NATIONAL BUREAU OF STANDARDS REPORT

3930

THERMAL CONDUCTANCES OF SIX INFRA INSULATIONS IN

CLOSED AIR SPACES FOR VARIOUS DIRECTIONS OF HEAT FLOW

by

H. E. Robinson

F. J. Powlitch

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
U. S. DEPARTMENT OF COMMERCE

Sinclair Weeks, Secretary

NATIONAL BUREAU OF STANDARDS

A. V. Astin, Director

THE NATIONAL BUREAU OF STANDARDS

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NATIONAL BUREAU OF STANDARDS REPORT

MBS Pf?®JECT NBS REPORT

1000-30-4^30 March 14, 1955 39^0

THERMAL CONDUCTANCES OF SIX INFRA INSULATIONS IN

CLOSED AIR SPACES FOR VARIOUS DIRECTIONS OF HEAT FLOW

by

H. E. Robinson

F. J. Powlitch

to

Civil Aeronautics Administration

Department of Commerce

<NBS>

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

The publication, repri

unless permission Isol 25.D.C. Suchpermi

cal ly prepared If tha

Approved for public release by the

Director of the National Institute of

Standards and Technology (NIST)

on October 9, 2015.

part, is prohibited

dards, Washington

rt has been specifiirt for its own use.
f0,y\:: :r-->

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THERMAL CONDUCTANCES OF SIX

CLOSED AIR SPA r- FOR VARIOUS

I INTRODUCTION

INFRA INSULATIONS IN

DIRECTIONS OF HEAT FLOW

The thermal insulating value of a wall , floor or

ceiling construction incorporating closed air spaces, such

as those formed by studs or joists, can be considerably

increased by the use of a reflective insulation installed

in the spaces. This report presents data as to the thermal conductance between the inner surfaces of the two

faces of a test panel having nominal 2x6 inch framing

spaced 16 inches on centers, with the original air space

(a) uninsulated (b) insulated with five differenct types

(Type 4-3, 4-PS, 4PX, 6PK, and 6PS) of Infra Accordion insulation (c) insulated with Infra Retardive (flat) insulation to form three reflective air spaces of equal

thickness. The results were obtained in a rotatable

guarded hot-box heat transfer apparatus, and include the

effect of the wood framing members. The conductances

were determined in most cases for five orientations of

the test panel, from horizontal with heat flow downward

to horizontal with heat flow upward.

The measurements were made at the request of the

Civil Aeronautics Administration of the Department of

Commerce.

II DESCRIPTION OF INSULATIONS,

TEST PANEL, AND TEST METHOD

Measurements were made of the thermal conductance

between the inner faces of a test panel, with various

orientations of the panel, and with the panel spaces

(a) uninsulated (b) insulated with five different types

of Infra Accordion Insulation, made by Infra Insulation,

Inc., New York, N. Y. , and designated by the manufacturer

as Types 4S, 4PS, 4-PX, 6PK and 6PS and (c) insulated

with two sheets of Infra Retardive (flat) aluminum foil

forming three air spaces of equal thickness. In addition,

preliminary tests were made on a "check panel" of 1 1/4-

inch plywood to compare results obtained in the test apparatus with those obtained by means of a guarded hot

plate thermal conductivity apparatus.

The several Infra Insulations were designed for

application between framing members spaced 16 inches

on centers. Figure 1(a) shows the construction of the
•
~2

5.5x8 ft test panel, the faces of which were 19-gage

galvanized sheet steel painted on all surfaces with a

flat white paint having a total emissivity at ordinary

temperatures of approximately 0.9. The faces were

screwed to the nominal 2x6 inch framing members over

stud-width felt strips. The metering or test area of

the panel was the central rectangle 32 inches wide

(two framing spaces) and 60 inches high; the surrounding

area of the panel served as a thermal guard for the

metering area. The metal sheet covering the test area

on each face was separated from the surrounding sheet

metal by a l/8-inch gap to minimize lateral heat flow;

ten thermocouples were permanently soldered to each

sheet, at the centers of the ten I6xl2-inch rectangles

into which its area could be divided, for measuring its

temperature during a test.

The several reflective insulation applications tested

are shown in Figures 1(b) to 1(g). Each figure shows the

construction or design of the insulation, and particulars

as to its dimensions and application in the test panel.

Each of the several insulations was installed in the test

panel by a representative of the manufacturer; all spaces

in the panel were insulated as similarly as possible.

The accordion insulations were fastened in place by means

of staples in two rows about 1/2 inch apart, staples in

each row being spaced 1 1/2 to 1 3/4 inches on centers.

The Retardive insulation (Fig. 1(e)) was fastened by means

of staples spaced 1 1/2 inches on centers, driven through

a reinforcing cardboard strip.

In all cases, the flat (stapling flange) side of the

insulation was toward the warm side, and the peak side toward the cold side. For the accordion insulations, the

flat side aluminum foil was 0.0009-inch thick, the peakside (or other) foil 0.0007-inch thick, and the accordionfolded blue paper 0.005 inch thick, approximately. The

Retardive aluminum foil was 0.0015 inch thick. All of the

aluminum foils had bright clean surfaces, both before and

after the tests.

Because the ends of the several accordion types of

insulation had to be folded for stapling at the end of

the framing spaces, some thinning of the insulation thickness at the ends occurred. The decrease in thickness was

slight except for Type 6PS, for which it was about one inch

at points a few inches from the ends.

-3-

Examination of the installed insulations after each

was tested indicated that little or no sagging or movement

of the exterior foils of the insulation occurred when the

panel was turned from one horizontal position to the other,

except in the case of Type 6PK, the peak foil of which

deflected downward about one inch in the center when the

peak side was downward.

It will be noted that with Type 4S insulation (Fig.

1(b)), a gap of triangular cross section existed between

the framing member and the flank of the insulation on the

peak side. The other types of accordion insulation

(Figs. 1(c) to 1(g)) were designed to have the peak flank

in close contact with the framing member. This was found

to be the case for all of these types, the gap being substantially closed except for Type 6PK. In this case, a gap of about 1/4 inch width at the peak existed for a

distance of about 3 ft along one flank side of one of the

metering area framing spaces. This may have been partly

due to the fact that the distance between the sides of

two framing members in the test panel was 14 11/16 inches

instead of the nominal 14 3/S inches. This spacing was

adjusted to 14 7/l6 inches for the t^sts of the Types 4PS,

4PX, and 6PS insulations, for which flank contact was

important.

The tests were made in a rotatable guarded hot-box

heat transfer apparatus which is wholly mounted on horizontal trunnions to allow the entire apparatus to be rotated for positioning the plane of the panel at any angle

up to 90 degrees from the vertical in either direction.

The hot-box apparatus conforms substantially to the

requirements of ASTM 236-54T, "Method of Test for Thermal

Conductance and Transmittance of Built-Up Sections by

Means of the Guarded Hot-Box”, except for the additional

feature of rotatability on trunnions.

The test panel was tightly held and sealed by gaskets between the "cold box" and the "guard box”, the

latter containing a "metering box" tightly pressed and

sealed by gaskets against the central area of the panel

32 inches wide and 60 inches high. The cold'- box was

held at a constant temperature of about 0°F by a refrigerating coil, and the metering and guard boxes were

4-

heated electrically to a constant temperature of about

70°F or more. All electrical input to the metering box

was measured by a calibrated integrating watt-hour meter;

the guard box was automatically maintained by a sensitive

differential thermostat at the same temperature as the

metering box to minimize heat flow between them. A sensitive compound thermocouple indicated the magnitude of

any unbalance, to compensate for which a small adjustment

was applied to the measured power input to the metering

box. Air temperatures were measured by means of thermocouples at ten points three inches from the panel surface

on each side. The air in the boxes was circulated gently

by fans (at about 40 fpra on the warm side and 90 fpm on

the cold side) in a direction to assist natural convection

on the faces of the panel.

Each test was begun after a state of steady temperature conditions had been reached and held for several

hours; the average data-taking period was about 20 hours

for each test. During this period, all observed air and

surface temperatures were constant to within 0.6 degree F

or less. All thermocouple data used for calculations

were read by means of a semi-precision manually-operated

potentiometer. A sensitive temperature-recorder was used

to record six selected temperatures continuously for information as to the constancy of conditions between manual readings.

Most of the tests were made with the air at about

70°F on the warmer side and 0°F on the colder side, except for those with heat flow downward and the tests

made with Infra Retardive forming three reflective air

spaces. In the two latter cases the air temperature on

the warm side was raised because the metering box fan

power input alone was slightly more than the heat required for a 70 degree temperature difference across the

panel

The thermal conductance of the insulated space was

calculated from the net measured heat input to the metering box divided by the area of the metering section of

the panel (13.33 ft^j and by the average temperature difference observed between the surfaces of the metal faces

which covered the stud spaces.
. .

. . }

.<•-
As a preliminary to these tests, tests were made in

three orientations on a "check panel" of plywood, 5*5 ft

xS ft x 1 1/4 inches in size, to compare the results

obtained in the guarded hot box apparatus with those obtained on a specimen of the same plywood tested in an 0-

inch guarded hot plate thermal conductivity apparatus.

The hot plate result, at approximately the same moisture

content and mean temperature as existed during the hot

box tests, indicated a thermal conductivity of 0,00 Btu/

hr ft 2 (deg F/inch), corresponding to a thermal conductance for the 1 1/4 inch panel of 0 o 64 Btu/hr ft 2 (deg F)

,

as compared to the average conductance of 0,65 obtained

in the three hot box tests. Since the difference would

be accounted for by a change of one percent in the moisture content of the plywood, the two results are considered satisfactorily concordant 0

- 6 -

III TEST RESULTS

The data and calculated results for all of the tests

are summarized in Table L The thermal conductances for

the several Infra Insulation applications are shown

graphically in Figure 2. Included in Table 1 and Figure

2 for purposes of comparison are similar data on Type 1+K

and Type 6S Infra Insulations obtained in tests conducted in 1949 and 1950 respectively. These two insulations resembled Type 4S (Fig. 1(b)) in that the flank

of the insulation on the peak side did not hug the side

of the framing member.

In Table 1 and in Figure 2, the thermal conductance

referred to is that between the inner or facing surfaces of the faces of the test panel. This conductance

includes the effect of the nominal 2x6 inch framing members, which occupied approximately the same proportion

of the test area as is ordinarily occupied in practice

by framing members spaced 16 inches on centers. The

effect of the framing members is slight in most cases

since their conductance is estimated to be about 0.14

Btu/hr ft^ (deg F).

IV DISCUSSION OF RESULTS

The test values of the thermal conductance of the

empty test panel, as recorded in Table 1, are in close

agreement with corresponding values calculated on the

basis of the convection data given in the paper "The

Thermal Insulating Value of Airspaces" (Housing Research Paper No. 32, Government Printing Office), for

the cases of heat flow horizontal and heat flow upward.

In the case of heat flow downward, the test conductance

is 9 percent lower than the calculated value. The calculations were based on an assumed total emissivity of

0.90 for the inner panel surfaces, and include the effect of radiation to and from the framing members considered as "refractory surfaces" (See "Heat Transmis->

sion" by W. H. McAdams, 3rd edition, pp. 69-76, 1954,

McGraw-Hill Book Co., Inc., New York, N.Y.).

The thermal conductances obtained for the test

panel with three reflective spaces of equal thickness

formed by two sheets of Retardive insulation (Fig. 1(e))

are from 1.8 percent (heat flow direction 45° upward) << to 6.8 percent (heat flow vertically downward) greater

than corresponding values calculated using the data

and procedure given in Housing Research Paper No. 32.

In view of the uncertainties involved in the calculations

in allowing for the effect of the framing members, and

the small conductance for heat flow downward, the test

and calculated conductances are in good agreement.

Considering the results shown graphically in Figure

2, it may be noted that for the insulations which had

conductances approximately equal to the value for three

reflective plane air spaces for heat flow horizontal,

the conductances are smaller for heat flow upward, and

larger for heat flow downward, than they are for the

three reflective plane air spaces. Their relatively decreased conductance for heat flow upward is believed to

be due to the interference of the accordion-folded blue

paper with convection currents between the aluminum

foils. Since convection is the predominant mode of heat

transfer for heat flow upward, such interference is of

major importance in reducing the conductance for this

condition. For heat flow vertically downward, these insulations have approximately the same conductance

(0.082 to 0.093), a result which would be expected if

convection were reduced to a quite small magnitude. The

fact that the conductances for heat flow vertically

downward are definitely greater than that for the three

reflective spaces is believed to be due in part to metallic conduction of heat from the warmer aluminum foil

to the cooler foil via the folded edges of the insulation where the foils lie in close thermal contact. An

estimate of the thermal conduction by this path yielded

a value equivalent to about 0.02 conductance units for

the Type 4PX insulation, which is approximately the

amount by which the insulation conductances exceed the

value for the three reflective air spaces formed by two

sheets of foil not in thermal contact at the framing

member.

The effect of closing the gap between the peakside flank of the insulation and the framing member is

very considerable, as shown by comparison of results

for Types AS with those for APS and 4PX, and those for

Type oS with those for 6PK and 6PS. The effect of the

gap is of course most marked for orientations where

convection in the outer air spaces is important; for

heat flow vertically downward, stratification of the air

inhibits the effect of the gap, and all of the insulations tend to yield about the same conductance.

The differences in conductance for Types 6PK and

6PS (apart from the probably smaller effect of the small

flank gap of the 6PK insulation, previously mentioned)

appear to be due to the greater thickness of the 6 PS

insulation, and to the separation between the peak side

foil and the blue paper of the Type 6PS design,, The

downward deflection of the peak-side foil of the 6PK insulation when it was underneath, as mentioned previously,

would cause it to simulate the Type 6PS insulation for

vertically downward heat flow, as the results indicate

was the case.

In considering the closed-air-space conductances

obtained with the various insulations in these tests,

it is important to appreciate the fact that the insulations were carefully installed under laboratory conditions. That is, the edges and ends were stapled tightly to the framing members, there were no perforations or

tears in the foil membranes, and care was taken (working

only on the stapling side of the test panel) to assure

good contact between the peak-side flanks of the insulation and the framing members in the cases of the insulations designed to have no flank gap.

The question may arise as to the conductances of

these insulations in closed air spaces formed by framing

of other dimensions, such as 2x4 studs, or 2x$ or 2x10

joists or rafters. The effect of changing the thicknesses of the outer air spaces as a result of changing

the framing dimension depends chiefly on the change in

the convection-conduction coefficient (he) the air

space with thickness. Data on this variation are given

in Housing Research Paper No. 32. Reference to this data

indicates that for an air space in a vertical orientation,

with heat flow horizontal, little change in overall conductance would occur for air spaces with framing between

4 and 10 inches in nominal size. A similar approximate

conclusion is reached for the cases of heat flow upward

or downward at a 45 degree angle. In the case of heat

flow vertically upward, the change in conductance of the

test panel due to use of 2x10 joists is estimated to be

a decrease of not more than 6 percent. For heat flow

downward, it is estimated that changing the joists of

the test panel from nominal 2x6 to nominal 2x10 framing

would result in a decrease of about one third in the conductance between the inner faces of the panel.
)

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TABLE I

SUMMARY

OF

TEST

DATA

AND

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Figure I (a) TEST PANEL (Upper Half)

In sulation Edg e Con struction

Insulated Space

Section A - A

End of Insulated Space

Section B-B

Stapling lines Glue line

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warm side

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THE NATIONAL BUREAU OF STANDARDS

Functions and Activities

The functions of the National Bureau of Standards are set forth in the Act of Congress, March

3, 1901, as amended by Congress in Public Law 619, 1950. These include the development and

maintenance of the national standards of measurement and the provision of means and methods

for making measurements consistent with these standards; the determination of physical constants and properties of materials; the development of methods and instruments for testing materials,

devices, and structures; advisory services to Government Agencies on scientific and technical

problems; invention and development of devices to serve special needs of the Government; and the

development of standard practices, codes, and specifications. The work includes basic and applied

research, development, engineering, instrumentation, testing, evaluation, calibration services, and

various consultation and information services. A major portion of the Bureau’s work is performed

for other Government Agencies, particularly the Department of Defense and the Atomic Energy

Commission. The scope of activities is suggested by the listing of divisions and sections on the

inside of the front cover.

Reports and Publications

The results of the Bureau’s work take the form of either actual equipment and devices or published papers and reports. Reports are issued to the sponsoring agency of a particular project

or program. Published papers appear either in the Bureau’s own series of publications or in the

journals of professional and scientific societies. The Bureau itself publishes three monthly periodicals, available from the Government Printing Office: The Journal of Research, which presents

complete papers reporting technical investigations; the Technical News Bulletin, which presents

summary and preliminary reports on work in progress; and Basic Radio Propagation Predictions,

which provides data for determining the best frequencies to use for radio communications throughout

the world. There are also five series of nonperiodical publications: The Applied Mathematics

Series, Circulars, Handbooks, Building Materials and Structures Reports, and Miscellaneous

Publications.

Information on the Bureau’s publications can be found in NBS Circular 460, Publications of

the National Bureau of Standards ($1.25) and its Supplement ($0.75), available from the Superintendent of Documents, Government Printing Office. Inquiries regarding the Bureau’s reports and

publications should be addressed to the Office of Scientific Publications, National Bureau of

Standards, Washington 25, D. C.