Ci^ >u-l

NATIONAL BUREAU OF STANDARDS REPORT

6283

THE EFFECT OF MOISTURE ON HEAT TRANSFER THROUGH

INSULATED FLAT ROOF CONSTRUCTIONS

by

H. E. Robinson and F. J. Powell

Final Report

to

Office of the Chief of Engineers

Bureau of Yards and Docks

Department of the Air Force

Washington 25, D. C.

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NATIONAL BUREAU OF STANDARDS
THE NATIONAL BLKEAU OF STANDARDS

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NATIONAL BUREAU OF STANDARDS REPORT

MBS PROJECT MBS REPORT

1006-12-4887 January 15, 1959 6283

THE EFFECT OF MOISTURE ON HEAT TRANSFER THROUGH

INSULATED FLAT ROOF CONSTRUCTIONS

by

H. E. Robinson and F. J. Powell

Heat Transfer Section

Building Technology Division

Final Report

to

Office of the Chief of Engineers

Bureau of Yards and Docks

Department of the Air Force

iMPOHtANT NOYICE

NATIONAL BUREAU Of STA

Intended for use within the C

to sdditlonal evaluation and re listing of this Reporti either li the OlHce of the Director, Nat

however, bjr the Government 1 to reproduce additional copiei

Approved for public release by the

Director of the National Institute of

Standards and Technology (NIST)

on October 9, 2015.

>rogreas accounting documenta

rmally published it Is subjected

reproduction, or op3n<llterature

slon Is obtained In writing from Such permission it not needed,

prepared If that agency wishea <NBS>

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
• >• '• ,

.v-3. f "'fl'

%r.

I

m

i
TABLE OF CONTENTS

Page

I. Introduction 1

II. Method of Investigation 2

III. Experimental Exposure Conditions 4

IV. Test Apparatus 5

V. Test Phases: Objectives and Specimens 7

VI. Presentation of Results 9

VII. Conclusions and Relevant Discussion of Results... 11

1. Effect of Moisture on Insulating Value,

and the Influence of Exposure Conditions

on this Effect 11

2. Effects of Vapor Barriers and of Dense

Concrete Decks 13

3. Behavior of Specimens Having No Dense

Concrete Deck or Vapor Barrier 15

h. Pressure-Relieving Vents 19

5. Roof Insulating Materials 20

VIII. Remarks and Comments 24

A. Test Conditions 24

B. The Moisture Problem 26

C. Self-Drying Roofs 27

TABLES I through VI - After Remarks and Comments

Figures 1 through 11

THE EFFECT OF MOISTURE ON HEAT TRANSFER THROUGH

INSULATED FLAT ROOF CONSTRUCTIONS

I. INTRODUCTION

The roof constructions used on many Government, industrial, and commercial buildings commonly consist of an

approximately flat roof deck of monolithic concrete or gypsum, pre-cast slabs or tiles, steel, or boards, protected

from the weather by a built-up roofing of several plies of

saturated roofing felt laminated by mappings of a suitable

compatible bituminous mastic. Thermal insulation of roofs

of this type is usually accomplished by placing on the roof

deck a layer of insulating material, which then is covered

by the roofing. Among the insulating materials used for this

purpose are factory-made board types (e.g., mineral or vegetable fiber insulating board, cork board, cellular glass or

plastic, and pre-cast insulating slabs of various kinds), and

pour ed-in-place or castable materials (e.g., lightweight

insulating concretes, gypsum, and insulating fills consolidated in place). The thicknesses of the various materials

are adjusted to yield the desired insulating value for the

roof, depending on available information on the thermal conductivity of the materials.

Such data are available for most roof insulating

materials in a substantially dry state , but useful information on their insulating value if moisture is present in them

is not available. The lack of information is only partly due

to the difficulty of conductivity measurements on moist

materials. It is due also to the facts that moisture contents in service are likely to be unpredictable, and that

because of moisture migrations in the material, the effect of

a given amount of moisture is markedly influenced by the

variations of temperature in the insulating material imposed

by the climatic exposure of the top surface, including solar

heating

.

It is highly probable that moisture will be present in

most insulated roof constructions, either from the outset, or

at some time in the service life of the roof. In construction practice, the chances are not inconsiderable that a board-type insulation, shipped from the factory in a subs tantially-dry condition, may be wetted by precipitation

or dew, in transport, site storage, or during application,

before it is protected by the built-up roofing. For hygroscopic insulating materials, exposure to the relative

humidity of the atmosphere during the construction process

may result in a significant moisture content in the material

- 2

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when the roofing is applied. Pour ed-in-place insulations,

such as insulating concrete or gypsum, which are pre-mixed

with water, are almost certain to contain water in excess of

that required for hydration or chemical combination, and

these, as well as insulations consolidated in place without

water, may be wetted by dew or precipitation before the

roofing is applied. In addition to these possibilities,

which may be referred to as leading to initial moisture in

roof insulation, there may also be accidental wetting of the

insulation by water entry through leaks or punctures in the

roofing, either during construction, or in service, and as

the roofing ultimately deteriorates. There may also be,

under some circumstances, a gradual accumulation of water due

to condensation of water vapor entering through the undersurface of the roof construction.

The effect of moisture in roof insulation permeable to

water or vapor is, in general, to seriously reduce its insulating value, apart from its tendency to cause physical

deterioration of the roofing, or of components of the roof.

Failure of a roof to develop its* expected insulating value

causes increased expense for heating or cooling buildings,

and may lead to inadequate capacity of heating and cooling

systems designed on the basis of the expected insulating

value. In order to obtain information on the effect of moisture on the insulating value of various kinds of insulated

flat roof constructions, by means of which designs and specifications might be improved or compensating estimates of

effective insulating value be made, an experimental investigation was undertaken at the National Bureau of Standards

under the joint sponsorship of the Corps of Engineers, the

Bureau of Yards and Docks, and the U. S. Air Force.

This report presents a summary of results and conclusions obtained from tests covering three years and a total of

46 insulated roof specimens, each l8 inches square, exposed

to successive periods of simulated summer and winter top-side

temperature conditions, each with simulated daily solar

heating of the top 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.

II. METHOD OF INVESTIGATION

A primary need in this investigation was to be able to

measure the effective thermal conductivity of the insulation

of a roof deck specimen, and to record its changes as the
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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, the specimens were mounted

horizontally between an upper and lower chamber (see Figure

2), in each of which air at controlled temperatures was

caused to flow parallel to the exposed contiguous face of each

specimen. Solar heating was simulated by raising the top-side

air temperature for six to seven hours per day, depending on

the seasonal condition. The daily air temperature cycles in

the two chambers for the summer and two winter exposure conditions used in the investigation are shown in Figure 1. The

temperatures shown are the averages of values taken from the

circular charts of the air temperature recorders. Departures

of a few degrees F from the given cycle temperatures occurred

from time to time in the course of the work, chiefly in the

initial stages of Phase 1, and purposely in exposure condition

0 of Phase 3*

The general arrangement of a roof deck specimen is shown

in Figure 4, which indicates the positions of thermocouples

and of a heat flow meter permanently attached on the roofing

of the specimen. If steady-temperature conditions prevail,

the observed heat flow meter reading and the temperature drop

across the insulation enable calculation of the effective

thermal conductivity of the layer of insulation.

As Figure 1 shows, the air temperatures in the chambers

were substantially constant from 2 to 8 A.M. Figures 5 to 8

give, for two insulations representing extremes in thickness

and heat capacity, temperatures and heat flow rates observed

at 75-minute intervals throughout typical 24-hour cycles of

the imposed summer and winter exposure conditions. 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 conductivity of the insulation at its existing moisture content and

distribution. It was expected that the thermal conductivities

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.

III. EXPERIMENTAL EXPOSURE CONDITIONS

If liquid water is present in a roof construction, the

distribution of moisture and the migration of vapor are subject to the saturated vapor pressures of water at the various

temperatures existing within the construction at points where

there is liquid water, as well as to such properties of the

insulation and other components as absorptiveness, capillarity

and vapor permeability. The movement of moisture as vapor,

and thus its effect on insulating value, is therefore affected

very substantially by the temperature differences within the

construction, and by the temperature level, since the vapor

pressure of water increases more than linearly with temperature. It was necessary in this work, therefore, to select

experimentally-f easible temperature conditions which would be

representative in significant respects of those to which the

generality of roofs in this country are exposed in service.

For summer conditions, it was assumed that the temperature of the roll roofing of a flat insulated roof would be

about 75°F at night, on the average. During the day when the

roofing was subject to solar heating, its temperature was

assumed to rise to about 1^5 to 150°F. For the condition

designated as winter, it was assumed that the 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 55°F

when heated by the sun.

The conditions selected for the air underneath the test

specimens were 90°F and 70 percent R.H. , corresponding to a dewpoint of 79°F and a vapor pressure of 1.00 inch Hg . These

conditions were maintained closely throughout all winter

exposures, but during the summer exposures, the bottom air

temperature tended to rise in response to heat transmitted

downward through the test specimens during the solar heating

period, as shown on Figures 5 and 7«

The temperature and relative humidity selected for the

bottom air are higher than might be expected in the majority

of buildings in winter, at least, but were adopted with the

view of speeding the rate of moisture accumulation in specimens under winter exposures.

The 79°F dewpoint maintained under the specimens was

always higher than the roofing temperature during the winter

conditions, and in the "night-hours" of the summer condition.
I
- 5

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and therefore during these times, the vapor pressure difference was such as to cause vapor flow into the specimen.

During the solar-heating hours of the summer exposure, however, the vapor pressure difference was directed outward. In

view of the much greater vapor pressures that would exist in

a wet 'S’P'eclmen at the relatively high temperatures prevailing

in the insulation during the solar -heating period, it was

estimated that the somewhat high bottom air vapor pressure

would cause only a small percentage reduction in the outwardlydirected vapor pressure difference. (See page 9? Phase 3.)

The settings for the controllers of the air temperatures

in the top chamber had to be adjusted to values a few degrees

different from those indicated above, in order that the

roofing might attain the desired temperatures. The relative

humidity of the top chamber air was not controlled, since the

roofing constituted an almost perfect vapor barrier between

the air and the specimen insulation.

IV. TEST APPARATUS

A schematic drawing of the apparatus used is shown in

Figure 2. Figure 3 is a photograph showing the top sides of

15 roof specimens installed in the apparatus.

The apparatus was constructed as two sections of a box,

with the insulated top half capable of being raised by cables

for access to the interior. The fifteen l8-inch square specimens, arranged in three rows of five each, were supported on

a steel grid frame in the bottom section. The areas at the

ends and sides of the frame were covered with insulated panels

to complete the division of the box into top and bottom chambers. Insulation was packed between and against the vertical

sides of the specimens to reduce transverse flow of heat and

edge effects in each specimen. Air in the top and bottom

chambers was circulated by blowers in the paths indicated in

Figure 2.

The air temperatures in the top and bottom chambers were

maintained at the selected values by means of electric heaters

controlled by sensitive mercury-in-glass temperature regulators. In the top chamber, low air temperatures were

obtained by means of the refrigerating coil in the air flow

circuit, with a small amount of electrical reheating to control to the selected air temperature. An electric time clock

and relays were used to switch automatically to the appropriate temperature regulators, heaters, and refrigeration controls to produce periodic temperature cycles each day, as

- 6

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shown in Figure 1. The relative humidity of the air in the

bottom chamber was measured and automatically controlled by

means of Dunmore-type hygrometer sensing elements, which

governed the moisture output of the humidifier by on-off

operation of a small blower in the humidifier.

All temperatures in the chambers and in the specimens

were measured by means of 30-gage copper-constantan thermocouples in conjunction with a manually-operated potentiometer

capable of being read to within one microvolt, corresponding

to a temperature within 0.05 deg. F. Continuous 24-hour

records of the air temperatures in the top and bottom chambers, and in the surrounding laboratory, were obtained using

circular-chart recording thermometers. The blowers circulated the air past the specimens at an average velocity on

the order of l4o ft/min. In passing from the first to the

second air temperature thermocouple in its path (see Figure

2), the air changed in temperature by less than 1/8 deg. F in

the bottom chamber, and in the- to-p chamber by less than 0.4

and 1.0 deg. F under summer and winter night-time conditions,

respectively

.

The apparatus operated automatically and continuously to

subject the specimens to the reiterated daily cycles of the

selected temperature exposure without shut-downs for weekends or holidays, at which times manually-made observations

of data were not taken.

The heat flow meters used were of the Gier and Dunkle

type, of bakelite 3/64 inch thick and 4 1/2 inches square,

having a stated response of approximately 1/8 millivolt per

Btu/hr ft^. The heat flow meter was cemented with a rubber

adhesive to the top of the roll roofing at the center, as

shown in Figure 4. Initial observations showed that the

meter reading fluctuated undesirably in response to rapid

slight fluctuations in the temperature of the air passing

over its surface. These fluctuations were substantially

damped out by covering the meter with a 4-inch square of 5/8-

inch steel. The thermal resistance of the meter and steel

together was estimated to be less than one percent of the

over-all resistance of a dry specimen, and thus to have negligible effect on heat flow measurements under steady conditions. Heat flow meter readings were measured manually, by

means of the potentiometer used for thermocouple readings.

The 15 heat flow meters were supplied with individual

calibration factors (with corrections for the meter temperature) which were approximately alike. Initial observations.

-7-

and some meter calibrations that were made subsequently,

indicated that, in general, the response was only about twothirds of that stated. However, since the results of this

investigation are expressed in relative terms, involving the

ratio of readings of the same meter at different times, the

calibration factor of the meter, if constant, is cancelled

out. As applied and used, each meter was cemented to a specimen, and was not subject to motion or high humidities,

and therefore its calibration factor should have been substantially unchanged. All meter readings were corrected for

mean temperature in accordance with the manufacturer's temperature correction curve.

V. TEST PHASES: OBJECTIVES AND SPECIMENS

The investigation was conducted in three phases, each

consisting of alternations of periods of exposure of 15

specimens to temperatures simulating winter or summer conditions. Each period consisted of repeated 24-hour cycles of

the appropriate daily temperatures to simulate night-time and

solar heating conditions, as indicated in Figure 1. The seasonal periods were of several weeks duration each, varying in

length as necessary to observe apparent trends in the insulating value of a majority of the simultaneously-exposed specimens . The three phases were aimed at somewhat different objectives, reflecting the changing emphases of the investigation

as it progressed. In accordance with the objectives, the

designs of the specimen constructions, or their initial and

later conditioning, were changed in the successive phases.

Summarized briefly, the chief objectives and the general

nature of the specimens employed in each phase were as

follows

.

Phase 1 . Investigation of the insulating value of roof constructions insulated with board-type or light-weight concrete

insulations placed on a 3~inch dense concrete deck slab, with

and/or without a vapor barrier, under simulated summer and

winter seasonal temperature conditions, including daily solar

heating. This was done with specimens assembled from materials

which were at "as-received" moisture contents, i.e., factorymade materials were taken from cartons stored a few months in

the laboratory; dense concrete slabs were air-dried approximately four months in the laboratory; insulating concretes

were air-dried in the laboratory from three to seven weeks.

- 8 -

Particulars of the individual specimens of Phase 1 are

given in Table I. The typical specimen design is shown in

Figure 4, which includes details of the edge-sealing of a specimen and the method of installing it in the apparatus.

Referring to Figure the asphalt edge-seal (7) was

approximately 1/8 inch thick, applied hot, and was additionally enveloped by a double wrapping of thin Saran plastic

film (6) se§led against the specimen faces by pressure and

mastic, as shown. The roofing was similarly held down at its

edges. The bakelite tubing inserts (13) were connected to

mercury manometers, to indicate internal pressures within

specimens, or to limp balloons of 0.00^-inch polyethylene to

allow for relief at atmospheric pressure of expanded gases

within specimens without entire loss of the moisture contained in the relieved gases. If a vapor barrier was used,

it was sealed around the inlet of the tubing insert by mastic.

Phase 2 . Investigation of the effects of known amounts of

moisture on the insulating value of various insulated roof

constructions, under the simulated seasonal exposure conditions. This was done using 11 specimens which were ovendried prior to installation, into 6 of which water was introduced during later exposure periods in incremental amounts of

1, 2, and 7 percent by volume of the insulation. Because of

the apparent high vapor resistance of the dense concrete

decks observed in Phase 1, the effect of omitting the decks

on the effective conductivity of the insulating material was

also investigated, using 6 specimens. Four specimens were

continued unchanged from Phase 1 for information as to the

effects of a prolongation of exposures.

Particulars of the individual specimens of Phase 2 are

given in Table II. The design of the specimens was similar

to that shown in Figure 4, except that no specimen had a vapor barrier, and for six of the specimens, the 3-inch dense

concrete deck was removed or omitted. Water added to selected

specimens was introduced through the bakelite tubing insert in

the dense concrete deck, which was kept closed except to admit

water, or through a small temporary hole in the roofing, which

was then sealed.

Phase 3 . Investigation of the effect of (a) omission of the

dense concrete decks of selected specimens, and (b) pressurerelieving vents through the roofing, on the effective conductivity of various types of insulations when subjected to a succession of alternated simulated summer and winter exposure

conditions

.

-9-

To investigate to some degree the effect of the bottom

air conditions in the apparatus, they were changed at week

lh7 from 90°F and 70 percent R.H. (vapor pressure 1.00 inch

Hg , dewpoint = 79°F) to 80®F and 50 percent R.H. (vapor pressure 0.52 inch Hg , dewpoint = 60°F) . At week l5l^ the bottom

air conditions were changed again, to 88°F and 39 percent R.

H. (vapor pressure 0.52 inch Hg , dewpoint = 60°F) , or, to

approximately the original temperature, but half the original

vapor pressure.

The specimens of Phase 3 are individually described in

Table ITT. All were oven-dried prior to installation; the

amount of moisture removed by oven-drying was re-introduced

into 10 of the specimens during a later exposure condition.

Twelve of the 15 specimens had 3-inch dense concrete decks,

8 had a vapor barrier, and 11 were equipped with openable

vents through the roofing. Specimen 32 was installed in

place of specimen 31 at the end of week 13^-

The design and installation of the specimens resembled

the details shown in Figure Exceptions were that the

edge-seal (7) was made not with asphalt but with 1 / 32 -inch

neoprene sheet cemented to the specimen with a rubber

adhesive, that 3 specimens were installed without a dense

concrete deck, and that no specimen had a tube inserted

through the deck. For each specimen equipped with a vent

through the roofing, the top of the insulation was scored

with a groove, 1/4- inch wide and 1/4 inch deep, outlining a square enclosing the central half of the face area, at one

corner of which a 3/l6-inch i.d. bakelite tube was installed

projecting through the roofing. The vent-tube was used also

as a means of introducing water into the specimen early in

Phase 3*

VI. PRESENTATION OF RESULTS

The summarized heat transfer data obtained in the investigation are presented graphically in Figures 9, 10, and 11,

in which, for each specimen, values of the ratio k/ko are

plotted as ordinates versus time as the tests progressed.

In the ratio k/ko, 4 is the observed effective thermal

conductivity of the insulation of a specimen at a given time,

and ko is the approximately minimum value of conductivity

observed for the specimen during its exposures. For each

specimen (except specimens 13 and 21), the value of ko

selected was the value of k obtained under winter exposure

conditions for the insulation when it was in an oven-dry

- 10 -

condition, or, if the specimen had not been oven-dried, when

its observed conductivity was minimal. As will be seen, the

observed conductivity under winter exposure conditions of an

insulation containing only moderate amounts of moisture

approached closely to that of the oven-dry insulation.

The values of conductivity k were obtained from the

experimental data in accordance with the equation

^ A(At)

where k = thermal conductivity, Btu/hr ft^(deg F/in.)

X = insulation thickness as installed, in.

q/A = heat flow* as indicated by the heat flow meter

attached to the roofing, Btu/hr ft^

At = temperature difference* across the insulation, deg F

*0bserved daily during the substantially steady temperature conditions prevailing just prior to the start of

the solar heating period.

Each plotted value of k/ko is the average of the (usually)

five values obtained during each week of the tests.

Use of the ratio k/ko to represent the data has several

advantages; (a) It indicates proportionate increases of

conductivity (e.g., a k/ko ot* 1*5 indicates a 50 percent

increase in effective conductivity above the winter value for

the substantially dry insulation), (b) relative changes of

conductivity of different insulations are readily compared,

(c) the absolute calibration constant of the heat flow meter

is not involved, provided only that it remained substantially

constant

.

The time scale of Figures 9, 10, and 11 is expressed in

elapsed weeks, starting from the beginning of Phase 1. The

time scale is also subdivided into periods designated by

letters, each of which represents a sustained seasonal exposure condition. Phase 1 comprised seasonal conditions A to

D; Phase 2, E to J; and Phase 3? K to Q. The type of exposure condition is indicated by the line spanning the lettered

period, a summer exposure condition being shown by a solid

line, and a winter condition by a hatched line. The exposure

conditions were substantially those shown in Figure 1 (the

more severe winter condition was used only in exposure period

C). Plotted data for individual specimens are identified by

- 11 -

distinctive symbols, and by the specimen number in accordance

with the listing and description of the specimens at the

right side of the figure.

Tables IV, V, and VI give data on the weights of the specimens at the start and end of their exposures, and on the moisture contents of major components of the constructions where

these could be ascertained. Final moisture content data for

insulations and concrete decks were determined from their

weights before and .afteh oven,-drying them at 2l5°F. The column

headed "Exposure Gain (or Loss)" indicates, as well as could be

determined, the over-all effect of the succession of imposed

exposure conditions on the weight of the specimen, an allowance

being made for the weight of water purposely added to it, if any.

VII. CONCLUSIONS AND RELEVANT DISCUSSION OF RESULTS

Conclusions are given below which summarize generally the

data obtained. Each conclusion is followed immediately by a discussion in detail of the specific data which it summarizes.

1. Effect of Moisture on Insulating Value, and the Influence

of Exposure Conditions on this Effect.

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 conductivity of a permeable insulation was considerably greater under the summer

conditions than under the 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 (e.g., an insulating concrete),

effective conductivities observed were as much as four times

the' value for the dry insulation, under the summer exposure

condition, and two times the value dry, under the winter

exposure condition. For nearly-dry insulations, the increases

and seasonal differences were much smaller.

A possible explanation for the greater effect of a given

amount of moisture under summer conditions is that for the

simulated summer exposure condition used, the temperature

gradients in the insulation definitely reversed direction

daily. The reversal would tend to cause moisture to migrate

back and fortn each day, thus keeping it distributed throughout the insulation during the summer condition and available

for latent heat transfer.

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Under the simulated winter condition used, there was no

daily reversal of direction of the temperature gradient

(except possibly in the upper portion of the insulation)

and the moisture would therefore tend to migrate to and

concentrate in the upper (colder) part of the insulation,

leaving the reSiainder relatively dry.

Evidence of the relative effects of approximately the

same moisture content in summer and winter is afforded by the

plotted data for practically all of the specimens which contained significant moisture.

For example, consider specimen 11 (Figure 10). In expoure period A (summer), values of k/kQ approximated 3*0. In

periods B and C (both winter), the values were about 1.8; on

resuming a summer condition (D), k/ko again approximated 3*0.

These tests were made with the insulating concrete and dense

deck at "as-received" .moisture contents (moderately air-dried).

At conclusion of D, specimen 11 was oven-dried (it lost 12 lb

in being dried; see Table IV), and assigned the new number 26.

In summer exposure E, the value of k/ko for the oven-dry

specimen 26 approximated 1.2, and in winter exposure F, about

1.0. After a cumulative amount of water equal to 10 percent

of the insulation volume (6.7 lb) had been introduced into

the originally oven-dried specimen during winter exposure H,

the value of k/ko rose sharply from about 1.1 to 2.0, and

slowly decreased, presumably as the moisture redistributed.

In summer period I, k/ko rose to values in excess of 2.0.

Quite similar behavior was recorded during the corresponding

exposure conditions for other insulating materials similarly

treated. See insulating concrete specimens 12(27) and

14(29), and board-type insulation specimens 2(17) and 5(20).

For the permeable board-type insulations, the behavior was

similar, although less extreme (in Phase 1, these insulations

contained initially only hygroscopic moisture, although the

dense concrete decks were only "air-dry").

A contrast to the performance in Phase 1 of permeable

insulations containing moisture is afforded by the practically vapor-impermeable board-type insulations of specimens

9 and 10 (Figure 11). These showed litti'e change in effective

conductivity under summer and winter conditions (A, B, C, D)

,

even though the dense concrete deck, and vapor permeation

through the -deck during B and C, would have been sources of

moisture to cause an effect during D if the insulations had

been permeable.

-13-

It should be noted that the only specimens having vapor

barriers in Phase 1 (3, 6 , 15) duplicated in C and D their

performances in B and A, respectively, and that they were the

only specimens showing a loss in weight in Phase 1 (see Table

IV), presumably as a result of drying of the dense deck, the

moisture in which could not enter the insulation during the

winter exposures B and C. Why a similar loss of weight did

not occur for specimens 9 and 10 , which had quite vaporresistant insulations, although no vapor barriers, has not

been accounted for.

Further evidence of the relative effects of moisture

under summer conditions as compared with winter conditions is

amply afforded in Phase 3? in which their originally-held

moisture was restored during exposure M to selected initially

oven-dried specimens, as indicated in Figures 9, 10, and 11.

In Phase 3 5 however, the effects change in some specimens

( 3 ^? 37 , ^6 ) because of apparent drying-out of the specimens

under the exposure conditions (see Conclusion 3)*

2. Effects of Vapor Barriers and of Dense Concrete Decks.

Changes in the effective conductivity of permeable roof

insulations with time (for the same exposure condition)

were almost inappreciable when a sheet vapor barrier

(0.08 perm, approx.) separated the insulation and the

dense concrete deck. In most cases, changes were also

materially recarded when the insulation was placed

directly on the deck without the sheet vapor barrier,

apparently because of the effective resistance to moisture transfer of the dense concrete deck.

For roof insulations that are dry, a vapor barrier and a (dry) dense concrete deck are advantageous, since they

help keep them dry. But for roof insulations which initially contain appreciable moisture, or those made wet

(as by 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 under-surface, 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 by moisture from the deck, which otherwise would occur. (Lt is estimated that the "air-dried"

3-inch dense concrete decks used in Phase 1 had moisture

contents of 3 -^ percent by weight, or approximately 1.3

lb of water per square foot of deck.

)

-IVIn Phase 1, three specimens (3, 6, and 15, Figures 9 and

11) had a sheet vapor barrier under the insulation, the others

did not. In exposure period D, the values of k/kg for these

three specimens were substantially the same as at the end of

exposure A, although they had been subjected for 19 weeks to

winter conditions B and C, during which moisture from the

dense concrete deck or the air below would ordinarily have

tended to migrate into the insulation. The similar specimens

1 & 2, k & 5, and 13 & l4, which had no sheet vapor barriers,

had average values of k/ko in period D appreciably greater

than those in period A, indicating increases in insulation

moisture content due to the winter exposures B and C.

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 vapor permeability.

Further evidence of the effect of a vapor barrier is

afforded by specimens 39 and kh. Both were oven-dry at the

start of exposure period K (Figure 11); specimen 39 had a sheet vapor barrier, hh had only a dense concrete deck.

Throughout all the exposures of Phase 3? these insulations

had k/ko ratios only moderately greater than 1.0; although

for both the ratios showed a tendency to increase slowly (in

successive similar exposure conditions). For specimen 39?

the increase of k/ko ^as relatively less than for kk, incicating the effect of the vapor barrier. It should be

noticed, however, that the dense concrete deck of specimen

kk 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 with sheet vapor

barriers was recorded for specimens 31 and 33*

On the other hand, k/kQ ratios for moist insulations

used with a sheet vapor barrier remained large throughout

their exposures. This resistance to change was also evident

for specimens with moist insulation over dense concrete

decks. For example, see specimens kl and k5 (Figure 11),

following the restoration of (original) moisture early in

exposure period M. Throughout the subsequent 5k weeks of

various exposures, the k/ko ratios for these specimens continued at approximately unchanged summer and winter values.

Approximately similar behavior is noted for specimens 38 and

kO, which had dense decks but no sheet vapor barriers, and to

which original moisture was restored in M, and for specimens

1, k, 8, and 13 in period D of Phase 1 and throughout Phase

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- 15-

These data, and the contrasting data discussed under

Conclusion 3? are evidence of the effect of a vapor barrier,

and of a dense concrete deck, to act as an impedance to

change of the k/ko ratio of the insulation for given exposure conditions, and also, presumably, to change of the

moisture content of the insulation.

3 . Behavior of Specimens Having No Dense Concrete Deck or

Vapor Barrier.

Apparent self-drying tendencies were observed for some

roof specimens consisting of roofing and 'a layer of insulating material installed without a dense concrete deck

or a sheet vapor barrier underneath.

For some insulations of only moderate permeability and

relatively high absorptive capacity^ installed in this

way, it was found that the originally high effective conductivity of the initially moist insulation tended to

decrease rapidly during the simulated summer exposures,

and to approach a value not much greater than that of the

dry insulation, presumably as a consequence of drying

through the undersurface. Increases of conductivity

during winter exposures were observable, but relatively

small and slow, even when the insulation was initially

oven-dry. The evidence obtained indicated that, for

suitable insulating materials (e.g., some insulating concretes), the effective conductivity, 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 general type of behavior was observed for all

specimens without dense concrete decks underneath, but

for those of high permeability and low absorptive

capacity, the changes in effective conductivity in each

seasonal exposure (especially winter) were extreme, and

the over-all average level of conductivity was unsatisfactory . Specimens of certain insulations used without dense concrete decks, initially at appreciable moisture contents,

attained low values of effective conductivity in one or

two alternations of the imposed summer and winter exposure periods. In contrast, for specimens of similar

insulations, also initially appreciably moist, placed on
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- 16 -

dense concrete decks with or without a sheet vapor

barrier, effective conductivities remained at substantially their original high values throughout the same

exposure periods.

In the following, the k/ko ratios for all specimens not

having a dense concrete deck are discussed, and comparisons

made with those for similar specimens having a dense deck and

in some cases a sheet vapor barrier also.

Specimens 37 and ^0 (Figure 10) were similar, except

that 37 had no dense deck. Both were initially oven-dried,

and both had water added to them at week 111 in the amounts

removed when they were dried (l 8.6 and 23*0 lb, respectively;

see Table VI). The k/ko ratios were approximately the same

for both specimens prior to week 111 ; addition of the water

caused an immediate sharp increase for both, but during the

remainder of summer exposure M, k/ko for specimen 37

decreased very considerably (from 3*6 to 2 . 0 ), while for hO

,

it showed only minor change (from 4.5 to 4.0, approx.). In

subsequent winter condition N, values of k/ko were markedly

different (1.65 for 37 versus 2.7 for 40). In summer,, condition 0 , k/ko for 37 dropped to 1 . 1 , while k/ko for' 40 reduced

to only about 3*5* In winter condition P, and summer condition Q, k/ko for specimen 37 reproduced closely the values

obtained in exposures K and L when it was substantially ovendry. But for specimen 40, k/ko in exposure P and Q had

values only a litcle smaller than those in exposures M and N,

when ^the specimen contained much moisture. (The changed bottom air conditions during weeks 14/ and l5l unquestionably

Increased the obtained values of k/ko for the following five

weeks, for all specimens containing much moisture at that

time, although the change had much less effect on simultaneously-exposed specimens containing little moisture, indicating th.e changed effect was due to a new moisture distribution.

Table VI shows that specimen 37 had an "exposure loss"

of weight of 15.2 lb, and specimen 40 a loss of 3*3 lb. The

final moisture content of the insulation of specimen 37 was

7.7 percent as compared with 63.4 percent, the presumable

moisture content at week 111 when its original moisture was

restored. For specimen 40, only the final moisture content

( 50.5 percent) of the insulation could be ascertained,

because separation of the insulation from the dense deck was

not feasible earlier.

Similar but not as rapid reduction of the k/ko ratio was

observed for specimen 34 (no deck), which compares with

specimen 38 (with a dense deck). Similarly also, compare

-17-

specimen ^6 (no deck) with specimen 4l (deck) and specimen ^5

(deck plus sheet vapor barrier), on Figure 11. It should be

noted, in passing, that specimens without decks (37, 3^, and

46, and also 36) showed a tendency for the k/ko ratio to

increase slightly during winter condition exposures (see

periods L, N, and P), and, apparently because of moisture

gains during these periods, to have k/kg values at the start

of the following summer exposure slightly greater than the

values at the end of the preceding summer exposure. For this

reason, specimen j,6 is of especial interest. It had no deck,

was oven-dried before installation, and had no moisture added

to it except as a result of its exposures. Specimen hh was

similar, but had a dense concrete deck. Tracing the values

of k/{ko for the two specimens from the start to the end

(periods, K to Q), it is seen that values for specimen 36 were

larger than those for specimen 4-4 far most of the exposure

periods, but at the end the values for specimen 44 were

slightly larger than those for 36.

It is instructive to compare specimens 36 and 46, which

were similar (no decks) and initially oven-dried. Their k/ko

ratios were almost identical until water was added to specimen 46 early in exposure M. As further exposures took place,

the k/kQ ratios for specimen 46 tended to decrease, and those

for 36 to increase slightly. At the end of summer period Q,

they were approximately 1.45 and 1.25, respectively, and the

final moisture contents of the insulations were 9*3 and 6.7

percent, respectively (Table VI). In short, without dense

concrete decks underneath them, the insulating concretes, one

initially oven-dry and one starting at 20.6 percent moisture

content, tended toward the same operating values (plateaus)

of k/ko and of insul^-ipn moisture content, approximately

attaining them in a relatively short time.

Although there was no other pair of like insulating concrete specimens without decks with which to examine this tendency for convergence to a plateau- value , it is evident that

specimen 37 rapidly approached k/ko moisture content

values typical of fairly dry insulation, and that specimen 3^

appeared to be tending in a similar direction, but at a considerably sl'ower rate. In connection with the levels of k/ko

and moisture content which appear to be the plateau- values in

the tests conducted, it is important to appreciate that the

bottom air dewpoint for exposures followrng week l47 was 60°F.

For lower bottom air dewpoints, which would be probable for

most buildings in summer and practically, certain for all

ordinary buildings in winter, the plateau- values of k/ko

insulating concrete moisture content should be lower than

those indicated.
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- 18 -

In contrast, consider specimens having dense concrete

decks under the insulation. In Phase 3, k/ko values

increased slowly for specimens 39 and to which no water

was added, and decreased very slowly for specimens 38, 4-0,

4l, and 45 , to which water was added. it is reasonable to

suppose that eventually the like specimens would reach common plat eau- values of k/ko and insulation moisture content,

but evidently the time required would be many times greater

than for specimens without dense concrete decks. For the

latter reason, it is not possible to estimate even roughly

the plateau- values that might be attained by the specimens

with decks; the slight evidence available from specimens 38

and 44 suggests they would be higher for specimens with decks

than for those without decks. Some additional information on

the trends of values of k/ko is afforded by the values in

Phase 2 for specimens 24 & 26 , and 25 & 27 (Figure 10), and

specimens 29 & 30 (Figure 11).

In Phase 2, three board-type insulations were made into

specimens without dense concrete decks or vapor barriers

underneath. Specimen I 8 was highly permeable to vapor, but

of very low moisture absorptiveness; specimen 21 was

moderately vapor permeable and of substantial absorptiveness; specimen 22 was of moderate to low permeability, and

moderately absorptive. Data for these specimens are given in

Figure 9* All were oven-dried before installation, but some

may have gained moisture hygroscopically before the test exposures were started.

Specimen I 8 showed, to an extreme degree, rapid increases

of k/ko during winter exposures, and rapid decreases during

summer exposures, in all periods of Phase 2. This behavior

would be expected for a material having its properties; it

was observed, in fact, just after week 53? that a quantity of

water had dripped from the undersurface of the specimen,

which probably was the reason the k/ko decreased sharply

for a time. 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 resistance underneath. Its performance in the latter case is shown by that of specimen 17,

which was initially oven-dry, and that of specimens 1, 2, and

3 in Phase 1, which were initially at "'as-received” moisture

contents

.

Specimen 21 changed moderately in k/ko values with different exposure conditions, which tended to increase slightly

during winter exposures. The value of ko used for this specimen was that of specimen 20, because it was believed the

- 19 -

exposed insulation of specimen 21 had gained moisture hygroscopically before the test exposures were started. The insulation appeared to be at approximately a plateau-condition

throughout Phase 2, for the exposure conditions imposed, but

it is believed that for longer winter exposures, some vapor

protection underneath would be necessary to avoid extremes of

the type evidenced by specimen l8. Performance of the insulation with a dense concrete deck underneath is shown by specimens 4 , 6 , and 20 (the latter initially oven-dried, and

later subjected to water additions).

Specimen 22 had no deck, and was oven-dried before

installation. Its resistance to vapor permeance, and moisture absorption capacity, were apparently sufficient to prevent significant changes in k/k© ratio throughout the exposures of Phase 2. It may be noted that specimen 8, installed

in an "as -received"' condition in Phase 1 and carried without

modification through Phase 2, also showed little change in

k/ko ratio, although values are higher under summer conditions than those for specimen 22, presumably because specimen

8 gained during winter conditions B and C some of the moisture initially in the undried concrete deck.

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 conductivity 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 a limp polyethylene balloon of about 0.1 cu ft

volume when distended. Observed changes of volume of the

balloon as a result of air volume changes in the specimen due

to the daily temperature changes were on the order of 75 to

100 cu in., during summer exposure A, for a cellular concrete

specimen which had a relatively large void volume. The mercury manometer readings indicated daily changes of internal

pressure in specimens for the same period equal to not more

than 3 Ih per sq ft. 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 lb per sq ft could be expected for the temperature

changes experienced by the insulations.

- 20 -

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

reducing the high k/ko values obtained with moist insulations.

Accordingly, such vents were installed in 10 of the Phase 3

specimens. These specimens, which were installed in ovendried condition, had restored to them during summer exposure

M the amount of moisture rem.oved by the oven-drying process.

Fifteen weeks later during winter exposure N, 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 trend of the k/ko ratios in

period M and subsequent periods. Referring to the plotted

values of k/ko for these specimens, no trend in period M

existing before the vent was opened was materially changed

after it was opened, for any of the specimens. The behavior

of the specimens having sheet vapor barriers (35? ^2, ^-3 , and

^5) in subsequent exposure periods was substantially the same

as that prior to opening of the vents. Similarly, the k/ko

ratios for specimens 38, ^0, and hi, which had dense concrete

decks but no sheet vapor barriers, and which, like specimen

h5, had relatively high moisture contents, also showed no

significant change as a result of opening the vents. In the

case of specimens 3^? 37? and h6 , which had no dense decks,

substantial trends of reduction of k/ko values existed prior

to opening of the vents, and the absence of sharp differences

in these trends in the subsequent exposure periods indicates

that the vents had little if any beneficial effect of this

kind.

A calculation of the probable drying rate of a cellular

concrete specimen as a result of the breathing action, based

on daily venting of air saturated at the higher mean temperature of the insulation and its replacement by dry air as the

specimen cooled (for the summer condition) indicated that

years would be required to effect a change in moisture content of only a few percent. The experimental evidence

appears to be in accordance with the calculated estimate.

5 . Roof Insulating Materials

The roof insulating materials used in this investigation

can be separated into a few groupings, those in each

group having common characteristics important in connection with moisture problems and the effect on insulating

- 21 -

value of moisture present in the roof construction. There

are differences between insulations in each group, but

such differences are less important than those between

groups

.

The first group comprises the insulations that are practically impermeable to vapor or moisture. In this investigation, these were cellular glass and expanded polystyrene . The second group consists of vapor permeable insulations

which are delivered and installed in a substantially-dry

condition. These included, in this investigation, corkboard, glass or mineral fiber insulating board, vegetable-^

fiber insulating board, and a compacted-in-place mineral

fill insulation.

The third group comprises permeable insulations mixed

with water in the process of application, and any of the

second group which, for any reason contain moisture, when

installed, in excess of their normal hygroscopic moisture

content. In this investigation, these included three

types of light-weight insulating concrete, and insulations

of the second group made wet by adding water.

Based on the data obtained in this investigation, the

common general characteristics of each group were as summarized below. (Since detailed references to the same

experimental data have been made in connection with preceding

conclusions, data relevant to the following summarizat ions

are indicated merely by citing the pertinent specimen numbers.)

Group 1 . Effective conductivities were substantially equal to

those of the dry materials, and were the same for both summer

and winter exposure conditions (within the precision of the

measurements). Their conductivity did not change with exposure time, under conditions where moisture was available from

the dense concrete deck underneath (9, 10).

Group 2 . Installed at air-dry or oven-dry moisture contents,

over a good vapor barrier, these materials maintained effective conductivities approximately corresponding to those of

the dry materials, throughout the duration of the tests (3 vs. 17, 6 vs. 20, 31, 33, 35, ^2, ^3)« Effective conductivities

were perceptibly greater under summer exposure conditions than

under winter exposure conditions (3, 6, 33, but not 31 )

There was some evidence that conductivities tended to increase

slightly with time, presumably as a result of slow moisture

gain through the vapor barrier (33, 35, but not 42 or 43, or

31 as long as it was observed).

- 22 -

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 increases in effective conductivity during summer

exposure conditions, but not during winter conditions (1, 2,

4, 7, 8). Effective conductivities increased slowly with

exposure time, the increases becoming observable during both

summer and winter conditions (1, 4, 8).

Upon addition of appreciable water to these insulations,

effective conductivity values increased markedly, and were

substantially greater during summer exposure conditions than

during winter conditions. Conductivity values with water

added in the amount of 10 percent of the insulation volume

were from 50 to 100 percent higher than values dry, under the

summer exposure conditions, and from 0 to 20 percent higher

under the winter conditions (17, 20). The specimens so

treated had oven-dried dense concrete decks but no vapor

barriers; if there had been vapor barriers to impede transfer

of some of the added water to the concrete deck, the effective conductivities would dlmost certainly have been still

larger.

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 conductivity values (l8, 21, but not 22). These changes were

apparently dependent on the permeability and moisture absorptive capacity of the insulation. (See discussion under Conclusion 3, pages 18 - 19 .)

The essential conclusions concerning the permeable insulations of Group 2 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 summer conditions), and (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 indefinitely 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 insulations of this kind of quite low

permeability and high moisture absorptiveness, they should

not be used directly exposed to the air underneath.

-23-

Group 3 » The water-mixed insulations of this group (insulating light-weight concretes) had free moisture contents

between 20 and 60 percent by weight after several days of

air drying (Table VI). When used as specimens at these airdry moisture contents, their insulating effects were from two

to four times less than their observed values when oven-dry,

depending on whether they were being subjected to winter or

summer exposure conditions, respectively (3^? 37, 38, ^0, ^1,

45 , 46). When the insulations were placed on dense concrete

decks, the impaired insulating values continued almost without significant improvement, throughout the period of observation ( 38 , 40, 4l, 45 ). The lack of improvement is considered due to the effective resistance or impedance of the

dense concrete decks to moisture escape from the undersurface of the insulation.

The same insulations, exposed to the air below without a dense concrete deck underneath, increased rapidly in insulating effect during exposure to a few alternations of the

imposed summer and winter exposure conditions (34, 37, 46).

In one case, the insulation attained, during the period of

observation, an apparent plat eau-value of 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 (37)* Similarlytreated specimens of other insulating concretes without decks

showed an approach to the same behavior, but at a less rapid

rate (34, 46).

Specimens of these insulations, without decks, installed

in an oven-dried condition, decreased perceptibly in insulating value when subjected to alternations of the test summer

and winter exposures, and tended to approach, from a dry initial

condition ,( 24, 25, 36), the approximate plateau- value of

insulating effect being reached from the other extreme by

similar insulations to which water had been added (34, 37,

30 , 46, respectively).
I
-2kVIII. REMARKS AND COMMENTS

A. Test Conditions.

Surprisingly large values of effective conductivity were

obtained in this investigation for moist insulations during

the imposed summer exposure condition. In many cases, they

were so much larger than the values obtained for the same

materials when dry that it seems necessary to attribute a large part of the increase 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 non-capillary 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 moisture corresponding to its hygroscopicity at humidities lower than 100

percent, and the over-all effective conductivity is only

moderately increased 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 distillation 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 affords reason to believe that the solar heating

period in the imposed summer exposure condition was the

essential cause for the large effective conductivities observed

with moist insulations. In this connection it should be noted

that for substantially dry insulations, effective conductivities

were approximately the same during the imposed summer and winter conditions (i.e., when there were, and when there were not,

daily temperature gradient reversals, respectively), and therefore that heat capacity effects at the times when conductivities

were determined were not the cause.

Whether the very high effective conductivities indicated

for moist permeable insulations in this investigation

actually occur in roofs in service - in other words, whether

this finding is important practically - depends therefore

upon whether daily temperature gradient reversals do occur in

roof insulations in service, during summer or other seasons.

-25-

There is little doubt that in summer, at least, in most areas

of this country, the top surface of an insulated roof is considerably hotter than its undersurface during the sun-hours

of the day, and that at night the undersurface of the roof is

warmer, for buildings not air conditioned, and possibly also

for air conditioned buildings, than its top surface, since

the latter is subject to comparatively rapid cooling by the

night air and by radiation to a clear sky. Accordingly, one

must conclude that free moisture (that in excess of the hygroscopic capacity 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 is pointed out that in this investigation, effective

conductivities 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. Consequent ly , latent heat transfer

must have been occurring at greater rates at these times than

at the time measurements were made. Because of heat capacity

effects, it is not feasible to estimate the over-all effective conductivity of the insulation during the times of

changing temperatures. Nevertheless, it seems reasonable

that at such times over-all effective conductivities of moist

insulations were not less than, and that probably they were

greater than, the values at the time observations were made.

In other words, the indicated effective conductivities are

probably less than the average values over the 24-hour daily

cycle

.

One further point should be made concerning the exposure

conditions used. The roofing surface 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

increase to some extent the "plateau-values" of effective
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- 26 -

conductivity 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 3^5 37, and possibly ^5, during weeks

15I-I 56 when the bottom air dewpoint was lowered to 60°F.

B. The Moisture Problem.

As mentioned in the Introduction (pages 1-2) there are

several avenues by which excessive moisture can become present

in an insulated roof construction in service. The degree to

which precipitation or roofing faults or punctures during construction may contribute moisture is an imponderable, or

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 gradient 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 plainly 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 (C), this appears to be a possibility.

The most difficult problem is that of in-leakage of water

through leaks in the roofing. The possible quantity is an

imponderable, except 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

insulation performance would require either that the insulation
*r

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- 27 -

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 common solution is required.

C. 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, i.e., 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 ventilating channels or ports in

the roof (which may constitute an effective means of drying a wet roof construction).

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 necessary to appreciate clearly what has been

shown - namely, and merely, that a dense concrete deck under

an insulating concr-ete impedes its necessary drying, and that

without this deck satisfactory drying occurs. Apart from the

important practical questions of the strength, and feasibility,

of a roof of monolithic insulating concrete, one must consider

that a form-board or decking is necessary to pour the roof.

The question enlarges therefore to a consideration of practical permapent deckings other than dense concrete, and what

properties "these mtist^have so as not to impede drying, or

allow excessive moj^-ture gain under winter conditions. It

would"in fact be desirable for a number of reasons 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 of movement

of moisture as water or vupor in materials is quite incomplete. When a problem involves differences in temperatures,

and changing differences, as in the case of roofs, our

present knowledge is not adequate for the complexities

- 28 -

involved, although it does indicate possible fruitful lines

of attack. For instance, the impedance 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 is believed necessary at present to

examine the various possibilities by means of roof exposure

tests

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TABLE II

Description of Roof Specimens I6 - 3 O

Phase 2--Exposure Periods E-J

Specimen Constructions

For 11 of the Phase 2 specimens, the materials were those

used in the corresponding Phase 1 specimens having a specimen

number less by 15 than the number of the Phase 2 specimen. Four

of the 11 were continued without change or modification from

Phase 1, and for this reason are referred to in Results by their

original numbers (1, 4, 8, 13). Seven of the 11 were modified by

being oven-dried, and 2 of the 7 were installed with the dense

concrete slab removed. As noted, 4 of the Phase 2 specimens were

fabricated with previously un-used insulating materials.

During exposure periods F, G, and H, water was added to the

insulations of 6 of the initially oven-dried specimens, in successive increments of 1, 2, and 7 percent by volume.

No specimen of Phase 2 had a vapor barrier under the insulation, and internal pressure connection tubes were not used. All

Phase 2 specimens were oven-dried at the conclusion of exposure

period J.

Details of Specimens

Insul. Dense Oven-Dried Water Added

Spec.

No. Type of Insul.

Thickness

in.

Deck

Used

Before

Expos. E

to Insul

Expos. F

( 16)1 Glass Fib. Bd. 1 3/4 Yes No No

17 Glass Fib. Bd. 1 3/4 Yes Yes Yes

18 Glass Fib. Bd. 1 3/4 No Yes No

( 19 )^+ Roof Insul. Bd. 2 1/4 Yes No No

20 Roof Insul. Bd. 2 1/4 Yes Yes Yes

21 Roof Insul. Bd. 2 1/4 No Yes No

22 Corkboard^ 1 3/4 No Yes No

(23)8 Corkboard 1 3/4 Yes No No

24 Perlite Conc.b 6 No Yes No

25 Vermiculite Conc.c 6 No Yes No

26 Perlite Cone. 5 3/4 Yes Yes Yes

27 Vermiculite Cone. 5 3A Yes Yes Yes

(28)13 Cellular Cone. 5 11/16 Yes No No

29 Cellular Cone. 5 11/16 Yes Yes Yes

30 Cellular Cone. 5 11/16 No Yes Yes

(a) New sample of corkboard, taken from same carton as the other

corkboard insulations.

(b) New perlite cone, insul.; same mix as for Spec. 11.

(c) New vermiculite cone, insul.; same mix as for ~Spec. -12.

(d) New cellular cone, insul.; same mix as for Spec. , 13 - 15 .
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TABLE IV

Specimen Weight Data

Phase 1--Specimens 1-15

Spec. WtT^, lb Chanee in Final Moisture

Specimen Construction

All specimens were newly constri

Phases 1 and 2. Two new types of in;

All specimens were oven-dried p]

selected specimens early

Details of Specimens

in exposure

Spec

No.

• Type of Insul.

Insul. I

Thickness D

in. 1

^1 Glass Fib. Bd. 1 3/4

32 All-Weather Crete 2 7/8

33 Roof Insul. Bd. 2 3/8

3*+ Perlite Conc.c 6

35 Roof Insul. Bd. 2 3/8

36 Cellular Conc.b 6

37 Vermiculite Cone.® 6

38 Perlite Cone.® 6 1/8

39 Cellular Conc.b 6

40 Vermiculite Cone.® 6 1/8

4l Cellular Conc.^ 6

^-2 Glass Fib. Bd. 1 3/^

^3 Fesco Bd. 2 1/2

Cellular Conc.^ 6

45 Cellular Conc.b 6

46 Cellular Conc.b 5 13/16

(a) Moisture content of

Nbisture content of

cone. Insu

dense deck

(b) Average moisture content for i

(c) Mix: 9^ lb Type III cement , 6

(d) Mix: 9^ lb Type III cement , 8

(e) Mix: 9^ lb Type III cement , 6

(f) Original moisture content was

weight of insulation.
•

•V
TABLE III

Description of Roof Specimens 31-^6

Phase 3 --Exposure Periods K-Q

Specimen Construction

All specimens were newly constructed, using materials from the same stock as those used for specimens in

Phases 1 and 2 . Two new types of insulating material were used in two specimens (32 and 4

- 3 ).

All specimens were oven-dried prior to exposure period K; the water removed by drying was restored to

selected specimens early in exposure period M.

Details of Specimens

Spec.

No. Type of Insul.

5 l Glass Fib. Bd.

32 All-Weather Crete

33 Roof Insul. Bd.

34 Perlite Conc.c

35 Roof Insul. Bd.

36 Cellular Conc.b

37 Vermlculite Conc.e

3« Perlite Conc.*^

39 Cellular Conc.b

4o Vermlculite Cone.®

4i Cellular Conc.^

42 Glass Fib. Bd.

43 Fesco Bd.

44 Cellular Conc.^

45 Cellular Conc.b

46 Cellular Conc.^

Insul.

Thickness

in.

Insul. Properties, Oven-Dry

Density Th. Cond. at 75°F

lb/ft 3 Btu/hr ft 2 (F/in.)

Dense

Deck

Used

Vapor

Barrier

Used

Vent

Through

Roofing

Moisture Content

of Insul. before

Drying^, %

Original

Moisture in

Insulation

Restored

1 3/4 12.0 0.27 Yes Yes No Negligible No

2 7/8 23.1 0.53 Yes (Primed) Yes 11 Yes^

2 3/8 12.3 0.33 Yes Yes No 7.3 No

6

3/8

30.0 0.84 No No Yes 39.1^ Yes

2 12.3 0.33 Yes Yes Yes 4.9 Yes

6 30.1 0.90 No No No 19.7^ No

6

1/8

29.3 0.98 No No Yes 63.4a Yes

6 30.0 0.84 Yes No Yes 12 . 8b Yes

6

1/8

30.1 0.90 Yes Yes No 35.2a No

6 29.3 0.98 Yes No Yes 20.54 Yes

6

3/4

30.1 0.90 Yes No Yes 10 . lb Yes

1 12.0 0.27 Yes Yes Yes Negligible Yes

2 1/2 9.6 0.36 Yes Yes Yes 2.7 Yes

6 30.1 0.90 Yes No No 10.4b No

6

13/16

30.1 0.90 Yes Yes Yes 26.5^ Yes

5 30.1 0.90 No No Yes 20.8a Yes

(a)

(b)

(c)

(d)

(e)

(f)

air

)

, Molstu^P measured at 7 days (1 day in mold, 3 in damp-room, 3 in lab. Moisture content of dense deck slabs at 7 days averaged 3

-4-^ of dry weight. moisture content for Insulation and dense concrete deck togethe?. Mix. 94 lb Type III cement, 6 ft^ perlite, l4 gal. water

^

Mix: 94 lb Type III cement, 8.4 lb foam, 5.2 gal. water Mlx;_ 94 Ib Tpe III cement, 6 ft^ vermlculite^ 19 gal. water.

weight^of*^insUlation^^^^^

Jaegllgible; water was added in exposure period 0 to equal 40 percent of dry

TABLE IV

Specimen Weight Data

Phase 1--Specimens 1-15

Spec. Wt^, lb Change in Final Moisture

Spec. Oven- Spec. Wt , Content,

No. Type of Insul. Start End dry lb Insul

.

Cone. Deck

1 Glass Fib. Bd. 96.6 _c - - - - 2 Glass Fib. Bd. 97.3 98.3 94.2 +1.0 - - 3 Glass Fib. Bd. 97.6 95.2 92.3 -2.4 0 3.2

4 Roof Insul. Bd. 98.6 - - - - - 5 Roof Insul. Bd. 99.2 100.3 96.2 + 1.1 - -

6 Roof Insul. Bd. 99.3 98.5 95.3 -0.8 6 .

0

3.2

7 Corkboard 92.9 93.9 91.3 +1.0 1.5 2.7

8 Corkboard 93.6 - - - - - 9 Cellular Glass 95.5 96.1 94-.

3

+0 .

6

0 2.3

10 Cell. Polystyrene 91.1 91.8 89.9 +0.7 0 2.4

11 Perlite Cone. 130.0 130.6 118.2 +0 .

6

_ _ 12 Vermiculite Cone. 140.1 140.6 120.3 +0.5 - - 13 Cellular Cone. 130.6 - - - - - 14 Cellular Cone. 130.9 132.0 122.0 +1.1 - - 15 Cellular Cone. 130.4 129.4 122.8 -1.0 l>+.3 2.8

(a) Percent of oven-dry weight.

(b) Specimen weight includes from 5 to 9 lb due to the roofing, heat

flow meter, seals, and wrappings.

(c) A dash in the table indicates that data were not taken at this

time because the specimen was not dis-assembled at the conclusion of Phase 1. Specimens 1, 4, 8, and 13 were carried

over to Phase 2 and remained installed in the apparatus at the

end of Phase 1.

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U. S. DEPARTMENT OF COMMERCE

Lbwii L. StfQiiSB, Secretary

NATIONAL BiJRtiAU OF STANDAHOS

A. V, Aatlh, Director THE NATIONAL RIJREAIJ OF STANRARI»S

The scope of activities of the National Bureau of Standards at its headquarters in Washin^'ton, D. C., and Us major laboratories in Boulder, Colo., is suggested in the following listing of the

divisions and sections engaged in technical tvork. In general, each section carries out specialized

research, development, and engineering in the field indicated hy its title. A brief 'description of

the activities, and of the resultant publications, appears on the inside front cover. WASHINGTON, D. C.

and liHoctronlcs* Resistance and Reactance. Electron Devices. Electrical In- strumentsi Magnetic Measurements. Dielectrics.. Engineering Electronics. Electronic Instru- mentation, Electrochemistry,

Optics and IHctrology* Photometry and Colorimetry. Optical Instruments. Photographic

Technology. Length. Engineering Metrology.

Heat* Temperature Physics, Thermodynamics. Cryogenic Physics, Rheology. Engine Fuels. Free Radicals Research.

Atomic and Radiation I*hyslcs» Spectroscopy. Radiometry. Mass Spectrometry. Solid

State Physics. Electron Physics. Atomic Physics. Neutron Physics. Radiation Theory.

Radioactivity. X-rays. High Energy Radiation. Nucleonic Instrumentation. Radiological

Equipment. ' Chemistl'y* Organic Coatings, ^urface Chemistry. Organic Chemistry. Analytical Chemistry.

Inorganic Chemistry. Electrodeposition. Molecular Structure and Properties of Gases. Physical

Chemistry. Thermochemistry, ^peclrbchemistry. Pure Substances,

IH^clianics* Sound. Mechanical Instruments. Fluid Mechanics. Engineering Mechanics. Mass

and Scale. Capacity, Density, and Fluid Meters, Combustion Controls. Organic and Fibrous IHatcrlals. Rubber. Textiles. Paper. Leather. Testing and

Specifications. Polymer Structure. Plastics. Dental Research,

metallurgy* Thermal Metallurgy. Chemical Metallurgy. Mechanical Metallurgy. Corrosion.

Metal Physics.

mineral Products* Engineering Ceramics. Glass. Refractories, Enameled Metals. Concreting

Materials. Constitution and Microstructure.

Building Technology* Structural Engineering. Fire Protection. Air Conditioning, Heating,

and Refrigeration. Floor, Roof, and Wall Coverings. Codes and Safety Standards. Heat Transfer,

Applied mathcnintics* Numerical Analysis, Computation. Statistical Engineering, Mathematical Physics.

Data Processing Systems. SEAC Engineering Group. Components and Techniques. Digital

Circuitry. Digital Systems. Anolog Systems. Application Engineering.

• Office of Basic Instrumentation. • Office of Weights and Measures. UOIJLDEII, FOMHIAIIO

t'l’yogcnic Fligiiie<*riug. Cryogenic E<pdpment. Cryogenic Processes. P 'operlies of Materials. Gas Liquefaction.

Itudio Propagation Physics. Upper Atmosphere Research. lonospheris Research. Regular Propagation Services. Sun-Earth Helationships. VlIF Ib'scarch. Ionospheric Connnunicalion

Systems.

l*r<»pugiiii<»u 9’AngiiH'‘4^ii*tiig. Data l{c<luclion I nstrunu'ittalion. Modnialion Systems.

!\avigalion SysUmis. l{a«lio Noise. Tropospheric Mtuisun'ineiils, 'rropospluM'ic Analysis. Radio

Systems Application Engineering. Radio-Mctmuologv.

Staodlirds. High Frc.picm y El.vli i. /d Siandards, Rndi<» Rroa»l< a.st Scr\ icc. High

f I coiicf'cy I'rqualance Slnndai d.s. i'JccI i ooic ( !<dihral ion ( !ciiici . Micro.ua\c I'liysics. Microwave

Ciic.oi :'-,‘andard.s.
'i

;