TECHNICAL INFORMATION ON BUILDING MATERIALS

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FOR USE IN THE DESIGN OF LOW-COST HOUSING

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THE NATIONAL BUREAU OF STANDARDS

UNITED STATES DEPARTMENT OF COMMERCE

WASHINGTON, D. C.

May 22, 1956.

THERMAL INSULATION

Summary and Conclusions

This is a summary and Brief presentation of conclusions, pertaining to thermal insulating qualities of various types of Building walls,

commercial insulating materials , and application of insulating materials

to Building wall^, derived from tests conducted By the National Bureau

of Standards.

Complete details regarding materials and walls tested, methods of

testing, and results are given in Bureau of Standards Circular No. 376,

"Thermal Insulation of Buildings", (OctoBer 17, 1929);^ Research Paper

No. 291, "Heat Transfer Through Building Walls", (August 6, 1930),^ "by

M. S. Van Dusen and J. L, Finck; and Letter Circular No. U21, "Thermal

Insulation", (July 13 , 193^)*^

^-Available from Superintendent of Documents, Government Printing

Office, Washington, D. C. (Price 5 cents)

^Available from Superintendent of Documents, Government Printing

Office, Washington, D. C. (Price 15 cents)

•^Available from National Bureau of Standards, Washington, D. C.

(Free)

052 93 6-

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various types of the same size, in the same locality, unless air leakage around doors and windows or through very poorly constructed walls

is excessive >

The difference "between insulating .values of the poorest and the

"best wall tested (representing extremes in ordinary uninsulated construction) is equal to the insulating value of approximately $/k inch

of good insulating material.

Commercial Thermal Insulating Materials

Light Fibrous or Cellular Materials: Differences in respective

insulating values of light fibrous or cellular insulating materials are not very great. Less than 11/2 inches of the poorest material is

equivalent in insulating value to 1 inch of. the best. In general, the

lighter the material per unit total volume the better its insulating

value per inch thickness. ......

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*" l i Stiff Fibrous Insulating Boards: Stiff fibrous insulating boards

having considerable structural strength are somewhat poorer insulators

than lighter and looser materials.

Dense Highly Compressed Wallboards: Dense. highly compressed

wallboards made of wood or other organic fibre are not as good insulators

as less compressed boards of the same- general character.

Heavy Plaster Wallboards: Heavy wallboards containing piaster in

one form or another are relatively poor insulators, although they, are very useful building materials.

Hetal Toil: Aluminum foil is used. to increase the insulating

value of air spaces by reducing heat transfer by radiation, therefore,

it is of value only when used in conjunction with air spaces, and has

no value when placed in continuous contact with solid material on both

sides, except insofar as it may. act as a building paper in preventing

air leakage. .

Paper with aluminum foil glued to bpth sides or a sheet of bright

tin plate would produce the same thermal insulating effect. Sheet

material coated with aluminum paint c.an also be used as thermal insulation, but it is considerably less effective than actual- metal foil.

Paint

As a film of paint is relatively thin, it can- offer but

slight resistance to heat flow by conduction. However, the color of

external paint has considerable effect on absorption of heat from the

sun. Light-colored paints absorb less heat from the sun and are therefore more desirable from a thermal insulation, standpoint. The reradiation of heat from the underside of a hot roof may be reduced by

coating the underside with paint containing metallic flakes, such as' aluminum paint

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Building Walls

Hollow Tile Walls; Of the tjrpes of 8" holloH tile walls tested

(single unit—6 cell; double shell unit—3 cell; and two 4" units hack

to hack-—3 cell) the differences in insulating values are negligible.

Brick Walls? According , to- tests, conducted on two kinds of brick . (very hard burned brick; and rather porous, dry-pressed brick), repre-,;

senting anproximately the two extremes in common brick manufacture j the

kind of brick used in a brick wall is of little importance from an insulation standpoint alone. ,

Temperature of Wall: The insulating value of all walls tested

increased as temueratures decreased; the increase , in general, being

more rapid in hollow walls than in solid walls.

Air Infiltration: Investigations carried out elsewhere indicate

that air infiltration through complete walls is usually so small as not

to materially increase heat loss from buildings. There is a possibility, however, that an individual hollow type wall may be subject to' air

penetration effects of considerable import.

Air Spaces: Although air is a very poor conductor of heat, the’

insulating value of an ordinary air space is rather small because of

the large transfer of heat by radiation and convection; for the same reason the insulating value of an air space is not proportional to its

width (thickness), as would be the case with a slab of uniform solid

material

Insulating values of air spaces in excess of about 3/4 inch remain

practically constant regardless of the width, narrower air suaces have

less insulating value. Insulating values of air spaces less than l/2

inch are approximately proportional to the width.

Under average conditions, the insulating value of vertical air'

spaces commonly found in building walls is equivalent to approximately

1/4 to l/3 inch of average insulating material (about 1 Btu per hour,

per square foot, per temperature difference of 1°F).

Furring; Furring materially increases the insulating value of

ordinary types of walls.

Choice of a Bui lding. Wall or Roof

A great many types of walls

and roofs are to be found in present day dwelling-house construction.

Tests show that, the insulating. value of one type may vary considerably

from that of another. From an insulation standpoint, in an actual

building, it has been demonstrated that hea.t' losses through and around

windows and doors tend to level out the effect of differences in the

properties of the walls themselves. Thus, it. follows that there are

no wide variations in the amounts of fuel required to heat houses of

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Appl ication of Thermal Insulation

Type vs. Thickness: Erom an insulation viewpoint only, the most

important question is the thickness of insulating material to he applied,

rather than which material to select.

CostThe real cost of an insulating material is not the cost per

square foot of commercial thickness, hut the cost per unit insulating

value of the commercial thickness.

Effectiveness

The percentage increase in the insulating value of

a building wall through the application of thermal insulation depends

(l) upon the original insulating value of the wall without insulation,

and (2) upon the amount of glass surface and air leakage around windows

and doors. A somewhat greater percentage saving in fuel may he obtained

by insulating a solid masonry wall than by applying the same insulation

to a frame or hollow tile construction.

Insulating Air Spaces: Application of l/2 inch of insulation in

the middle of a frame wall air space is equivalent to applying slightly

more than 3/4 inch at some other place in the wall, as the insulating

layer not only furnishes its own insulating value, but it also divides

the air sp-: .ce into 2 parts, each of which has approximately the same insulating value as the original air space. Application of a sheet of

aluminum foil in the middle of an air space in excess of 1 l/2 inches

across, dividing the original air space into 2 parallel spaces, in- creases the insulating value of the original air space by an amount

equivalent to about 1 l/4 inches of insulating board.

If one interior surface of an air space in excess of about 3/4

inch across is lined with aluminum foil, the insulating value of the

air space will be increased by an amount roughly equivalent to that of

l/2 inch of insulating board. Little additional insulating value is

gained by lining the opposite interior surface with foil.

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Aluminum Foil Insulation

(The following transcript of National Bureau /of Standards

Letter Circular No. U65, dated June U f 1936, contains additional information concerning Aluminum ^oil Insulation .

)

At the time Circular C37& of the National- Bureau of Standards, was

prepared (1929), insulating materials of the reflective type- bad no, ;V. come into prominence. This letter circular's to he considered as a supplement to Circular C37& pending its revision. •

:

" i- V- , In Circular C3 76 it is stated: Commercial' ‘insulating materials. can he divided into two general grouus -

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; y (1) fibrous materials either in loose form or fabricated into . soft flexible quilts confined between relatively thin layers of uaper

or textile, and

(2) more or less rigid boards- in which the components are bonded

together in some way.

To this classification, must now be added a new and -distinctive

group, as follows:

(1) reflective materials, such as aluminum foil, deriving their

insulating properties from the fact that they reflect radiant hea.t.

Since the 'principles involved in the use of aluminum- foil or other

bright metal sheet as thermal insulation are not generally- understood,

a brief discussion will be given here. Aluminum foil is used to in- . crease the insulating value of air spaces by reducing heat transfer y

radiation. It is of value only in conjunction with air spaces, ana has

no value when placed in continuous- contact with solid material on hot

sides, except in so- far as it may act as a building uaper in preventing

air leakage.

Clean metallic'- surfaces in general- are good, reflectors and uoor

emitters of radiant heat. Since a large proportion of the heat transfer across air spaces bounded by non-metallic materials takes place by

radiation, the use of aluminum as one or both boundaries of a space

will materially reduce the heat transfer across the space. It will be

evident that the insulating effect does not depend on the thickness . of

the metallic foil, while the insulating value of ordinary types of insulating materials depends mainly on their thickness . The insulating

value of air spaces bounded on one or both interior surfaees with

aluminum foil increases with increasing width of space up to about

3 /U inch width. Spaces wider than about Jb inch have substantially

the same insulating value, regardless of width.

There is little definite information on the permanency of the

reflective surfaces of aluminum under various conditions of use over

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long periods. Installations .are reported where no apureciable deterioration of the aluminum has occurred over a considerable period of years.

Thin layers of dust readily visible to the eye do not cause any very

serious lowering in the reflecting power. If aluminum is wetted over considerable periods of time, there is possibility of corrosion. The

reflectivity is not auureciably reduced until the 'aluminum is covered

by an easily visible layer of corrosion products (Gregg, Refrigerating

Engineering, May, 1932).

The use of lacquer to- resist possible corrosion under severe conditions of use reduces the reflecting uower to a greater or lesser

extenb. The effect of a very thin coat of lacquer is small, but relatively thick lacquers, even though they are almost invisible to the

eye, may seriously reduce the effectiveness of the foil.

The effect of reduced reflectivity on heat transfer across an air

suace is less marked the narrower the suace, since heat transfer by

conduction and convection -plays a more important role than radiation in

the case of narrow air spaces.

The available data in the literature on the insulating value of

aluminum foil used in various ways show considerable variation. The

numerical data in the following table are therefore necessarily only

approximate, but are sufficiently accurate for practical purposes in

connection with house insulation. In this table the data in Column 1 on clean aluminum foil are calculated from test results. The data in

the other columns are calculated from the figures in Column 1, assuming

that the reflecting uower of clean aluminum is 95 uercent. All air

spaces are assumed to be 3/4 inch or more in width. The figures given

reuresent the number of inches of insulating material having a thermal

conductivity of 0.3 Btu/hr sq ft (deg E/inch) to which each combination

of reflective material is equivalent.

A material having a conductivity of 0.3 Btu has been selected as a basis for comparison because this figure is about the average of

insulating materials now being sold.

The figures in the table represent the total insulating values of

the air-suace combinations described. Since the original single airspace bounded by ordinary materials such as paper has insulating value

equivalent to about l/4 inch of insulating ma.terial (Column 4), the

insulating value which is actually added by the use of reflective

material is about l/4 inch less than the figures given in the table.

There are a number of ways in which insulating value may be

expressed. The method of exmressing insulating value in the table

has been chosen as being most readily intelligible to persons not

familiar with the technical side of the subject. Anyone who wishes

to have the results expressed in terms of heat transfer in Btu per

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hour per square foot and per degree F temperature difference, may do

so hy dividing 0.3 (the conductivity of an average insulating material)

"by the numbers given in the, table, for example, the number 1.0 reuresents 0.3/1. 5 or 0.2 Btu per hour, per square foot and per degree F

temperature difference.

Aluminum foil is also auplied in a crumpled form so that it. is

self- spacing. If two or three crumpled sheets are hung in the air suace

of a frame wall, there is so little contact between the sheets that the

insulating values are . es sentially the sane as those given for the suaced

sheets.

Insulating value

in inches of insulating material

Clean Reflecting Aluminum Ordinary

Combination aluminum surface

(95$ re- (85$ re- fleeting) fleeting)

paint paper

( 65$ re- ( 10$ re- flecting) fleeting)

1 Space, reflective

material on one side 0.75 0 . 6 l O.U5 0.26

1 Space, reflective

material on both sides .SO 0

1

—

• .54 .26

2 Spaces, formed by

dividing wide space by

1 reflective sheet 1.50 1.21 .90 • 53

3 Spaces, formed by

dividing wide space

with 2 reflective

sheets 2.30

H Spaces, formed by

dividing wide space

with 3 reflective

sheets 3«I0

1.90 1 .U3 .79

2 . 6 l 1.97 1.05

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