, and the physical properties of thermal conductivity, water

vapor permeance, and liquid-water and hygroscopic-moisture absorption of the materials.

Criteria for design of self-drying insulated

flat-roof constructions were developed and

limits of performance as obtained by laboratory

tests were suggested.

The authors recommend field tests on full- scale buildings to compare results with laboratory results on relatively small specimens as presented here.

This work was conducted at the National

Bureau of Standards with partial sponsorship

by the Office of the Chief of Engineers, U. S. Army; the Bureau of Yards and Docks, U. S. Navy; and the Office of Civil Engineering, U. S. Air Force.

The authors acknowledge with appreciation

the services of L. S. Crockett and D. R. Showalter, who constructed the apparatus, made the

majority of the test specimen installations, and

recorded the data. They also thank T. W.

Reichard for making the concrete specimens

and determining their compressive strengths;

T. W. Watson for determining the thermal conductivity of all materials by the guarded hot

plate method ; and W. C. Cullen for measuring

water vapor permeances.

Acknowledgment is made to the several as- sociations, institutes, and manufacturers who

freely contributed information and materials

for this investigation.

Table of Conversion Factors to Metric ( S.I. ) Units

Physical

Quantity

(and symbol used in paper)

To convert from

to multiply by

Length X inch meter 2.54* X 10-^

foot m 3.048* X 10-' Area inch 2 m- 6.4516* X 10-«

foot 2 m2 9.290 X 10-2 Volume inch^ m^ 1.639 X 10-5

foot^ m^ 2.832 X 10-2 Temperature Fahrenheit Celsius = (^F-32)/1.8 Temperature difference Fahrenheit Kelvin K = (AW/1.8

Pressure inch Hg (60F) newton/m^ 3.377 X 10^ Mass Ibm kg 4.536 X 10-1 Mass/unit area M lbm/ft= kg/m2 4.882 Moisture content rate Ibm/ft^ week kg/m's 8.073 X 10-« Density p Ibm/ft^ kg/m^ 1.602 X 101 Thermal conductivity k Btu/hr ft2 (F/inch) W 1.442 X 10-1 mK

U-value U Btu/hr ft^F W 5.678 m^ K

Thermal resistance R F/ (Btu/hr ft^) K/(W/m2) 1.761 X 10-1 Heat flow Btu/hr ft^ W/m2 3.155 Water vapor:

permeability M grain kgm/Ns 1.457 X 10-1= hr ft2(in.Hg/in.) permeance P,P grain kg/Ns 5.738 X 10-11

hr ftHin.Hg)

(perm)

J 'Exact value; others are rounded to fourth place.

47
150

140

130 120

110 u.

o - 100 ^ 90 ^ 80 < 70

IJJ 60

Q.5 50

40

30

20

"T—I—I I r

J \ L J I L I I I I I I I I 9 10 II 12 I 2 3 4 5 6 7 AM PM TIME OF DAY

9 10 II 12 I 2 3 4 5 6 7 AM

Figure 14. Daily temperature wave forms used to simulate winter,

spring, fall, and summer exposure conditions 07i the insulated flat-roof specimens for the calorimeter method of measiirement.

r

A

1, Insulated test room

2. Specimen on scales

3- Water pump

4. Storage tank

9. Thermostat

5. Sink

6- Heat exctianger

7. Immersion heaters

8. Air cooled refrigeration

10. Solenoid valve

Figure 15. Apparatus for measuring the effect of moisture on heat transfer, using the calorimeter method.

48
1. Specimen

2. Calorimeter plate

3. Guard insulation

A. Platform scale

5. Tubing to drill-hole

6. Water in

7. Water out

8. Vapor and water proofing

9. Ten differential thermocouples

10. Thermocouple

Figure 16. Construction and installation details of a typical specimen used with the calorimeter

method.
.Y GLASS FIBER

INSUL. BOARD

Expanded Shale Concrete

-WINTER 38-75F

SUMMER 75-I38F

NO VAPOR BARRIER

0.9 0.8

0.6

0.5

90F. 30% RH

(DEWPOINT 54.5 F)

LU 5 o

z^ 4

<r> IT i

_li

2

ir^ I

n

. 1st , EXF

c 1st

J ^OSUREc 2nd , PERIODS

2nd 3rd WINTER SUMMER WINTER

A

SUMMER

- Water

dded 10%

WINTER

Conl

1 38°F Cont.

1 B 1 Vol. (5.0 lb)

0 o

• ( jsign R- Dry (C=(3.14 )

\— • •

o

• ••

ecirr

^Glass

•

Fiber.

Sp en - V ' •

Nnsulation i

f

r

icrete,

C

fy Concret ^ Y 1

'

' 1 I'M r '

1 1

' '

1 I'M 1

' '

1 1

' ' 1 \\*' '

1 1

' '

1

11,80 1082

9.83 8.85 7.87 6.88

5.90

4.92

3.93

2.95

1.97 098

0

2 X

I- LxJ

O 03 5 <

10 15 20 25 30 35 40 45 50 55 60 65 70

TIME - WEEKS

Figure 17. Effect of moisture and exposure conditions on the thermal resistance of Specimen 1.
2y, MINERAL WOOL

INSUL BOARD

I " Expanded Shale Concrete

- WINTER 38 - 75 F SUMMER 75 - 138 F

NO VAPOR BARRIER

. 90 F, 30% RH

(DEWPOINT 54,5°F)

-^

< 1st

, EXPOc

!=•

>

1 SURE PERIODS

2nd 2nd 3rd WINTER SUMMER WINTER SUMMER <— WATER

Added 10%

WINTER

I38>

Cont. 38°F Cont. 1 BY Vol. (6.4 !b.)

o

• \

o

• o

^Design R Dry (C = 0.12) ^ • Oct* S pecffnen- •

MINEF :al wo

DRY

DL

1 r

^Insulation

1

CONC RETE, [)RY

ete /

1

' ' 1

1

1

' ' 1

1 1

' '

1 [ 1

'

' 1 f

1 ' '1

\

\ " I 1

1 '

'

1 f1

' '

1

1 1

' '

1 11

' '

1 1

1

' '

1

1

1

' ' 1 1' ' 1 1 ' ' 1 20 25 30 35 40 45 50 55 60 65

TIME - WEEKS

70

Figure 18. Effect of moisture and exposure conditions on the thermal resistance of Specimeri 2.
WOOD

INSUL, FIBER BOARD

r'Exponded Shale Concrete

-SUMMER 75-138 F WINTER 38-75F

- NO VAPOR BARRIER

_ 90F, 30 % RH

(DEWPOINT 54.5°F)

4

^ 1st EXPOSLJRE PE

, 2nd RIODS

2nd 3rd WINTER SUMMER WINTER

Ac

SUMMEF

- Water ded 10°/

WINTER

o r58F

XXlt.

1 38°F Cent i

By Vol. (6,4 b) c

0

\\

^Design R Dry (C = 0.I4)

\

Sf ecimen - 0

iwooo

'I '8 o• -*-^

1,

Inci ilntlnn

BOAR

FIBER

D DRY

e

]

CONOR :te, DRI

Concrete

I'M 1

' '

1 11

' '

1 I'M I'M 1

' ' 1 [

1

' '

1 T'^lj 1

' ' i| 1 ' '

1 I'M 1

' ' 1

17.03

16.14 15.24

14.35 13.45

12.55 &5

11.66

1 10.76 t-z 9.86 LlI 8.97 NTi

8.07 CO

7.17

LlJ 6.28 q:

Z3 5.38 tl/l 4.48 O

3.59

2.69

1.79 0.89 0

10 15 20 25 30 35 40 45 50 55 60 65 70

TIME - WEEKS

Figure 19. Ejfect of moisture and exposure conditions on the thermal resistance of Specimen 3.
6.0"PERLITE CONCRETE

1/2' GYPSUM FORMBOARD

WINTER 38 -75 F SUMMER 75-138 F

- NO VAPOR BARRIER

- 90 F, 30 % R.H.

(Dowpoltil ,54.5 'F)

1 St 1st EXPOSURE PERIO

Z nd — 2 nd _

3 rd

ADDED

VOL.

B)

SUMMER WINTER SUMMER

GYPSUM FORM

BOARD

REMOVE

WINTER SU VIMER «—WATER

3.4% BY

(3.7 L D

^DESIGN R-DRY (C=(5.12 )

"^PERLIT E CONCRETE, DRY

=ECIMEN

=ERLITE C DNCRETE Vgypsu (1 FORMBOA !D. DRY '-•--•-^ ^GYPSUM FORMBOARD —

1

'

'

1 11

1

' ' 1 I'M I'M 1

' '

1 M '

4 4.70

4 1.90

39. I I

36.32

33.52

50.73 2794

25.14

22.35

19. 55

16. 76

13. 97

I I. 17

8.38

5.59

2.79

0

50.56

47 40

I- 44.24 5

UJ 41.08 ^

>- 37.92 g

34.76 ^

O

3 1.60 <

28.44

H" 25.28 S

HZ

22.12 O

18.96 ^

=) 15.8 ^

O

12.64 S

9.48

6. 32

3. 16

0

10 25

TIME -WEEKS

Figure 20. Effect of moisture and exposure conditions on the thermal resistance of Specimen A.
6.75 VERMICULITE CONCRETE

WINTER 38-75F

SUMMER 75-I38F r

NO VAPOR BARRIER

0.5" GYPSUM BOARD

UJPbJ

tr 2 <

UJ I

CD (r

90F, 30% R.H. (Dewpoint , 54.5 °F)49.10 45.00

40.91

J 36.82 ^ 32.73 \ 28.64

\ 24.55

J 20.46

^ 16.37 t 12.27 \

:

EXPOSL RE PERIODS

*

1 ST SUMMER1 ST WINTER

2 NO SUMMER

ULITE CO —DRY —kCRETE,

esign R Dry (C=C).20)

/ Specimen

*

^Vermiculite Cor c. ^GYPSU 1^ BD. DRY ^^^ypsum Bd. MMM 1 M M 1' 1 M 1 M M MM

4.09 0

10 15 20 TIME -WEEKS

Figure 21. Effect of moisture and expo- sure conditions on the thermal

resistance of Specimen 5.
VERMICULITE CONCRETE

1/4" CEMENT ASBESTOS BOARD

WINTER 38- 75 F SUMMER 75- 138 F

NO VAPOR BARRIER

90 F, 30 % R,H, (D«wpolnl.54.5 "Fl

1 St 1st ^ EXPOSURE PERIO _ 2 nd 2 nd 3rd SUMMER WINTER SUMMER

CEMENT

ASBESWINTER SUNDMER — WATER ADDED 9% BY VOL.

(11.5 LB)

)US BOARD REMOVED

- DESIGN R -DRY ( C=

(

3.21 )

IVVERMICUL ITE CONCRETE, DRY

SPECIMEN

"^VERMICULmE CONCRETE

-CEMENT AS lESTOS BD., )RY ^^CEMENT ASBESTOS BOA ?D

1

1

' M 1' M 1 M '

1

1 1

' M 1 M '

1 1

' '

1 I'M I'M 1 ' '1

UJ

41.12 45.47

39.25 43.41

37.38 41.34

35.51 39.27

33.64 37.21 — 31.77 35.14

29.90 33.07

28.04 31.01

26.17 28.94

24.30 26.87

22.43 24.80

20.56 22.74 <

J* 18.69 20.67 h"z

16.82 18.60

UJO

14.95 16.54 o

13.08 14.47 UJ

cc

-D 11.21 12. 40 h-<no

9.35 10.34 s

7 48 8.27

6.61 6.20

3.74 4. 13

1.87 2.07

0 0

20 25 35 40 TIME-WEEKS

Figure 22. Effect of moisture and exposure conditions on the thermal resistance of Specimen 6.

55
PERLITE CONCRETE

1 POLYSTYRENE

-WINTER 38-75 F SPRING-FALL 56- 106 F SUMMER 75-138 F

— NO VAPOR BARRIER

-90 f; 30% R.H

(DEWPOINT, 54.5 F)

10X106-

9

6

54

3

2

I 0 70

6.5

6.0

5.5

5.0

4.5

40

3.5

3.0

2.5

2.0

1.5

1.0

0.5

0

5 50 55 60 65 70 75 80 TIME - WEEKS

Figure 23. Effect of moisture and exposure conditions on the thermal resistance of Speci- men 7.
3,0° PERLITE CONCRETE

\% INSULATING BOARD

WINTER 38- 75 F SUMMER 75- 138 F

NO VAPOR BARRIER

90 F, 30 % R,H.

(DewpoinI , 54.5 °F)

1 St 1st ^ ^ EXPOSURE PERIO 2 nd _ 2 nd 3 rd

ADDED

BY VOL.

B)

SUMMER WINTER SUMMER WINTE R

PAIC

PRAY

<TED TWO OATS

SUIV1MER

1— water

14.5 %

(12.3 L

^DESIGN R-DR * (C-0.I2E )

^ INSULA" ING BOARD, DRY

; IPECIMEN

0 o o r-o-o-D-5-c 0 o

U ^ ' 0 o

o

"

• ^ INSULATI iJG BOARD

-•-•-•-•-< .•-•-•-•-^ ^PERLITE CONCRETE , DRY • • « t

'

i-> ,-«.».,

' PERU fE CONCR :te

1

'

'

1 I'M 1

' '

1 1

' '

1 I'M I'M M '

1 1

' '

1

1 ' '1 1

' '

1

TIME -WEEKS

Figure 24. Effect of moisture and exposure conditions on the thermal resistance of Specimen 8.
3 PERLITE CONCRETE

If WOOD FIBER INSULATING BOARD

-WINTER 38-75 F SPRING -FALL 56-106 F SUMMER 75-138 F

-NO VAPOR BARRIER

-90 F, 30 7oR.H.

(DEWPOINT, 54.5 F)

1

1 1 M MMIMM MM Li U MM MM M Ml 1 iMJ MMJ MM MM MM [MM MM 1 11 M MM

. INSULA riNG BC ARD TOP Pi :rlite CONOR ETE -y

/

1 4—f—MID -PERL TE CO JCRETE

f f

* I. .J

6-5

si

o

I-<no

30 35 40 45 50 TIME -WEEKS

Figure 25. Effect of moisture and exposure conditions on the thermal resistance of Specimen 9.

58
5.5 ^ 5.0 <

u. 4.5 O

g 4.0 H 3.5

h.> 3.0

u>m

-I Z.5 2 2.0

UJ

I-z 1.5

1.0

0.5

0

3.0° VERMICULITE CONCRETE

iVz INSULATING BOARD

WINTER 38- 75 F SUMMER 75- 138 F

NO VAPOR BARRIER

- 90 F, 30 % R. H. (Dewpoirt , 54.5 "F)

1 St \ St - EXPOSURE PERIO

Z no 3 rd

ADDED

BY VOL.

B)

SUMMER WINTER SUMMER WINTE R

« SPRAY

PAINTED TWO COATS

sup MER ^ WATER

172 %

(14.1 L

^DESIG M R-DRY (C;-ai7)

/ SPECIME N ^ >-•-•-•' ^ INSULA riNG BOAR

.-,-•-«-•- >-• ^INSULATII IG BOARD, IRY

Wermicul ITE CONCR ETE, DRY ^VERMICU LITE CONC ?ETE

> » « 1 MM MM MM MM [mm

43.64 K

39. 67 5

UJ 35.70 *

31.74 g

27.77 ^

O

23.80 §

19.84 *t

I- 15.87 S

11.90 O

o

7.93 UJ

(T

O

0 s

0 5 10 15 20 25 30 35 40 45 50 TIME -WEEKS

Figure 26. Effect of moisture and exposure conditions on the thermal resistance of Specimen 10.
2"GYPSUM CONCRETE

INSULATING BD.

WINTER 38-75 F SUMMER 75-138 F NO VAPOR BARRIER 90 30 % R.H.

(Dewpoirl , 54.5 °F) k

EXPOSURE PEF?IODS

1 ;t summer 1 ST WINTER

-" WATER ADDED

10% BY SPEC. VOL (8.57 LB.l

2 ND SUMMER 2 ^ WINlDrER -SPRAY

PAINTED COATS

3

su^ RDMER ^HeO

ADDED

24.1 %

BY VOL (2a S LB.)

ITING BO ARD, DR1 I. - DESIGN R DRY C = 0.I25

- SPECIM LN

C

p? o-n

O or

0 .dock/ w

• -6 ^

•INSULA IN6 BO iRD ¥-

M CONC )ETE, DR ^GYPS IM CONC RETE L,.

I'll] MM I'M MM MM MM MM I'M MM I'M MM MM MM

44.75

41.03 37.30 33.57 29.84

26.1 I 22.38

I 8. 65

I 4.92

11.19 7 46 3 73

0

10 15 20 25 30 35 40 45 50 55 60 65 TIME-WEEKS

Figure 27. Effect of moisture and exposure conditions on the thermal resistance of Specimen 1

60
2 "gypsum concrete WINTER 38-75 F SUMMER 75- 138 F NO VAPOR BARRIER 90 F, 30 % R.H. 8.0 7 5

6.5

2.5 2.0

1.5 LO

0.5

uj" 4 o

i 3

in

a: 2

_j<

a.UJ

n

A

EXPOSU RE PEf?IODS

-1 ST Wl

1 ST SUMMER

NTER —WATER ADDED

10% BY SPEC. VOL. (9.1 LB)

2 NO

WINTER

2 ND SUMMER — WATER ADDED 20% BY VOL.

I1B.3 LB.I

3 RD WINTER — SPRAY

POINTED

2 COATS

3SUARD

IMER WER

ADDED

%

BY VOL.

120.6 La;

r

OD-F BER CEK ENT BD.,1 ;CIMEN

^ DE IIGN R D RY (C'O 165)

0

o

<}*''

1

-o^ —W. •^^

i

'i ^ W )0D-F1BE ^ CEMENl BD. >***•*'

^OYPSUI 1 CONCRI TE. DRY ^GYP SUM COl ICRETE

I'M I'M MM I'M MM MM I'M M M 1 M M MM MM I'M MM

50.29

47. 15 44.00

40.86 37 72 3457

31.43 28.29

25.14 22.00

18.86 15.72 12.57

9. 43

6. 29

3. 140

0 5 10 15 20 25 30 35 40 45 50 55 60 65

TIME -WEEKS

Figure 28. Effect of moisture and exposure conditions on the thermal resistance of Specimen 12.

61
WOOD-FIBER CEMENT

WINTER SUMMER•75F I38F

m ^

1.0 0.9 as

0.7

0.6

0.5

0.4 Q3

0.2

0.10

CO

CO

LU

LU

EXPOSURE PERIODS

90F, 30% R.H.

(Dewpoinf , 54.5 °F) 1706 , 15.35

I 1365

I 11.94 b 10.23 C 8.52

1^ 6.82 Q 5.12 F

c 3.41 I

1.71 '

0

— SPRAY

PAINTED

2 COATS

2 NO

SUM/ilER WATER AOOED

4.6 %Br

VOL

(Z.SLB.f

DESISN R DRY (C-0.201

^WOOD-FIBER CEMENT BO., DRY

WOOD- FIBER CEMENT 8D.

C AIR-SURFACE

10 15 TIME-WEEKS

20 25

Figure 29. Effect of moisture and expo- sure conditions on the thermal

resistance of Specimen 13.
3 PERLITE CONCRETE

I" Glass Fiber Formboard

WINTER 38-75F

SUMMER 75-I38F

• NO VAPOR BARRIER

90F, 30%RH

(OEWPOINT 54.5° F) 43.83

38.35

yi o O CD s <

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80

TIME -WEEKS

Figure 30. Effect of moisture and exposure conditions on the thermal resistance of Specimen IJt.

63
PERLITE

3" CONCRETE

"g

1 \lz GLASS FIBER BOARD

WINTER 38-75 °F, SPRING a FALL 56-l06°F "summer 75-I38°F

-NO VAPOR BARRIER

90°F, 30% R.H. (OEWPOINT, 54.5 "F)

10x10°9

87

10 6 1 5 ° 4

3

2

I 0

4.0

< 3.5

iLl o: 3.0

u.o

o

n: 25 ^ 2.0

UJ

a:

=> 1.5 I-<ni io

5 0.5

0

1

1

1

1 I'M I'M 1

1 ' 1 1

' '

1

1

1

' 1 1 1 ' '

1 1

1 1 1 1

' '

1

1

1 '

1 M '

1 1

' ' 1 «

1

t'-y

;

:i— MID- GLASS FIBER BOARD'' PE RLITE

T C0^ CRETE

TOP PERLITE CONCRETE

- 39.0

34.13 ,

29.25 •-

1

2 O

24.38 z*

19.5

14.63

9.75

4.88

0

UJ

I I

'

I

I

I

I I I

I

I

I '

I

I M M I

I '' I

I M I I

I

I

I I

I

I

I

'

I

I

I

I ' I

I

I

I I

I

I

I '

I

I

I M

0 5 10 15 20 25 30 35 40 45 50 55 60 TIME - WEEKS

Figure 31. Effect of moisture and exposure conditions on the thermal resistance of Specimen 15.

64
3" PERLITE CONCRETE

WINTER 38-75 F SPRING- FALL 56 - 106 F SUMMER 75- 138 F

-COTTON SCRIM

-NO VAPOR BARRIER

ly GLASS FIBER FORMBOARD

-90F, 30% R.H. (DEWPOINT, 54.5 F)

Z

S2

iij si

(£h-

<OO

K

COO

CO

(OUJ

a:

-I<

(£X

9

8

7

6

54

3

2

I0

3.5

MM I'M MM 1

1 '

1 MM MM 1

1 M 11 M MM MM I'M

I GLASS -IBER B

I

J TOP PE RLITE :qncbe F 1

r

1 MIO PEF ILITE C< ^NCRETI

-I

1 -*

I

I ji M I M M I

I

I M I

I

I M I

I

I M I

I

' M I M M I

I I M I

I

1 >

I 60 65 70 75 80 85 90 95 100 105 110 115 TIME -WEEKS

Figure 32. Effect of moisture and exposure conditions on the thermal resistance of Specimen 16.
3" VERMICULITE CONCRETE

I" Glass Fiber Formboard

WINTER 38-75F

SUMMER 75- 138 F

NO VAPOR BARRIER

90 F , 30% R,H. (DEWPOINT 54.5 F)

T 37.73

TIME -WEEKS

Figure 33. Effect of moisture and exposure conditions on the thermal resistance of Specimen 17.
-WINTER 38-75 F SUMMER 75- 138 F

40.34

TIME - WEEKS

Figure 34. Effect of moisture and exposure conditions on the thermal resistance of Specimen 18.

67
GLASS FIBER BOARD

3" PERLITE CONCRETE

WINTER 38-75 "F, SPRING a FALL 56-106 "F, SUMMER 75-l38°F

NO VAPOR BARRIER

_ 90°F, 30 % R.H. (DEWPOINT 54.5 °F)

- 10x10*

J

I 9

8

i 7

I in 6

' o

; 4

3

> 2

I

I

I 0

2.0

2 1.5

a:<o 1.0

0.5

I'M I'M 1

' '

1 1

' ' 1 I'M 1' ' 1 1

' ' 1 I'll I'M :^ 1'

' 1

-PERLITE CONCRETE

SLASS "IBER B OARD —

*^ \

19.47

9.73

LjJ —

i>o o<

s

Figure 35. Effect of moisture and exposure conditions on the thermal resistance of Specimen 19.

68
3 PERLITE CONCRETE

-WINTER 38- 75F

SPRING -FALL 56-I06F SUMMER 75- 138 F

NO VAPOR BARRIER

— 90F, 30 % R.H. (DEWPOINT, 54.5F)

10 X I06 9

87

6

54

3

1

1 1 1 I'M 1 1

' '

1 I'M 1

' 1 11 I'M 1 1

' '111 1

' '

1 1

1 '

1 I'll I'll 1

' '

1 1

' '

1 1 1

' ' 1 1' MJ 1 ' '

1

BOTTOj 1 PERLT tCONC RETE — h

HD PER JTE CC NCRET

--INSULATtN( 1 ^OARI t

TIME- WEEKS

Figure 36. Effect of moisture and exposure conditions on the thermal resistance of Specimen 20.

69
l" POLYSTYRENE

3" PERLITE CONCRETE

WINTER 38-75 F SPRING- FALL 56- 106 F SUMMER 75 -138 F

NO VAPOR BARRIER

_ 90F 30% R.H. (DEWPOINT 54.5F)

10X109

87

6

54

3

2

I 0

MM MM MM 1 ' M J MM [MM MM 1 ' '

1 MM MM MM M 1 M MM M 1 1 MM MM MM

—BC TTOM PERLn E

LYSTYF ENE

2 -Ml 1 PERU FE

Si

si

o

I-o

TIME WEEKS

Figure 37. Effect of moisture and exposure conditions on the thermal resistance of Specimen 21.

70
3" PERLITE CONCRETE

WINTER 38-75F

SPRING- FALL 56-i06 f SUMMER 75- 138 F COTTON SCRiM NO VAPOR BARRIER

90 F, 30%R.H.

(DEWPOINT 54.5 F)

W

LlI to <ioo

LlI <

OC LlI 3 CC

I- <

,

. (E COm

I'M I'M 1

' ' |-

1

' '

1 1

' '

1 1

'

'

1 1

' '

1 I'M 1 1

' ' M 1' ' 1 I'M

BOTT pM PER JTE-^ — W

eo MCRETE

GLA

i SS FIBE OARB—

R / MID-PE (LITE C 0NCRE1 E

TIME-WEEKS

Figure 38. Effect of moisture and exposure conditions on the thermal resistance of Specimen 22.
6" PERLITE CONCRETE

WINTER 38- 75F

SPRING- FALL 56- 106 F SUMMER 75-I38F

- 90 F, 30% R.H. (OEWPOINT, 54.5 F) MM MM M M 1 1

1 M MMIMM 1'

1 1 'Mil ' ' 1

M_.P_ER

1 ' M |! ' M 11 MM MM I'M

-BOTTO LITE CONCRETE--^

rop PE ILUE C 3NCRETI I - t-

-MID P!;rlite CONCRE r ^

U 10X10® oz 9

8

CO 7 CO

RE to 6 i

5 < O 4 o 3

q:

1- 2

0

6.0

5.5

5.0

4.5

4.0

3.5

3.0

2.5

2.0

1.5

1.0

0.5

0

lo* Lt^ i_ lo* ISt-)^ ALL Wl

EXPOSURE PERIODS 1 I38fJ WINTER SPRlN(; SUMMER F 3NTINU0US

1 1

WTER

'

t

SP 1ING ' Su"mMER t

1 WATER ADDED

I IRQ o/_ ov cDcr TEST VOL. (17 5 .8.1

— -DESIB N R DF Y (C=< .18)

^SPECIMEN

ID, OR E OTTOM PERLTC, DRY

TDP PEF LITE — BOTTOM

vtx. "'JO

PERLITE

M. PERUT AR SURFACE M'MMM

0 5

1

^ ' ! 01

1 !

1

, 1

I 1 5 20 2

' '

1

, M '

1

1 M M . M '

1 5 30 35 40 4 M '

1

j M '

1 M 1

1

j M '

1 1

' '

1

1

[

' '

[ 5 50 55 SO 65 70 7 M ' 1 M '

1 5 80 8 TIME -WEEKS

Figure 39. Effect of ^noisture and exposure conditions on the thermal resistance of Speci- men 23.
- WINTER 38-75 F SPRING -FALL 56-106 F SUMMER 75-138 F

-90 F, 30% R. H. (DEWPOINT, 54,5 F)

10X109

87

to s 5 5 § 4

3

2

I 0

3.0

.— BOl TOM P :rlit TO ' PERL TE

1 J>

1

'

I

i

...

TIME- WEEKS

Figure 40. Effect of moisture and exposure conditions on the thermal resistance of Specimen 2U73
WINTER 38-75F

SUMMER 75-I38F

TIME- WEEKS

Figure 41. Effect of moisture and exposure conditions on the thermal resistance of Specimen 25.
WINTER 38-75 F SUMMER 75- 138 F 2/2" MINERAL WOOL BD.

2 GYPSUM CONCRETE

•NO VAPOR BARRIER

90 F, 30 % R.H. (Dewpoint 54.5 °F)

w'

1209

11.28 10.48 9.67 8.86

8.06

7.25

6.45 5.64

4.83 4.03

3.22 2.42

1.61 0.81 0

°5o

>-

tt Q

O CD s <

15 20 25 30 35 40 45 50 55 60 65 70 75 80 TIME -WEEKS

Figure 42. Effect of moisture and exposure conditions on the thermal resistance of Specimen 26.
Z\ INSULATING BOARD

2' GYPSUM CONCRETE

WINTER 38-75F

'summer 75-I38F

NO VAPOR BARRIER

90F, 30% R.H.

( Dewpoint, 54.5°F)

TIME -WEEKS

Figure 43. Ejfect of moisture and exposure conditions on the thermal resistance of Specimen 27.
SPEC. 19 60

40

20

0 -20

-40

140

130

120

110

100

90

80

7060

40

20

0 20

-40

-60

140

130

120

1 1 1 1

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 ! 1 M

\ OUT QI

\ ROOF ^

! . 1 MMVi/m^i I 1

INTO ROOF

1 M M 1

SPEC. 19

I I I I I I I I I M I M I I I I I I M I

OUT OF ROOF t

M M I M M M I I I M I I I I I I

1 1 1

1 M 1 1 M 1 M M M M 1 1 1 1

^^^^^^^^^^INDOOR SURFACE

/ I^MID-PERLITE

/ \ ^INTERFACE, PERLITE-GLASS

/ \ FIBER

/ \ TOP SURFACE

/ \ X

MM M 1 1 1 1 1 1 1 1 M M 1 1 1 II SPEC. 15

I I I I I M I M M I M I I M M I I

I I M M I I II II II II M II M I

1 1 1 M 1 1 1

1 1 1 1 M M M M II

, indoor surface _

j!L

^INTERFACE, PERLITE- \ GLASS FIBER

Jf \ -~_«^MID-PERLITE

/ .^TOP SURFACE

1 1 1 1 1 1 M 1 1 M II 1 II

0 2 4 6 10 12 14 16 18 20 22 24 0 SUMMER

10 12 WINTER

14 16 IB 20 22 24

ELAPSED TIME, HOURS

Figure 44. Temperatures and top surface heat flux results for one day of a summer and winter exposure condition with specimens at low moisture contents. Specimen 19 contains a deck made from 3-inch-thick perlite aggregate insulating concrete that was covered with a lV2-inch-thick glass-fiber formboard. Specimen 15 materials were identical to Specimen 19 except that the glass-fiber formboard was the deck and the insulating concrete was used as a top cover.

77
A. MAXIMUM FABRICATION TIME I WEEK

B. MOISTURE CONTENT EQUILIBRIUM , DEFINED BY RATE OF LOSS , 0.05 Ib/tf^ WEEK

C. M,/M(; £ 2.0

D. WINTER MOISTURE REGAIN , AVERAGE RATE OF GAIN <: 0.05 Ib/ft^ WEEK

E. PERIOD OF SELF- DRYING OF WINTER REGAINED MOISTURE

F PERIOD OF SELF- DRYING FOLLOWING WATER ADDITION (10% BY VOLUME) IN SIMULATION OF ROOF LEAK a AVERAGE VALUE OF R, FOR 37 WEEKS. RATIO R,/Rq S 0.6

Figure 45. A typical laboratory performance characteristic of a speci- men that satisfies the suggested criteria for design of insulated self-drying flat-roof deck constructions.

78
Announcement of New Publications in

Building Science Series

Superintendent of Documents,

Government Printing Office,

Washington, D. C. 20402

Dear Sir: Please add my name to the announcement list of new publications to be issued

in the series: National Bureau of Standards Building Science Series.

Name

Company

Address

City State Zip Code

(Notification key N-339)
I

J

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1^
ORM NBS-114A (1-71) U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO. NBS-BSS-37

2. Gov't Acccssicm No.

4. TITLE AND SUBTITLE

The Effect of Moisture on the Heat Transfer Performance

of Insulated Flat-Roof Constructions

7. AUTHOR(S)

Frank J, Powell and Henry Eo Robinson

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

I Office of the Chiet ot Engineers, U.S. Army; the Bureau of Yards

and Docks, U.S. Navy; the Office of Civil Engineering, U.S.

j Air Force. C/o Mr. Stanley Howell, Chm., Room 1C31, Y&D Bldg.

I Department of the Navy, Washington, D.C. 20390.

15. SUPPLEMENTARY NOTES

3. Recipient's Accession No.

5. Publication Date September 197

L

fi.Perfonning Organization Code

8. Perfornung Organization

10. Project/Task/ Work Unit Nc 4212133

11. Contract/Grant No.

N/A

13. Type of Report & Period Covered

Final

14, Sponsoring Agency Code

u??-^^^*^!.^ (A 200-word or less factual summary of most significant information. If document includes a significant bibliography or literature survey, mention it here.)

I A solution to the problem of unwanted moisture in the thermal insulation of flat

{[roofs was found during a recently completed laboratory investigation of the effects of

I moisture on heat transfer through these constructions. The objective of the research

I was to ascertain how much the insulating performance of conventional constructions, hav-

; ing insulation over concrete decks, was affected by moisture. This is presented as Part

I lo Also, the objective was to investigate properties of materials, their arrangement and

dimensions which would yield a construction having an adequate degree of self-drying a- bility, combined with low winter moisture regain rate. This is presented as Part II

o

The results show that the best insulating and moisture performance was obtained by utilizing the heat of the summer sun on the roof to vaporize and transfer to the room beneath any free moisture contained within the construction. Roof specimens made from

moderately vapor-permeable materials without conventional vapor barriers were, in winter,

able to accommodate the small quantity of slowly accumulated condensation without dripping or severe loss of insulating value. During the nine-year investigation, the performance characteristics of 73 insulated roof deck specimens were obtained and two new

methods of measurement were developed. Criteria for the design of self-drying insulated

flat roofs were developed and limits of the parameters containing the main variables tha

:

affect performance were suggested.

This paper presents complete results of the research which was sponsored jointly by

the National Bureau of Standards, the Army, the Navy and the Air Force.

17. KEY WORDS (Alphabetical order, separated by semicolons)

Plat roofs; heat transfer; moisture transfer; thermal insulation.

18. AVAILABILITY STATEMENT

fxH UNLIMITED.

1 1 FOR OFFICIAL DISTRIBUTION. DO NOT RELEASE

TO NTJS.

19. SECURITY CLASS

(THIS REPORT)

UNCLASSIFIED

21. NO. OF PAGES

81

20. SECURITY CLASS

(THIS PAGE)

UNCLASSIFIED

22. Price 75 cents

USCOMM-DC 66244-P7I

n. S. GOVERNMENT PRtNTrNG OFFICE : 1971 O - 425-845



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