U, S. DEPARTMENT OF COMMERCE

R. P. LAMONT, Secretary BUREAU OF STANDARDS

GEORGE K. BURGESS, Director

MISCELLANEOUS PUBLICATION, BUREAU OF STANDARDS, No. 132

PROPERTIES OF

FIBER BUILDING BOARDS

By

C. G. WEBER

F. T. CARSON

L. W. SNYDER

December 1, 1931

UNITED STATES

GOVERNMENT PRINTING OFFICE

WASHINGTON : 1S31

For sale by the Superintendent of Documents, Washington, D. C. Price 5 cents

M132

PROPERTIES OF FIBER BUILDING BOARDS

By C. G. Weber, F. T. Carson, and L. W. Snyder

Wall boards and insulating boards, the two principal classes of fiber buildingboards, have been studied at the bureau to obtain information relative to the

properties of these important building materials. Tests were made on 15 samples of wall boards and 8 samples of insulatingboards, representative of the commercial products in common usage. Weight,

thickness, density, strength, absorption of moisture, and changes in dimension

with changes in moisture content were studied. Special apparatus was de- veloped for determining the strength and for indicating changes in dimension

with changes in moisture content. Some discussion of heat insulation, air

infiltration, and susceptibility to molds is included. There was no marked difference in the strength of the different boards

studied. The average thickness of wall boards was less than that of insulatingboards, but the density of the wall boards was greater and the strength was

approximately the same for both classes. There were no marked differences as regards absorption of moisture or changes in dimension with changes in moisture content, except that the crosswise expansion of the laminated or

built-un boards was greater than that of homogeneous or 1-piece boards. The difference in thermal conductivity between the various insulating boards

has been found by previous work at the bureau to be small, and the average

wall boards are so thin and of such density as to be considered of relatively

much smaller value as heat insulators. The results of laboratory tests of the various boards are tabulated, the

test methods used are described, and the different properties considered important are discussed in detail. CONTENTS

Page

I. Introduction 1 II. Description of fiber building boards 2

1. Types in common use 2

2. Fibrous raw materials 2

3. Manufacture 3

4. Uses 3

III. Previous bureau research 4 IV. Samples used in studies 4 V. Properties studied and test methods used 4

1. Strength 6

2. Moisture absorption and expansion 8

3. Insulating values 12

4. Dimensions 13

5. Air infiltration 13

6. Susceptibility to molding 13

VI. Relative properties of wall and insulating boards. _ 14

I. INTRODUCTION

Fiber building boards have recently assumed a position of importance among the materials used in the building industry, es- pecially in connection with house construction and remodeling.

83557°—31 1
2 BUREAU OP STANDARDS

While such boards are usually thought of as lumber substitutes, they

are in reality manufactured lumber designed for specific applications

in building construction. There are two classes of boards in general

use, namely, wall boards and insulating boards. Wall boards are manufactured for use as wall, ceiling, and partition finish on the in- teriors of buildings, while insulating boards are made primarily for

use as heat insulation within walls, partitions, and roofs of buildings.

Insulating boards may have a surface suitable for interior finish and

so serve a double purpose, and wall boards may be so constructed as to have good insulating value. The two classes, therefore, apparently

overlap to some extent. Although both classes of fiber building boards are widely used

and the manufacture of these products has become a very important

industry,1 little published information on their properties has been

available. Both classes of boards have been studied in the paper

section of the National Bureau of Standards for the purpose of

obtaining definite information as to their properties other than in- sulating. The tests herein reported were made over a period of

years, and some of the information has already been published in an

article on fiber wall boards.2 The fiber building boards are rather

recent developments. Wall board found its first extensive use during the late World War when, because of its comparatively low cost and the ease and speed of application possible, it was used very ex- tensively in cantonment and other war-time construction. Fiber

insulating board has come into extensive use during the past decade.

II. DESCRIPTION OF FIBER BUILDING BOARDS

1. TYPES IN COMMON USE

There are two definite types of fiber building boards. One type

is a laminated product that consists of several sheets of thin fiber board laminated or pasted together with an adhesive. The other

type is a homogeneous board which is a 1-piece product. The wall

boards are nearly all of the laminated type, and boards manufactured

primarily for heat insulation are of the homogeneous type. The laminated boards are generally of much higher density than the homogeneous boards used for insulation and for that reason, are usually

of greater strength in proportion to thickness. The homogeneous

insulating boards, being of much lower density, contain a proportionally greater number of dead air spaces. By reason of these

properties, a large proportion of the boards intended for use as wall boards are of the laminated type and those intended for use

for insulating purposes are of the homogeneous type.

2. FIBROUS RAW MATERIALS

Low-grade vegetable fibers are largely used in the manufacture

of both wall boards and insulating boards. Such boards were formerly made almost exclusively from ground wood pulp, and some

1 0. R. Sweeney and W. E. Emley, Manufacture of Insulating Board from Cornstalks,

B. S. Misc. Pub. No. 112.

2 B. W. Scribner and F. T. Carson, A Study of Fiber Wall Boards for Developing Specification Standards, Paper Trade J., Sept. 26, 1929.
FIBER BUILDING BOARDS 3

well-known boards are still made principally of this material.

Ground wood pulp, as the name implies, is wood (usually spruce or

fir) which has been reduced to a pulp by contact with a revolving

abrasive stone. Such pulp contains all of the materials present in

the original wood. Some boards contain chemical wood pulp in

mixture with ground wood, and some boards are made from waste,

such as newspapers, composed largely of ground wood and chemical

wood fibers. Chemical wood pulp is prepared by cooking wood by

a chemical process to remove the noncellulose material, hence the

fibers composing the boards made from ground wood pulp, from

old papers, and from combinations of these materials are essentially

the same. Kecently, there has been an increasing use of fibers other

than wood, especially fibers from waste plant materials. Among the

samples studied were boards composed of fibers from bagasse 3 (extracted sugar cane), straw,4 cornstalk,5 licorice roots, and sawmill

waste.

3. MANUFACTURE

The two types of boards (laminated and homogeneous) are made

by radically different processes. The laminated board is made by

pasting together three or more sheets (usually four) that have been

made on ordinary cylinder paper board machines, while homogeneous board is formed"in one continuous sheet of the desired thickness on a screenlike cylinder or is molded in sheets under pressure in

steam-heated platens. Both types are dried in ovenlike dryers on

which careful control is maintained to bring the board to a predetermined moisture content. Water resistance is produced in most

boards by incorporating rosin size before forming the board, and in

addition to this " internal sizing " some boards are given surface

treatments with paints, gums, oils, or waxes to further increase the

water resistance. Some of the laminated boards have outside plies

that have been given special sizing treatments and some have outside

plies of better quality than the center plies to improve the appearance and facilitate decorating. In some cases, the boards have a

" liner 55 consisting of a sheet of paper of better quality than the

board itself pasted on one or both sides of the board. This is done

to improve the appearance and to reduce the air permeability.

4. USES

In addition to the conventional uses as a finishing cover for in- terior walls, ceilings, and partitions (wall board), and for heat in- sulation in walls and roofs of buildings (insulating boards), fiber

boards, especially the homogeneous type, are used for a variety of

purposes, such as a base for inside plaster and outside stucco, for

sound deadening, and for the improvement of acoustics in auditoriums, radio broadcasting studios, etc.

3 Hind, Celotex and the Man Behind It, Sugar Central and Planters News, vol. 7, No. 1

1926.

4 Gibson, Insulating Board from Straw, Ind. and Bag. Chem., vol. 22, p. 223 ; March,

l«7t>0.

5 Sweeney and Emley, Manufacture of Insulating Board from Cornstalks, B. S. Misc. Pub. No. 112.
4 BUREAU OF STANDARDS

III. PREVIOUS BUREAU RESEARCH

On account of the necessity of purchasing enormous quantities of

wall boards for use in temporary structures during the World War,

the War Department requested the National Bureau of Standards in

1917 to find what type of wall board was best suited for the purpose

and to develop suitable purchase specifications. This led to an

extensive study of wall board by Clark and Conley.6 The following

abstract of the article is given, as it contains much valuable information and is no longer available for distribution

:

Nine different fiber wall boards and two piaster boards were studied. Tests were made of weight, bursting strength, thickness, ash content, thermal conductivity, and absorption of moisture. The last-named property was tested by

immersing a specimen in water and by noting the time of absorption of a drop

of water placed on the surface of the board. The conclusions reached were

:

The relative thermal conductivity of fiber boards is of little importance as the boards studied showed no marked differences; the same is true of combustibility; rosin sizing distributed through the board is considered essential for

protection against warping; surface sizing is considered unimportant; sodium

silicate is thought to be inferior to starch as the adhesive for laminated boards,

for the alkalinity of the former is injurious to rosin sizing; plaster board is deemed superior to fiber board for hospitals, owing to its sanitary and fire- resistant character ; fiber wall board is considered superior to plaster board for general use owing to the greater ease of applying it and the small loss due

to breakage.

When this pioneer investigation was initiated, it was found that

most of the boards on the market were deficient in interior sizing.

The incorporation of rosin sizing in the paper stock was not generally practiced at that time. Most of the manufacturers depended on surface sizing only. It was found that nothing but a waterresistant material incorporated throughout the board would give

satisfactory moisture resistance. Several manufacturers were in- duced to increase the degree of sizing in their products. So treated,

their products were found by actual usage tests to warp much less than the products made by the procedure then current. The present

standard commercial practice of adequate rosin sizing resulted

largely from this initial development work.

IV. SAMPLES USED IN STUDIES

Fifteen samples of commercial wall boards and eight samples of

commercial insulating boards were obtained from manufacturers.

Thirteen of the fifteen wall boards were of the laminated type and

all of the insulating boards were of the one piece or homogeneous

type. One sample of gypsum board was included for comparative

purposes. With the exception of this one sample, all of the boards were vegetable fiber products and the samples were representative

of the various types of wall boards and insulating boards in current

use. V. PROPERTIES STUDIED AND TEST METHODS USED

The properties considered important as regards usage of the boards

appear to be strength, moisture absorption, and dimensional changes.

6 F. C. Clark and A. D. Conley, Study of Commercial Wall Boards, Paper, vol. 25, No. 23; Feb. 11, 1920.
FIBER BUILDING BOARDS 5

and in the case of insulating boards, insulating value. The results of tests to determine the properties of the samples (other than

insulation) are given in Table 1 (wall boards) and Table 2 (insulating boards). The tests for the two classes of boards were made at different times and the tests applied were not quite the same in

both cases. The preconditioning of the boards listed in Table 2 was

accomplished by exposing the specimens for 72 hours in a cabinet at

a relative humidity of approximately 32 per cent. Most of the

samples listed in Table 1 were preconditioned at a relative humidity

of about 25 per cent by exposure to the atmosphere of the laboratory

during cool weather. A few, tested during the earlier experiments

and during the warmer months, were conditioned at about 50 per

cent humidity.

Table 1.

Some properties of fiber wall boards

Bureau of Standards No.

72 hours in i 30 minutes

saturated | immersion

water ! in water at vapor ! 75° F.

O 35 = 8

Pi ?

Time for absorption ofO.lcroJ water on

surface

Flexural * strength, rupturing load—

©

23 '$ W s

OQ

5120. 5121. 5122-

5123.

5124- 5125.

5126. 5127.

5128.

5129.

5405.

5406. 5733. XX 3. Average

g/cm3 0. 75 .75 .60

.55 .60

.60

.65

.55 .50

.50

.55

.65 .26

Lbs. 618

552 572

547 548

587

529

487 477 677

550 680

565

1, 112+

617

1, 843

Inch

0. 16

. 14

. 18 .19 .18

. 18 .16

.17

.18 .27

.19

.25 .20

.37

.45 .38

P.ct. P.ct.

0. 70

.75

.75

.55 .60

P.ct.

88

9592

46

P.ct

65 1.00

.65

40

. oo .50

I .65

I .30

.36

. 11

90

Hours I-Ivurs 0.8

40

.57 11.8 56 66

2.3

6.5

7 +

7+

6.0

7 +

7+

7+

7.0

5. 8 5.8

6.5

6. 3

6.5 6.0 7+

5. 8 5.8

Lbs. 9.4

9.4

12. 12.2 Ms 4

15. 10.2

15.9

11.3 25.0

15. 3 26.1

15. 1 44. 9

10. 6

Lbs.

'4. 9 5.4

7.3

7.4

7.0 9.0

5.5

8.5

7.4

15.4 8.6

13.0 8.3

35.4

10. 6

Lbs J

443

392

357

291

316 389

345

381 224 353

342 458

334 600+

333

5. 8 16. 3 i 10. 2 370

1 Rupturing load at midspan of a specimen 3 inches wide laid flatwise on parallel supports spaced 12 inches apart.

2 Approximately pounds per square inch,

a Gypsum plaster board (nonfibrous).

« Exclusive of data for the last board (nonfibrous).
6 BUREAU OF STANDARDS

Table 2.

Some properties of fiber insulating boards

Bureau of Standards No.

Density

at 32 per cent R.H.

Weight

at 32 per cent R. H.

(1,000 square

feet)

Thickness

at 32 per cent R.H.

72 hours in saturated water vapor

Flexural strength

Increase in weight

Expansion Rupturing

loadi

Deflection at rupture Width

of board

Thickness of board

LengthwiseCross- wise LengthwiseCross- wise

311001 311002

311003

311004

311005

414031

g/cm*

0. 34 .26

.29

.32 .22

.27 .37

.28

Pounds

806

603

687

745 595

626

918

711

Inch

0. 45 .44

.45 .45

.44 .44

.49

.49

Per cent 10

14

16

12

1518

13

14

Per cent

0. 25

2.25 .40

.55

.45

.55 .40

.50

Per cent

6

5

7

66

Pounds

13.4 9.8

17.2 13.3

12.8

14.6 24.8 7.0

Pounds

12.2 9.5

13.4 9.2

10.0

14.2 24.0

8.2

Inch

0. 70

.65 .70

.70

.60

.50

.65 .60

Inch

0.80

.80 .80

.80

.70

.60

.65 .70

311006 311007Average

6

7 .29 711 .46 14 .42 6 14.1 12.3 .64 .73

1 Rupturing load at midspan of a specimen 3 inches wide laid flatwise on parallel supports spaced 12 inches apart.

2 Expansion of this board rather variable, 0.20 to 0.35 per cent.

1. STRENGTH

All structural work involves the question of strength. Although

the application of fiber boards adds materially to the rigidity of

the structure of which they are component parts, it appears that a board strong enough to permit easy handling of large sheets inci- dent to their erection will have sufficient strength for ordinary

structural requirements. Since the resistance to flexural breaking

is the property most essential in the economical handling of such

boards, the samples were tested for flexural strength.

The flexural-strength test is commonly made by supporting a specimen of material on two parallel knife-edges placed some distance apart and applying a load to the specimen at midspan through

a third knife-edge parallel to the other two. In testing fiber boards,

it was found that small iron rods or pipes were more satisfactory

than knife-edges and three-fourths inch pipes were used. The dis- tance between the supports wr as 12 inches, the test specimens being

3 by 18 inches. The apparatus is illustrated in Figure 1, clamp A

being securely fastened in the upper jaw of a breaking-strength

tester and clamp B in the lower or loading jaw. Clamp B must be

so adjusted as to be parallel to the line of movement of the lower

jaw, and the instrument reading must be corrected for the weight

of the device. A point is marked on the specimen equidistant from

its ends and the specimen is so placed that this point is under the

center of the rod in the lower clamp. The method and rate of

loading are the same as outlined in the standard method for determining tensile breaking strength.7 If a suitable tensile testing machine is not available, the test may be made using the form of apparatus illustrated in Figure 2. In using this apparatus, the load is

7 Paper-Testing Methods, Xockwood Trade J. Co., 10 East Thirty-ninth Street, New

York, N. Y.
FIBER BUILDING BOARDS

Figure 1.Fleamral strength apparatus far use with tensile test machine

bucket

Figure 2.Flexural strength apparatus for using shot hucket
8 BUREAU OF STANDARDS

applied by adding shot to the bucket at a slow, uniform rate of

speed until the specimen breaks, when the load required to break the

specimen is recorded in pounds. Thickness and bursting strength were obtained with standard instruments and in accordance with

standard methods.7

2. MOISTURE ABSORPTION AND EXPANSION

Fiber boards are subject to dimensional changes with changes in

the relative humidity of the surrounding atmosphere. Excessive

expansion or contraction of a board after its installation may result in buckling or pulling away from its fastenings, and consequently

Time in hours of exposure in saturated water vapor. Figure 3.

Some typical curves showing the rate of expansion of waU boards

in saturated water vapor the boards are sized to impart moisture resistance. Tests were made

to determine the increase in weight and expansion when exposed to saturated water vapor and when immersed in water. The boards were first conditioned for 72 hours in an atmosphere of approximately

30 per cent relative humidity, after which the expansion resulting

from exposure to saturated water vapor and immersion in water

for given periods of time was measured. For preliminary measurements, a set of extensometers 8 was designed by means of which

expansion could be determined at any time without disturbing the

specimens. (See fig. 4.) A separate unit was provided for each

7 Paper-Testing Methods, Lockwood Trade J. Co., 10 East Thirty-ninth Street, New

York, N. Y.

8 Suggested by P. W. Codwise, Beaver Products Co.
FIBER BUILDING BOARDS 9

specimen, B, and several units were- placed side by side, each specimen resting on one long edge. Each extensometer served not onlyits primary purpose of indicating the expansion of a specimen, but

also as a spacer and guide for the next specimen. The extensometer consisted of a metal frame, at one end of which was a pointed lever L. The shorter arm of the lever bore against one end

of the specimen and the longer arm served as a pointer as it moved

along the scale, S. The scale was graduated directly in percentage

of a definite length to which the specimen was cut. By means of a second lever, K. at the opposite end of the specimen, the reading

could be made zero at the start of a test. After the procedure was

established, expansion was determined by the simpler method of

measuring the distance between reference marks before and after

exposure.

The rate of absorption was determined in two different ways—by

increase in weight after immersion in water for a definite period of

time and by the time required for complete absorption of a measured

quantity of water on each side of the board. The time required for

Figure 4.

Extensometer absorption of a measured quantity of water was obtained by measuring onto the surface of the board 0.1 ml of water and noting the

time required for complete absorption of the water as indicated by no further reflection of light from it. The side designated in the

tables as " front " is the side apparently designed to be exposed on erection.

The connection between change in moisture content and change

in dimensions is a complex one, depending upon such factors as

density of the board, structural arrangement of the fibers, the nature of the fibers, modification of characteristics by treatment in

fabrication and by sizing materials, bonding strength of fibers, etc. But in the attempts to remedy the evil of undue dimensional changes

attention has been largely confined to the rather obvious; factor, the

proper sizing of the material. Because of the presumption that the

moisture absorption is governed by the sizing of the board and be- cause sizing is added to paper, in general, in order to impart water

resistance, the test long used for judging the moisture absorption of

building boards and likelihood of warping has been the rate of absorption of water by a specimen immersed in water. But, since most
10 BUREAU OF STANDARDS

of the trouble resulting from dimensional changes comes from moisture absorbed from or given up to the atmosphere, some experiments were made upon both laminated wall boards and homogeneous insulating boards to find out if there was a relation between the rate of absorption of water and the expansion in a humid atmosphere.

The results are shown in the two tables. The tests made upon the

laminated boards were made several years ago, and those upon the

insulating boards more recently.

In Table 1 are shown the percentage increase in weight as a result

of immersing the boards in water for a definite period of time, the

percentage expansion under the same conditions, the time required

for the absorption of a measured quantity of water placed on the

surface, the gain in weight by absorption of moisture for a period of

72 hours from an atmosphere containing saturated water vapor, and

the percentage expansion under the last-named condition. The absorption by immersion in water was found to be extremely variable

in boards from the same source and presumably alike. There is observable a slight correlation between the absorption by immersion

and the expansion in saturated water vapor, but there are too many

gross discrepancies toi justify the use of the simpler immersion test to replace the direct measurement of expansion resulting from the absorption of moisture from the atmosphere. Making comparisons

likewise of expansion resulting from immersion, or absorption on

the surface, with the expansion in saturated water vapor, one observes no definite correlations. The measurement of expansion in saturated

water vapor, although objectionably time consuming and requiring more apparatus than the simpler tests, was deemed the most trust- worthy of the tests considered for predicting the probable relative

tendency of boards to be unsatisfactory. In Figure 3 are shown

some typical curves of the rate of expansion of wall boards in satu- rated water vapor.

As may be seen from Figure 3 most of the boards were approaching

a state of equilibrium at the end of 72 hours exposure to the saturated

water vapor, and the relative grading of the boards would probably

not be much different for a longer period of exposure. Subsequent

tests were therefore made after exposure of the boards for 72 hours

in saturated water vapor.

There are certain facts which point to the desirability of further investigating the optimum conditions for making the expansion tests of boards. When saturated water vapor is used there is danger of condensation of water on the specimens resulting from a temporary lowering of the temperature such as might occur during

the night when the test has to be run continuously for a 3-day

period. Wall boards used indoors are seldom subjected to such an

extreme condition as immersion in saturated water vapor. As may

be seen from Figure 3 the time of exposure is very arbitrary. It might be that an investigation of the average limits of relative

humidity fluctuation and the period of time during which boards

are subjected to extremes of relative humidity in actual use would

suggest testing conditions more in line with the conditions under

which building boards are used. In some respects it appears that

air conditioned by a saturated solution of sodium chloride, which
FIBER BUILDING BOARDS 11

gives a relative humidity of about 75 per cent for the laboratory

range of temperatures, might have been a wiser choice.

The results in Tables 1 and 2, however, for the expansion of building boards as a result of absorbing moisture from the atmosphere

were obtained by the following procedure : Specimens 3 by 12 inches were cut from the samples with the longer dimension crosswise of

the board as it is usually obtained and erected. At points approximately 10 inches apart heavy marks were made with a glass-marking

pencil. Within each of these mark;s a fine line was engraved with a razor blade, these fine lines serving as the reference marks between

which the expansion was measured. The length of the specimens

preconditioned at 32 per cent relative humidity, maintained by a

solution of magnesium chloride, was measured between the two en- graved lines on each specimen and the result recorded to the nearest

0.005 inch. The magnesium chloride solution was then replaced

with water and the boards exposed for 72 hours to saturated water

vapor circulated through the cabinet. At the end of this 72-hour

period the distance between the two engraved lines was again

measured. A steel scale graduated in hundredths of an inch and a magnifying lens were used in making the measurements. The re- sults obtained in this manner were reported as percentage expansion

over the length at 32 per cent relative humidity. The dimensional

changes of building boards as depicted in the tables are representative of practically all types of commercial fiber building boards.

The last board in Table 1 is not, strictly speaking, a fiber board, al- though it is faced on both sides with thin fiber boards. The body of

this board is gypsum and it was included as a matter of general

interest because such boards are widely used in the same manner as the fiber building boards.

An important determining factor in the dimensional changes in

fiber building boards is the density of the boards. Other things

being equal a dense board may be expected to expand and contract more than a porous, bulky board. It is interesting to observe in

Figure 5 that there is a rough proportionality between density and

expansion in saturated water vapor. Figure 5 is a plot of the pertinent data of both tables. The nonfibrous board of great density

and low expansion is of course most out of line. Of the fiber boards,

certain of the insulating boards which have a relatively high ratio

of expansion to density are considerably out of line. The influence

of density upon dimensional changes in building boards is probably

to be explained structurally. When a vegetable fiber absorbs moisture from the atmosphere it is taken into the walls of the fiber and

causes swelling. The fiber increases in diameter but changes little or not at all in length. If the structure is of an open nature with

many voids and if the contact bonds between the fibers is not too

unyielding, most of the expansion of the fibers can take place into

the voids with relatively little effect upon the gross dimensions of

the board. A board, on the other hand, in which the fibers are

closely packed must suffer dimensional changes in order to accom- modate the lateral expansion of the fibers. Incidentally, the expansion of fibers in the lateral and not in the longitudinal direction is the reason why the laminated boards undergo a greater percentage
12 BUREAU OF STANDARDS

dimensional change in the crosswise than in the lengthwise direction, for, as a result of the conditions of fabrication, more fibers are laid lengthwise than crosswise of the boards. This is one of the

two principal reasons that measurement of dimensional changes has

been confined to the crosswise direction of the boards, the other reason being that the ends of the boards are usually covered by baseboards or molding and the materials there are not abutted in such

a maniier as to result in warping when expansion takes place.

3. INSULATING VALUES

The heat-insulating properties of boards were not studied in this

investigation. Thermal conductivities of different fibrous materials

Density in g/cm5

Figure 5.

Relation of density to expansion caused by absorption of moisture from the atmosphere

have been studied in the heat transfer section of the bureau and the

results obtained were reported by J. L. Finck.9 It was found that

the insulating value of a fibrous product depends more on such

factors as the density of packing and arrangement of fibers than on the choice of raw material. Density is an important factor since,

above a certain minimum not attained in any boards studied, thermal

conductivity increases with the density at an approximately uniform rate. The results of experiments on fiber arrangement indicated that the orientation of the fibers has much effect on conductivity. Boards having fibers for the most part parallel to the sur9 J. L. Finck, Mechanism of Heat Flow in Fibrous Materials, B. S. Research Paper

I\o. 243.
FIBER. BUILDING BOARDS 13

faces of the board, and therefore perpendicular to the line of heat

flow, have conductivities definitely lower than boards having no

definite orientation of fibers. The present methods of manufacture

of fiber boards tend to orient the fibers so that they are most effective

for insulation. At densities found in homogeneous fiber insulating

boards, 30 to 50 per cent of the heat transferred through the boards

is by conduction in the fibers, and the remainder by conduction

through the inclosed air. At such densities convection and radiation are unimportant.

4. DIMENSIONS

The boards are commonly made 32, 48, and 64 inches wide and 6

to 16 in length, all intermediate lengths being in even feet. Thickness varies widely. Laminated wall boards are made in standard

thicknesses of %, A, and % inches and are made quite accurately

to those thicknesses. The homogeneous boards are usually of greater

thickness, and, while there is no standard practice, such boards are usually made slightly less than one-half inch in thickness. The

processes used in making homogeneous boards do not permit accurate

control of thickness.

5. AIR INFILTRATION

Since infiltration of air does not appear to be an important factor

in ordinary usage, no tests of air permeability of fiber building board

were made. Such boards are not, except in unusual cases, installed

exposed and unprotected. Even when used as external sheathing,

fiber boards must be protected against the weather, and any coating

suitable for that purpose would be fairly impervious to air. Such

data on the air permeability of fiber boards as are available indicate

that heat transfer by air infiltration through any of them under the

usual conditions is of little consequence compared to the transfer by

conduction. There are insulating boards through which air passes

readily; however, such boards are not intended for use where they

will be the sole protective covering, but are normally covered by

other protective structural components.

6. SUSCEPTIBILITY TO MOLDING

In common with other materials composed of vegetable fibers, the

fiber boards are subject to attack by molds under certain conditions.

It does not appear, however, that these products are less resistant to the growth of fungi than other components of structures, such as wood and wall papers. Conditions of moisture and temperature that

will permit the growth of molds in the fiber boards will also permit

the growth of rot-forming fungi in untreated wood. If necessary

or advisable for special uses, means of preventing the formation of

molds in fiber boards by the use of preservatives can doubtless be

developed, but such measures do not appear necessary for use under

usual conditions.
14 BUREAU OF STANDARDS

VI. RELATIVE PROPERTIES OF WALL AND INSULATING

BOARDS

The relative properties of the two classes of boards are illustrated to some extent by a comparison of the average values for wall boards

given in Table 1 with the average values for insulating boards

given in Table 2. The values for the average density, thickness,

and breaking strength are significant for comparative purposes. The

average density for wall boards is twice that for insulating boards,

the average thickness of wall boards is slightly less than one-half

that of insulating boards, and the average flexural breaking strength

is practically the same for both classes of boards. There is no marked difference as regards increase in weight or expansion of the

two classes of boards in saturated water vapor. The crosswise ex- pansion of laminated boards was slightly higher than that of the

homogeneous boards, as might be expected when the relative densities are considered. The surface of a hard, dense board is best

adapted to decorative finishes ; therefore, the wall boards which are intended for interior finish are made of sufficient density to obtain

the desired surface finish. The insulating boards designed primarily for heat insulation must be of low density in order to have

optimum insulating properties ; therefore, the surface finish on boards

of this kind does not usually equal the finish obtained on wall boards.

Washington, June 6, 1931.

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