U. S. DEPARTMENT OF COMMERCE

BUILDING

MATERIALS

AND

STRUCTURE

REPORT BMS3

Suitability of

Fiber Insulating Lath

as a Plaster Base

by LANSING S. WELLS

and D. c. SMITH

ureau of Sta

SEP 2 2 1938

dV

is? m
The program of research on building materials and structm-es imdertaken by the

National Bm-eau of Standards is planned with the assistance of the following advisory

committee designated by the Subcommittee on Design and Construction of the Central

Housing Committee.

TECHNICAL RESEARCH GROUP

Harold D. Hynds, Chairman

Walter Junge, Federal Housing Administration, Vice Chairman

Pierre Blouke,

Home Owners, Loan Corporation.

C. W. Chamberlain,

Procurement Division.

Charles E. Mayette,

United States Housing Authority.

Vincent B. Phelan,

National Bureau of Standards.

A. C. Shire,

United States Housing Authority.

George W. Trayer,

United States Forest Service.

Elsmere J. Walters,

Quartermaster Corps, War Depart'

ment.

Sterling R. March,

Secretary.

The program is administered and coordinated by the following stafif committee of

the Bureau.

Hugh L. Dryden, Chairman

P. H. Bates W. E. Emley A. S. McAllister

H. C. Dickinson G. E. F. Lundbll H. S. Rawdon

The Forest Products Laboratory of the United States Department of Agriculture

is cooperating with the National Bureau of Standards in studies of wood constructions.

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UNITED STATES DEPARTMENT OF COMMERCE • Daniel C. Roper, Secretary

NATIONAL BUREAU OF STANDARDS • Lyman J. Briggs, Director

BUILDING MATERIALS

and STRUCTURES

REPORT BMS3

Suitability of

Fiber Insulating Lath

as a Plaster Base

by LANSING S. WELLS and D. C. SMITH

ISSUED AUGUST 23, 1938

The National Bureau of Standards is a fact-finding organization;

it does not "approve" any particular material or method of construction. The technical findings in this series of reports are to be construed accordingly

UNITED

FOR SALE

STATES GOVERNMENT

BY THE SUPERINTENDENT OF

PRINTING OFFICE • WASHINGTON • I938

DOCUMENTS, WASHINGTON, D. C. • PRICE 10 CENTS
Foreword

The present report describes an investigation conducted under an allotment from

the Works Progress Administration during the fiscal year 1937. As this study is closely

related to the general objective of the Bureau's research program on building materials

and structures, the results obtained are made available by publication in this series of

reports.

Many of the techniques used in the building industry have been developed

empirically as a result of experience, including successful and unsuccessful results. Most experienced bulding contractors have probably learned how to secure successful

results in the application of plaster to fiber ins\ilating lath, but there seems to be little published information to guide those who have not had previous experience. This

investigation was undertaken to supply that information and resulted in the definite

recommendation that only quick-setting strong plaster be used for plastering over fiber

insulating lath and that the thickness of the plaster be not less than one-half inch.

Lyman J. Briggs, Director.
Suitability of Fiber Insulating Lath

as a Plaster Base

by LANSING S. WELLS and D. C. SMITH

CONTENTS

Page

Foreword II

I. Introduction 1 II. Materials 2

1. Fiber insulating boards and lath 2

2. Plywood 2

3. Gypsum wall boards and gypsum

lath 2

4. Cement-asbestos board 2

5. Compressed-fiber board 2

6. Sand 3

7. Gypsum plaster 3

8. Calcined gypsum 3

9. Hydrated lime 3

III. Properties of the boards 3

1. Density and strength 3

2. Expansions and contractions 4

ABSTRACT

The investigation pertains to the properties of wall and plaster boards and the suitability of fiber insulat- ing boards as a plaster base. Such properties as density, strength (flexural and tensile), and linear changes

accompanying changes in relative humidity and wetting

and drying were studied. The boards were mounted

on steel frames and the extent of buckling resulting

from changes in relative humidity were determined.

Methods of successful application of plaster on in- sulating lath were determined by experiments in which

the thickness, sand content, strength, and time of set of the plasters were varied independently.

I. INTRODUCTION

Among the newer building materials which are

of interest in connection with low-cost houses,

prefabricated houses, and the remodeling of old

houses are the numerous types of waU, insulating, and plaster boards. Wall boards are used

as interior and exterior finishing materials; in- sulating boards, largely as interior finishing

surfaces or between waUs as sheathing; and

plaster boards, as a base for plaster finish.

Page

2. Expansions and contractions—Con.

(a) With changes in relative humidity 4

(b) With wetting and drying 5

3. Comparison of linear changes of painted with unpainted boards. 6

(a) With changes in relative humidity 6

(b) With wetting 6 IV. Experiments on unplastered boards when

attached to studding 7

V. Experiments on plastered fiber insulating

lath 12

1. Three-coat plastering 12

2. Two-coat plastering 15

VI. Sunmiary and conclusions 16

Fiber boards, especially the homogeneous

type of low density, may combine two functions,

serving as a base for plaster and also affording

thermal insulation. However, in common with

many other materials, including wood, these

boards change in dimensions with changes in

moisture in the surrounding atmosphere, and

this characteristic property must be considered

in the application of the material if satisfactory

results are to be obtained. Doubtless most

buUding contractors have learned through ex- perience methods of insuring the desired results, but there is a decided lack of published

information on the suitability of these boards

as plaster bases. Therefore, an investigation was undertaken to secure information on conditions necessary for the successful use of fiber insulating lath as a plaster base.

The properties of some fiber insulating boards

have been determined previously,^ but there

have been many changes in the methods of

1 Misc. Pub. NBS M132, Properties of Fiber Building Boards (Dec.

1, 1931).

[1]
manufacture. In any case, it was considered

desirable to determine the density and the

flexural and tensile strengths of the boards to be used in the experiments. The expansion

and contraction of the boards as influenced

by changes in moisture was also studied. The

boards were then attached to rigid steel frames

by two methods of attachment and the buckling

resulting from changes in relative humidity was

determined. These tests gave considerable in- formation as to the behavior of the boards

themselves and laid the foundation for the

study of their suitability as plaster bases.

This study was carried out by actually constructing plastered panels. Methods of secur- ing satisfactory results were determined by

experiments in which the thickness, sand content, strength, and time of set of the plasters were varied independently.

There is included also in tliis paper the limited investigation made of gypsum plaster

boards or lath, as well as that of certain wallboards other than those of the fiber insulating

type.

II. MATERIALS

1. Fiber Insulating Boards and Lath

From among the numerous wall and plaster

boards available, a limited number of homogeneous fiber boards were purchased for these

experiments. The boards purchased were

chosen as illustrative of various materials and

methods of manufacture. The individual speci- mens were not specially selected, and no at- tempt was made to systematically sample the

product, since the tests were not intended as a comparison of materials commercially available, or as a study of the effect of material and

methods of manufacture on the physical properties. It should be clearly understood that

the data on samples obtained in the manner

described are not adequate for such purposes.

The data given later in the paper refer to the

samples actually used, and the information as

to composition and method of manufacture is given as descriptive of those samples. It cannot be concluded that the data apply to all boards of the corresponding composition and

method of manufacture wliich are sold commercially.

The insulating fiber boards were of the usual

sizes, 4 feet wide by 8 to 12 feet long and

inch thick, and the insulating lath, 18 inches

wide by 48 inches long by inch thick.

The materials from which the fiber insulating

boards were manufactured and the key numbers which will be employed hereafter to designate the boards are:

1. Wood fiber.

2. Exploded wood fiber.

3. Balsam-wood fiber.

4. Western wood fiber.

5. Extracted sugar-cane fiber (bagasse).

6. Largely of pulp from waste paper.

7. Straw fiber.

8. Ground wood-pulp fiber.

2. Plywood

Realizing that investigations of plywood are within the field of the Forest Products Laboratory, only limited tests for comparative purposes were conducted on this material. Plywood boards are, of course, not used as plaster

bases, but their movements attending changes

in the relative humidity of the air and wetting

and drying were studied. The boards investigated were manufactured with three plies of Douglas

fir, using casein glue, and were K-inch thick.

The sample used in this study will be designated

hereafter as: 9. Plywood.

3. Gypsum Wall Boards and Gypsum Lath

Inasmuch as boards with a core of set gypsum and a covering of paper are used in much

the same manner as the fiber boards, two types were included. The designation of the boards

(^8 inch in thickness) wUl be: 10. The usual

type of gypsum wall board and gypsum lath;

11. A light-weight gypsum board having some

waste-paper pulp in a porous gypsum core.

4. Cement-Asbestos Board

This is a heavy dense, hard, strong board

made by pressing together in a hydraulic press a mixture of portland cement and asbestos.

The cement-asbestos board which was ji inch

in tliickness will be recorded henceforth as: 12. Cement-asbestos board.

5. Compressed-Fiber Board

The dense compressed fiber board investigated will be designated: 13. Pressed board.

[2]
Besides the above boards, the foUowmg materials were used:

6. Sand

III. PROPERTIES OF THE BOARDS

1. Density and Strength

Clean Potomac River sand was used in pre- paring the plaster panels. The sieve analysis

of this sand was as follows:

U. S. standard Sieve No.

Sand

passing

U. S. Standard Sieve No.

Sand

passing

% %

8 100. 0 50 14. 0

16 95. 0 100 1. 5 30 75. 5

7. Gypsum Plaster

The gypsum plaster for preparing the plaster

jj panels compHed with Federal Specifications

I; SS-P-401. The physical properties of the

i plaster were: Time of set (neat) 25 hours; and

1 tensUe strength, neat (average five briquets)

I 330lb/inl

I 8. Calcined Gypsum

The calcined gypsum (plaster of paris) for

I the finishing coat of plaster for the plastered

panels complied with the Federal SpecificaI tions SS-G-901. The physical properties of

I the material were: Time of set, 29 minutes;

l tensile strength (average six briquets) 585

I Ib/in^, and compressive strength (average six

i 2-inch cubes) 3,840 Ib/inl

9. Hydrated Lime

The hydrated lime used in preparing the

finishing coat of plaster complied with Federal

Specification SS-L-351 (Fmishing Lime). It had the following properties:

Chemical composition

Constituent Amount Constituent Amount

CaO

%

48. 1 Loss on igni-

%

MgO 33. 6 tion - 16. 4

SiOz 0. 2 CO2 1. 2 R2O3 . 4

Total 99. 9

_ _ 0. K. /Through No. 30 sieve 100. 0% Fmeness: j^hrough No. 200 sieve 96. 9%

Plasticity 530

In addition to the density and strength

(flexural and tensile), table 1 gives data relative

to the weight, tliickness, and deflection at point

of rupture of the eight types of fiber insulating

boards. The data are based on tests of at least three samples of three boards. In general, over the range of relative humidity studied, the

flexural strength of the fiber insulating boards

decreased as the relative humidity of the air

3^1 1 1 \ 1 i1 i

1 1 \ 1 \ 1 32

4 1 20 L—I 1 1 1 ^ ^ 1 1 1 1 1 1 40 SO 60 70 60 go 100 Re/ahVe hum/cf/'/y, percent

Figure 1.

Flexural strength of fiber insulating boards as a function of the relative humidity of the air at which

the boards were conditioned.

(in wliich they were conditioned) increased.

(See fig. 1.) A comparison of the average

flexural strengths at 40-percent relative humidity with those at 80 and 100 percent shows

that there was a decrease in strength of about

20 and 50 percent, respectively, at the higher

humidities.

The gypsum wallboards were tested according to the procedures given in Federal Specifications SS-W-51a (similar to methods of American Society for Testing Materials: C36-34).

The flexural strength was determined on speci-
mens 12 inches wide by approximately 16

inches long, supported on fixed parallel bearings

spaced 14 inches on center and loaded on a

similar bearing midway between the supports.

Board 10 broke at an average load of 115

pounds with the knife-edges across the fiber of surfacing and at 47 pounds with the knife-edges

parallel to the fiber of surfacing when tested at

40-percent relative humidity. The corresponding loads for board 11 were 136 and 44 pounds.

Measurements were not made of the strength

of the gypsum wallboard at the high humidities. It is known, however, that the flexural strength of these boards, when wet, is low.

Wallboards 10 and 11 weighed 1,475 and 1,335

lb/1,000 ft^ respectively.

Table 1.Properties of the fiber insulating hoards used in this investigation "

No. Composition of board

Density

at 65%

relative liumidity

Weight

at 65%

relative humidity

Thickness at 65%

relative humidity

Flexural strength, cross- wise, at relative humidities of:

40% 65% 80% 100%

Deflection at rupture

at 65%

relative humidity

Tensile strength at 65% relative hu- midity

Cross- wise Lengthwise

12

34

5

6

7

Wood fiber Exploded wood fiber Balsam-wood fiber Western wood fiber Extracted sugar-cane fiber Largely of pulp from waste paper Straw fiber Oroimd wood-pulp fiber

e/cm3

0.26

.36

.31 .31

.27

.46

.32

.30

lb/1,000

ft 2709

910

1, 143 798

766

m.

0.51

.51 .47

.54

.49

.49

lb

15. 0 15.0

18.0

12.0

11. 5

31. 0 16.0

13.0

lb

13. 0

15. 5 16.0

12.0

11.0

26. 0

15. 5 11.0

lb 12.0

15.0

13. 5 10.0

9. 0

22. 5

13. 0

9. 5

lb

9. 5 8.0

9.0

6.5

7.0

14.5 8.0

5.0

m.

0. 64

.38

.65 .60

.80

.72

.65

.58

lb/in.

!

164

162

254

173

152

323 288

117

Ib/in.i223

153

162 173

199 340

413

157

" Thickness, flexural strength, deflection, and tensile strength determined according to procedure given in Federal Specification for Fiber-Board;

Insulating LLI^F-321a. Flexural strength made on specimens 3 inches wide and 18 inches long placed on horizontal parallel supports 12 inches apart applying load at midspan. Measurements of flexural strengths at relative humidities of 40-, 80-, and 100-percent are not required in the specification.

2. Expansions and Contractions

(a) With Changes in Relative Humidity

Since wallboards in use are continuously

subjected to changing atmospheric conditions,

expansions and contractions are constantly

taking place. A standard meter rule obtained

from the length section at the National Bureau

of Standards was used to measure the linear

changes on specimens 4 feet square. The

k.

o

i2

OS

0.4

0.3

0.2

) >-

) < >- > ^ (

>

(

TIfAE IN DArS

Figure 2.Expansion of fiber insulating boards on

changing the relative huynidity from 25 to 90 percent

reference lines were approximately 97 cm apart

and were made with a razor blade on a waxed

surface produced with a soft wax pencil. A

magnifying lense was used to facilitate the

readings, recorded to the nearest 0.2 mm.

Three sets of reference lines were made for measurements in each of the two directions

lengthwise and crosswise. Four or more boards

of each type were measured.

Because the linear changes attending cycles

of changes in relative humidity were to be

ascertained, it was necessary first to know how

long the boards had to be conditioned at a

specified humidity to reach dimensional "equilibrium." Figure 2 shows the rate of expansion

of the boards on changing the relative humidity

from 25 to 90 percent at a temperature of

approximately 70° F. It can be seen that

most of the expansion had occurred at the end

of 3 days. In a few cases the expansions lengthwise were slightly less than those crosswise;

however, the differences were small and accordingly the data obtained from the measure- ments in one direction (crosswise) only are

given. It should be noted that three of the

four curves plotted in figure 2 pertain to more

than one type of board. In these cases the

[4]
variations between the dift'ei'ent types of boards were no greater than were those of separate

boards of the same type.

When the boards were conditioned in air maintained at 100-percent relative himiidity,

condensation of moisture occurred and no con- stant measm'ements could be obtained until

the boards were actually wet. Figure 3 shows graphically the linear expansions which occurred when boards conditioned

in air at 25-percent relative humidity were re- conditioned at relative humidities of 60 and 90

percent, respectively. Here, as in figure 2, cer- tain curves pertain to more than one type of

board. The spread is represented by the closed

vertical bar and tlie curve passes through the

average values.

Fiberboards in practical use are subjected to numerous cycles of changes in relative humidity.

To determine whether the boards returned to

their original dimensions, in going from cycle

to cycle, the specimens were subjected to three

cycles of changing humidity. The boards were

0.6

0.5

0.3

0.2

0.1

'—6

^3J 5

Cj>

H0j/hl2

30 40 50 60 70 80 90 RELATIVE HUMIDiryj PERCENT Figure 3.

Percentage expansions of wallboards accom- panying increases in relative humidity.

Numbers 1 to 8, inclusive, fiber insulating boards; 9, plywood; 10 and 11, gypsum wallboard; 12, cementasbestos board; and 13, pressed fiberboard.

first conditioned in air at 25-percent relative

humidity, then at 90 percent, and the expansions noted. Figure 4 shows the expansions

and contractions which accompanied three of

these cycles. Apparently the absorbed moisture at 90-percent relative humidity causes the

fibers to swell, with a resultant rearrangement

on drying, thus giving an actual slirinkage to the boards. The kind and mctl)od of sizing

used in the fabrication of the boards no doubt

are factors involved in this sluird'cage. Tlie

over-all movements (expansion j)his contraction)

which occur after the first cycle are greater

than the expansions shown in figures 2 nnd 3.

7^ \ 6 A,

-ij 7

r

90 eS 90 25 RELATIVE HUMIDITy^ PERCENT Figure 4.

Dimensional changes of fiber insulating boards accompanying cycles of changes in relative humidity.

Shrinkage did not occur when the boards were carried through 3 cycles of changes from

25-percent relative humidity to 60 percent and

back to 25 percent. The results of these

cyclical changes ai'e not shown.

(6) With Wetting and Drying

As there is no assurance that fiberboards will always remain dry even though they may be

used in the interior of a buUding, some experiments were carried out to determine the extent

of the expansion and contraction wliich accompanies wetting and drying. Figure 7 shows the

rate of expansion of several boards when they were soaked in a tank of water. It should be

noted that the total expansion and rate of

expansion of the boards varied rather widely.

The length changes residting from 3 cycles of wetting and drying are indicated in figure 5. The boards were dried in air at a relative humidity of 25 percent at the start and at the end

of each cycle until the board attained dimensional

equilibrium. From figures 4 and 5 it may be

seen that wetting and drying caused about

twice the dimensional changes (expansion, contraction, and resulting shrinkage) as raising the

relative humidity to 90 percent, then drying.

5]
V-7 f

7, ,« / (// —6/

1

D

CyCLES D

iv=tveT Figure 5.

Dimensional changes of fiber insulating boards resulting from alternate wetting and drying.

3. Comparison of Linear Changes of Painted

With Unpainted Boards

(a) With Changes in Relative Humidity

It has been shown that fiber boards undergo

marked expansion with an increase in the rela- tive humidity of the air. Obviously, if the

expansion can be decreased there will be less buckling when the boards are fastened, as in a

wall. Accordingly, a few experiments were

conducted on the total expansion and rates of

expansion on boards that were painted.

Four different types of fiber insulating boards were so chosen that they included the range of

expansion of the boards being studied. Speci- mens (4 by 4 ft) were first given a coat of sizing

composed of one part of turpentine to four of

spar varnish, by weight. After drying, this was followed by three coats of aluminum paiat

composed of 2 pounds of aluminum bronze per

gallon of spar varnish. The sizing and paint were applied to both sides and to the edges of

the specimens.

Since fiber insulating boards are very porous

the covering capacity of paint is low. Table 2 contains data on the covering capacity for each

coat, expressed in the square feet per gallon.

Figure 6 shows that the rate of expansion of

the boards on changing the relative humidity

from 40 to 90 percent was materially less for the painted than for the unpainted boards.

In two of the four types studied the ultimate

expansion was approximately the same, the

slight difference bemg no greater than would be

expected in individual boards. In two types

the ultimate expansion of the painted boards

was somewhat less than that of the unpainted.

Painting might in part be advantageous in that

it decreases the rate of expansion and therefore

minimizes the effects of rapid changes in relative

humidity.

Table 2.

Covering capacities of coats of paint on some

fiber insulating boards

Fiber

insu- lating board numbers

Composition of board

coat

(1:4 turpentine- varnish mixture)

First coat of alumi- num

paint

Second

coat of alumi- num

paint

Third

coat of alumi- num

paint

Total

for the

three

coats of alumi- . nurn

paint

ftt/gal ft'/gai ft'/gal ft'/gal ft'/gal 2 Exploded wood fiber.

.

105 111 370 560 74

5 Extracted sugar-cane

fiber 108 136 280 410 75

0 Largely of pulp from

waste paper. 150 235 358 545 113

7 Ground wood-pulp fib- er 130 158 320 545 88

(6) With Wetting

Figure 7 shows the linear expansion resulting

from soaldng both the painted and unpainted

boards in water. As in figure 6, painting decreased the rate of expansion. It did not,

however, eliminate expansion and in several

instances the ultimate expansion of the painted

and unpainted boards was the same. Much

that has already been said relative to the effect of painting on modifying movements accompanying changes in relative humidity is equally

applicable here.

/

7

o

6U D

5

5

5 bU n

0—

1

( 0 4

/ k y

(

p -D UP^UNPAINTED

30 40 50 TIME IN DAYS Figure 6.

Expansions of painted and unpainted fiber insulating boards resulting from an increase in relative humidity from 40 to 90 percent.

[6]
^ 0..

7U

\ —

1/

V, r—

/

f

PAINTED UP=UNPAINTED i

'/

a 12 16 TIME IN DArS Figure 7.

Expansions of painted and unpainted fiber insulating hoards soaked in water at 70° F.

IV. EXPERIMENTS ON UNPLASTERED

BOARDS WHEN ATTACHED TO

STUDDING

It has been shown that the various boards

undergo marked expansions and contractions

mth an increase or decrease in the relative

humidity of the air or with wetting and drying.

To determine how much these movements

would be reflected in buckling of the boards

when they were attached, special frames were

constructed for panel tests. The frames were made of l}^-inch angle-iron (welded together as shown in fig. 8) because wooden frames would

have introduced the variable of expansion and

contraction accompanying changes in moisture

content of the wood. They were so designed

that a board 4 feet square could be attached

to the uprights spaced 16 inches on centers

thus simulating the spacings of studding com- monly used in house construction. The flats which served as bracing for the frames and pre- vented outward movement of the "studding" were spaced about %^ inch from the face of the

frame so that the boards were free to buckle

inward as well as outward.

The board specimens were bolted to the

frames with }^-inch diameter steel stove bolts

having heads }^ inch in diameter. They were

attached only along the uprights at the bolt

holes spaced 6 inches on center, except as indicated otherv/ise in figure 8, thus allowing

freedom for buckling between the studding.

It is realized that this method of attaching the

boards is not comparable to nailing the boards

to wooden studding. It was evident, as the

investigation progressed, that the manner of

attaching the boards had a marked efi'ect on

the extent of buckling. The manufacturers

of the boards are cognizant of this and usually

recommend that the boards be not butted

together. However, the boards may be so firmly naded that there can be but little side

and end movement although space has been

allowed for such.

Two methods of mounting the boards were

used. In one, the boards were firmly attached

along the outside studding by means of strapiron side strips (fig. 8) 1 inch wide, inch

thick, and 48 inches long placed directly on

the face of the boards. When these strips were bolted in place little or no movement in

the boards could occur along the outside.

Consequently, buckling across the studs was

greater than when the boards were less firmly

attached. In the second method, the strips were raised from the surface of the board with

steel washers, inch wide. Thus the boards were held in place only by the bolts and not

by the iron strips. This prevented binding of

the outside edges except where bolted and

permitted a relief of some of the strains set up

at the points of attachment either by tearing

away or compressing the fibers of the boards.

The boards thus mounted were subjected to changes in relative humidity and the extent

of buckling between studduig determined by

measuring the height from the surface of the

board to a reference straightedge placed across the frame. A strip of 1-inch angle brass served

as the reference straightedge and was pro- vided with brass blocks soldered at the ends.

Thus the straightedge was raised above the

surface of the board when the blocks rested on the iron strips bolted along the outside

edges of the frame. The height from the ref- erence to the attached board was measured at

15 points, 9 midway between the studding and

6 directly over the studding.

A study was first made of the extent of buckling of the boards when mounted on the frames

by the first method. The boards were conditioned in air at a relative humidity of 35 percent

until dimensional "equilibrium" had been at- tained before bemg attached to the frames.
1

j£cr/on on aa'

3^.

Figure 8.^

Details of steel frame for test panels.

The height from the reference straight edge to the board was then measured at 15 points.

The test panels were then transferred to a room

where the relative humidity was 60 percent and

after the boards had been similarly conditioned

at this humidity measurements were again

made to determine the buckling. The test panels were next removed to a third room where

the relative humidity was 90 percent and

similar measurements were made. The buckling was arbitrarily taken as the average of the

three highest measured points. The results are shown in table 3. Figure 9 shows the budding

which occurred during the increases in relative

humidity from 35 to 60 and 90 percent, respectively. Certain of the boards were grouped

together since the variations between these

difl'erent types were no greater than those of

separate boards of the same type. The grouping is not the same as that indicated in figure

3, which shows the expansions of the unattached

boards.

[8]
Table 3.

Buckling measurements on various wall boards when attached to 4- by 4-i'nch angle-iron frames

with iron strips binding the two outside edges The figures represent the average of three points sliowing the greatest

rise. Numbers 1 to 8, inclusive, fiber insulating board; 9, plywood; 10 and 11, gypsum wall boards.

[Starting relative humidity 35 %j

No. Composition of board

Wood fiber.

Exploded wood fiber.

Balsam-wood fiber.

Western wood fiber.

Extracted sugar-cane fiber.

.

Largely of pulp from waste

paper

Straw fiber

Ground wood-pulp fiber

Plywood

Usual type of gypsum wall board A light weight gypsum wall board

Cycle number

Buckling at rela live humidities of—

60%

0.03

.05

90%

0. 11

. 12

.41 .45

.31

.31

.05

. 10

.05 .07

100%

.70

.61

.73

.83

.36

.27

Buckling left after drying back to 35% rel- ative humidity

in.0.03 .05

.03 .03

.12

.17

.06

. 12

.07

.10

.05

.08

.03 .04

The panels were next conditioned at a relative

humidity of 35 percent and the buckling wliich

remained was determined. This is recorded

under column 6 of table 3. On repeating the

cycle still further buckling occurred (table 3). Figure 10 shows the buckling across the studding after the panels had been conditioned a second time at 90-percent relative humidity.

The more usual type of buckling was the for- mation of two convex surfaces and one concave

surface, the latter occurring between the two

inside studs. In some cases, three convex sur- faces resulted but, whether two or three, there

seemed to be no correlation between the manner

in which the boards buckled and any other property. The alignment of the forces at the start

0.6

as

0.4

0.3

.'0 o.z

0.1

y

- 30 40 50 60 70 80 90 lOO

Fe/a/'jVe humic/i/y, percen/

FiGtTRE 9.Buckling {rise in inches across 16-inch

studs) of wall boards resulting from an increase in

relative humidity.

Boards were attached to steel frames, 4 feet square, with outside edges of the boards firmly bound. Numbers 1 to 8, inclusive, fiber insulating boards, and 10 and 11 gypsum wall boards. may have been the determining factor. Panel 7 shows a board which buckled with the formation

of two convex surfaces (the concave not evident

in the photograph) and panel 8 the same type

of board exhibiting three convex surfaces. The

greater rise usually occurred when two convex

surfaces were produced.

After the boards had been exposed to 2 cycles of changes in relative humidity (1 cycle,

35 to 60 to 90 and back to 35-percent relative

humidity) the panels were transferred to a room in which the relative humidity was maintained at 100 percent. The data relative to the budding wliich occurred are contained in

table 3. Figure 11 is a photograph of the

panels taken in a manner similar to that of

figure 10. A comparison of these two photographs shows the rather general increase in buckling accompanying an increase in the rela- tive humidity from 90 to 100 percent.

A study was then made of the extent of budding of the boards mounted on the frames

by the second method, previously described.

Table 4 contains the data relative to the rise in inches which occurred when the boards were

restrained only at the bolts. It should be

noted that the boards were first conditioned at a relative humidity of 45 percent rather than

35 percent as in the studies made by the first method. This was imavoidable since the rela-

[9]
Figure 10.

Panels showing the buckling of wall hoards

across the studding after the 'panels had been kept a second time at 90-percent relative humidity.

Boards originally attached after being conditioned at 35-percent relative humidity.

Panels 1 to 9, fiber Insulating boards as follows: 1 wood fiber, 2 exploded wood fiber, 3 balsam-wood fiber, 4 Western wood fiber, 5 extracted sugar-cane fiber, 6

largely of pulp from wastepaper, 7 and 8 straw fiber, 9 ground wood-pulp fiber. Panel 10, plywood, panel 11 usual type of gypsum waU board and panel 12, a lightweight gypsum board.

tive humidity of the room increased during the

time between conducting these two sets of ex- periments and there was available no ready means of reducing it to 35 percent. It is doubtful if this made very much difference

since the extent of buckling was small when the

relative humidity was under 90 percent and did

not rise rapidly until the boards had been conditioned above this relative humidity. Figure

12 shows the results graphically. A comparison

of tliis figure with figure 9 shows that when the

boards are given the freedom to take up some

of the stresses by tearing or enlarging the holes

thi'ough which the bolts pass (method 2, fig. 12)

the buckling is considerably less than that

which results when they are more firmly at- tached (method 1, fig. 9). Tliis reduction in

buckling is particularly noticeable at relative

humidities below 90 percent.

Table 4.

Buckling measurements of various wall boards when attached to 4- by Ji.-inch angle-iron frames in such a way that none of the edges is bound

The figures represent the average of three points showing the greatest

rise. [Starting relative humidity 45%]

No. Composition of board

Wood fiber.

E.xploded wood fiber.

Balsam-wood fiber

Western wood fiber...

Extracted sugar-cane fiber.

Largely of pulp from waste

paper.

Straw fiber.

Ground-wood-pulp fiber...

Cement-asbestos board

Pressed board

Cycle number

Buckling at rela- tive humidities

of:

60%

0.01 .04

.02

90%

0.06

.06

. 16

. 10

.04

. 12

100%

0. 14

.35

.30

.49

.04

.24

Buckling

left after drying back to 35% rel- ative humidity

0.03

.04

.02

.03

.05

.05

.05

.06

All of the experiments on buckling described

up to tliis point pertained to boards which had

been conditioned at a fairly low relative humidity before being attached to the frames. The

following experiments pertain to boards conditioned at a high relative humidity (90 percent)

before being attached to the frames. Since it

is evident that boards so attached would tend

to shrink, as the relative humidity subsequently

decreased, the boards were attached (second

method) in two sections and a study made of

the change in the width of the space between

these two sections as the relative humidity of

the air decreased. One section was 16 by 48

inches and the other 32 by 48 inches. A space

[10]
Figure 11.

Panels showing the buckling of wall boards

across the studding after the panels of figure 10 had been

conditioned at 100-percent relative humidity.

about /g inch in width was left between these

two sections along one of the uprights of the

frames which served as one of the studs.

Narrow strips of paper were pasted over tliis joint after the boards had been attached at a

relative humidity of 90 percent. The strips were then severed along the joints with a razor blade and the resulting separation of the adja- cent halves of the paper measured after the

panels had been conditioned at a relative humidity of 50 percent. The measurements showed

that the increase in the space between the two

sections of the various boards ranged from 0.05

to 0.12 inch. With one exception the magnitude of the separation of the various boards was in the same order as their linear contractions, as indicated by the data contained in

table 3. This exception was board 6, the fiber

insulating board having the greatest density

and tensile strength, crosswise (table 1). With this board there was not as much give

around the holes where the boards were bolted

to the frames as with the other boards.

After reference points for buckling measure- ments had been taken, the panels were again

conditioned at 90-percent relative humidity.

No convex or concave buckling occurred, indicating that the boards on expanding with a change in relative humidity from 50 to 90 per- cent closed up the enlarged holes formed when

the boards contracted from a relative humidity

of 90 to 50 percent.

These experiments suggest the method of

first conditioning wallboards at a high relative

humidity before attaching them to prevent

budding. This would require conditioning rooms at the job. An alternative would be to dampen the boards before they were used and

to then allow them to take up the added moisture. This, however, introduces an objection

since carpenters would find it difficult to saw dampened boards.

Fiber boards should never be wet and then

allowed to dry out before being attached to the

studding. The reason is obvious when one

considers the shrinkage (figs. 4 and 5) which

0.6

0.4

0.3

0.1

0

<

<

40 SO 60 70 80 90 100 RELATIVE HUMID ITYj PERCENT

Figure 12.

Buckling {rise in inches across 16-inch

studs) of wall boards resulting from an increase in relative humidity.

Boards were attached to steel frames, 4 feet square, with boards attached by bolts only, and outside edges not bound.

[11]
follows the first cycle of wetting and drying.

A shrinkage as large as 0.3 inch over a 4-foot

span resvilts when some fiber insulating boards

are wet and dried. If now, these boards are attached whUe in the contracted state, the

expansion with resulting buckling should be

greater than with the original boards which

have not been subjected to the first cycle of

wetting and drying. Experiments verified

this.

V. EXPERIMENTS ON PLASTERED

FIBER INSULATING LATH

1. Three-Coat Plastering

Fiber insulating lath are usually 72 inch thick,

18 inches wide, and 48 inches long. They are attached to studding so that the vertical joints are staggered. Most of these boards are shipZ60

240

220

200

180

b 'SO

140

120 k 100

80 ^ 60

40

200

12

(1:2)

(lu')efc., preoporfions /

?/- /<? sane

(1:3,^/

(I- 4)A

(/£)/

(1:6) /

14 16 18 20 22 24 26 28 30 3Z 3i Gypsum p/as^er /n .sanc/ec/ mix, percen/

Figure 13.Increase of tensile strength of plasters with an increase in the -percentages by weight of gypsum plaster in the sanded mixes.

lapped, and each manufacturer appears to have some slight modification of design. The properties of the four different makes of lath wliich were available for test were practically identical

with those of the corresponding waUboards.

Consequently, the wallboards of the remaining

makes were cut into strips 18 by 48 inches for

the plastering experiments. The lath were

bolted to the frames by method 2 of attaching,

so that the middle course gave a vertical joint over a stud.

Insulating lath thus attached budded to about the same extent as did the fiber wallboards when the relative humidity was increased an equal amount.

The investigation pertains entirely to the use

of gypsum plaster over these bases. It is generally recommended in specifications for plastering that the scratch coat on lath be composed of 1 part of gypsum plaster to 2 of sand,

by weight, and that the brown coat be 1:3. Leaner mixes are sometimes used, however,

and occasionally the plasters are rather badly

oversanded. Consequently, plastering experiments were made with the following four proportions of plaster to sand: (1) Scratch 1:2,

brown 1:3, (2) scratch 1:3, brown 1:5, and (3) scratch 1:4, brown 1:6.

Figure 13 shows how the tensile strength of

the set plaster decreased with an increase in

sand content. In making the briquets, the

mixes were brought to standard consistency

as described in ASTM Standard Methods of

Testing Gypsum and Gypsum Products,

ASTM. Designation C 26-33.

Although it is usually recommended that

the thickness of the plaster over lath be inch,

yet in actual practice the thickness is often

less than this. The experiments on buckling

previously described indicate that tliicker rather than thinner plaster should be applied.

Experiments were conducted on plaster y% and

% inch in tliiclmess. In the plastering experiments the insulating

lath were conditioned for a week in a room

where the relative humidity was about 50

percent before they were bolted to the frames.

The scratch and brown coats were then applied

separately. To prevent the coats from drying

out before the plaster had set and hardened

satisfactorily, accelerator was added to the

plaster, which was slow setting. About 1 percent of finely ground, set plaster of paris

accelerated the reactions so that each coat set in about 3 hours. After the brown coat had

dried, the finish coat of lime putty gaged with

calcined gypsum (plaster of paris) was applied.

This coat,which was about Xe inch in thickness,

[12]
was composed of 3 parts of plastic lime putty

to 1 part of gaging plaster by volume.

The panels were conditioned for 2 weeks in a room where the relative humidity was 50

percent. They were then stored for periods of a month in each of two other rooms where the

relative humidities were 90 and 100 percent,

respectively. Buckling measurements were

taken at intervals during these storage periods,

after which the panels were returned to the

first room. These plastering experiments brought out some rather definite results. In the first place,

the panels plastered with the third mixture

(scratch, 1:4 and brown, 1:6 by weight) all developed cracks along the horizontal joints of

the lath. Some also had cracks along the vertical joints. Cracks appeared in many panels

while the brown coat was setting. In general,

the bond between the scratch coat and the

plaster base was poor. This plaster was very

unsatisfactory. It is realized that this was a badly oversanded plaster. Nevertheless, the

results show what may be expected when such

weak plasters are used. An interesting point

is, that despite the fact that they gave such an

unsatisfactoiy performance, their strengths were of the order of magnitude of the minimum

requirements of ASTM Standard Specifications

for Gypsum Plasters, ASTM Designation:

C 28-30. The 1:4 scratch coat had a tensile

strength of 91 lb/in. ^ (fig. 13) ; the minimum of a

1 : 2 ready-sanded scratch reqiiired by the

ASTM specification is only 75 Ib/in^. The

1:6 brown coat with a tensile strength of 45

lb/in. ^ was only slightly below the stipulated

minimum of 50 lb/in. ^ for a 1 : 3 mix. It can be

seen from figure 13 that the tensile strength of

the 1 : 2 plaster was 240 and the 1 : 3 plaster

145 lb/in. ^, values roughly three times those of

the respective minimum requirements. Incidentally, through the strength of the neat

plaster used in these experiments was over

twice that of minimum requirements for neat

plaster, it was, nevertheless only about 10

percent greater than the average strength of

the numerous brands tested at this laboratory

during the past 5 years. The second factor worthy of note was the general satisfactory condition of the panels plastered

with % inch of plaster where the scratch coat

was 1:2 and the brown coat 1:3. Since the

plaster set within 3 hours there was but little noticeable buckling after the scratch coat had

been applied and wiiat little had taken place was straightened by the application of the

brown coat. After the finish coat had been

applied and the plaster dried, there was no

further buckling greater than 0.02 inch in the

plastered surface even after the panels had been

kept in the room where the relative humidity was 100 percent. After the panels had been

dried again no cracks developed in the plaster

along the horizontal or vertical joints of the

lath, or in any other portion of the panel.

Several of the panels plastered with % inch

of plaster (scratch 1:2 and brown 1:3) developed cracks along the horizontal joints after

the panels had been dried. The cracks appeared in the plaster which was applied to those boards having high tensile strengths and

high expansions with change in moisture content. However, there was but little buckling of the

plaster.

These experiments suggested the idea that

after the insulating lath had been "put in a

plaster of paris cast" of sufficient strength

they could not buckle. Since rather quicksetting plaster had been used the cast had

hardened rapidly enough to prevent much

budding. In order to obtain a comparison of

the flexural strengths of the set plasters with

those of the lath on which they were applied

several attempts were made to separate test specimens of set plaster from the plaster base.

This idea was soon abandoned as it was almost

impossible to remove test specimens without

cracking the plaster.

Consequently, the lath with the adhering

plaster were detached from the frames and

specimens, 3 by 18 inches, were cut crosswise

of the panel. The specimens were then divided

into three groups, one of which was conditioned

at 60-percent relative humidity, one at 80

percent, and the third at 100 percent. The

specimens of the board with the adhering

plaster were then tested for flexural strength on a 12-inch span by applying loads at midspan

following the same procedure used previously

in determining the strength of the boards

without the attached plaster. Three or more

specimens were broken in each case. Table 5

[13]
contains the data relative to the flexural

strength, expressed as the average load in

pounds, thus obtained for the boards with and

without plaster. As indicated in the table, some of the specimens were tested with the

face down (with the plaster in tension), others

with the face up. The flexural strengths of the

different unplastered boards are within a pound of those previously obtained and re- ported in table 1 . Because the fiber insiilating

lath deflected from ji to 1 inch before breaking

and the plastered lath broke with less deflection, the values recorded in column 6 of table 5 (where the load was applied against the backing)

may be considered as a fairly good measure of

the strength of the plaster. It should be noted

that, except for board 6, the flexural strength of

the plastered boards was greater than that of

the unplastered boards. The strengths were

always greater with the backing in tension

(face up) than with the plaster in tension.

Table 5.

Comparison of flexural strengths of plastered

and unplastered insulating lath (Determined according to Federal Specification LL-F-321a for the

insulating fiber boards. Flexural strength made on specimens 3 inches wide and 18 inches long placed on horizontal parallel supports 12 inches

apart applying load at midspan).

Plaster (gyp- sum: sand by

weight)

Scratch

coat Brown

coat

Fiber

insu- Relalating

lath Composition of tive board humid- num- ity ber

Average flexural strength

Fiber board

alone

Plaster and

board

Face downFace up PLASTER H INCH THICK

% lb lb lb 1:2 1:3 1 Wood fiber 60 13.8 25. 0 94

1:2 1:3 1 do 80 11.8 24.0 90

1:2 1:3 1 do 100

60

9.0 65

1:3 1:5 6 Largely of pulp 27.5 80 from waste paper.

1:3 1:5 6 do 80 22.0 20.0

1:3 1:5 6 do 100 14.0 14. 5

1:4 1:6 5 Western wood 60 11.0 16.0

fiber. 1:4 1:6 5 do_ _.. 80 9.0 16. 5 1:4 1:6 5 do 100 6.0 14.0 PLASTER H INCH THICK

1:2

1:2

1:3

1:3

1:3 1:4

1:4

1:4

1:3

1:3

1:3

1:3

1:5 1:5

1:6

1:6

1:6

Exploded wood

fiber. do Balsam wood fiber do

Extracted sugar- cane fiber. do

do Wood fiber. do

do

60

10060

80 100

15.5

14.0 12.0

6. 5 11.0

9.0

6.0

13.8

11.8 9.0

31.0 28.2

24. 4

14.0

13.0

13.3 18.0

11. 2

93.0

91.0

81.0

Even with a plaster that sets within 3 hours

there is bound to be some expansion of the

board. As the board subsequently dries the

set plaster tends to be placed under tension at the joint. Comparing the tensile strength of

the boards (table 1) with those of the plasters

(fig. 13) it can be seen that the plasters containing 4 parts or more of sand to 1 of gypsum are weaker in tension than the boards. These

oversanded plasters (mix 3) are the ones which

fafled even when % of an inch of plaster was

applied to the boards inch in tliickness. It

will be recalled that none of the panels plastered

with % inch of strong plaster developed cracks

along the horizontal joints but that several did

when but % inch of plaster was applied, and

that these cracks occurred on boards having the

higher tensile strengths.

It is obvious from these experiments that the

slower the time of set of the plaster the greater

the expansion of the boards before the plaster

hardens and consequently, the greater the tensile deformation on the plaster at the joints

when the board subsequently dries. As the

time avaflable for the investigation was limited

it was not feasible to conduct extensive experiments on three-coat plastering using plasters

having various times of set. Some experiments were conducted, however, on two-coat plastering where such plastei-s were used and the re- sults of these experiments wifl be described later in this paper.

The panels plastered with the second mixture

(scratch, 1:3 and brown 1:5 by weight) were

intermediate in their performance between those

plastered with the strong plaster (scratch 1:2 and brown 1:3) and those with the badly over- sanded plaster (scratch 1:4 and brown 1:6).

The bond of the scratch coat to the plaster

boards was not as good as with the richer

plaster nor as poor as with the more oversanded

plaster. Although none of the panels plastered

with % inch of the strong plaster (mix 1) cracked, yet several of those with the second

mix did develop cracks. On the other hand, the

cracking of the panels with the latter mixture was not as extensive as with those panels plastered with the badly oversanded plaster (mix 3). Since this investigation is primarily con- cerned with fiber insulating lath as plaster bases

the experiments on gypsum lath were limited.

14
No buckling or cracking was evident in tlie %

inch of plaster applied separately as a scratch coat (1:2) by weight and a brown coat (1:3)

when the plaster set within 3 hours, even with

the use of %-inch gypsum plaster boards, which

were 32 inches wide and 36 inches long.

2. Two-Coat Plastering

It has been shown that with three-coat plastering unevenness of the scratch coat resulting

from buckling can be straightened with the

brown coat. With two-coat plastering, wherein

the brown coat is applied over the scratch coat

by "doubling back" before the scratch coat has

set, this straightening cannot be accomplished.

Furthermore, by the application of two coats of

wet plaster, more water is initially available to bring about buckling than when the scratch coat is allowed to set and harden before the

brown coat is applied. Consequently, it would

appear even more desirable to use a quicksetting plaster with two-coat work than with

three. The investigations with two-coat plastering, therefore, were confined primarily to determining the relation between the extent of

buckling and the time of set of the plaster.

Figure 14.Four panels

shovoing that buckling

across the studding was

insignificant when plaster

which set within 2 hours was used hut with slower

setting plaster the buckling was considerable.

Plaster on panels 1 and 2 set at 2 hours, on

panel 3 at 12 hours, and

on panel 4 at 24 hours.

FiciUKK 15.

Panel showing cracks which developed in

slow-setting sanded plaster along the horizontal joints of

the insulating lath.

In these experiments the boards were at- tached to the frames by method 2. The time

of set of the plaster was then regulated so that

separate batches set at approximately 2, 12,

and 24 hours. Figure 14 illustrates four panels,

the buckling of wliich occurred across the

studding. The first two panels from left to right show that the buckling of two of the lath

covered with plaster (1 plaster:2 sand by

weight) which set in approximately 2 hours was

so slight that there was no visible unevenness

of the surface of the plaster. The third and

fourth panels, wherein the plaster was applied over the same make of lath as that used in the

second panel, show that the buckling increased

as the time of set of the plaster increased, re- sulting in uneven plaster surfaces. Figure 15 shows the cracks which developed

in slow-setting sanded plaster along the horizontal joints of the fiber insulating lath. The

cracks appeared even before the plaster had set and resulted from shear produced by a dift'er- ential buckling in the separate lath. As the

panel dried the cracks widened. In some

instances irregular cracks appeared in the

plaster along the studding. These cracks were formed before the slow-setting plaster had hard-

[15]
Figure 16.

Panel showing horizomal cracks alorig the joints of the insulating lath and irregular compression

cracks along the studding which developed in the slowsetting sanded plaster.

ened and, since the edges of such cracks were

raised, they probably were caused by the compressive forces at the studding. These compression cracks also widened as the panels dried

with an attending shrinkage of the boards.

This type of cracking is illustrated by the vertical cracks in the panel pictured in figure 16,

which also shows the more regular cracks along

the horizontal joints of the boards.

VI. SUMMAKY AND CONCLUSIONS

A study was made of the properties of wall

and plaster boards, particularly in regard to the

conditions necessary for successful use of fiber insulating lath as plaster bases.

Eight types of fiber insulating boards were

investigated, one of plywood, two of gypsum

board, one of cement-asbestos board and one

of compressed fiber board.

The thickness of the eight fiber insulating

boards ranged from 0.47 to 0.54 inches and the

weight from 699 to 1,143 lb/1,000 ft^ when

measured at 65-percent relative humidity.

The flexural strengths of the fiber insulating

boards were measured at relative humidities of

40, 65, 80, and 100 percent and the tensile strengths at 65 percent. The average flexural

strengths at 80- and 100-percent relative humidity were about 20 and 50 percent lower,

respectively, than those at 40-percent relative

humidity.

Studies were made of the rate of expansion

occurring at 70° F when boards were transferred from a conditioning relative humidity of

25 percent to one of 90 percent. From 3 to 4

days were required for the boards to reach a

state of dimensional equilibrium after the

transfer. The resultant expansions ranged from

about 0.07 percent for the gypsum boards and

cement-asbestos board to about 0.47 percent

for some fiber insulating boards, the range for

these latter type being from 0.26 to 0.47 percent.

When the boards were kept at 100-percent

relative humidity or placed in water the expansions were about 25 to 95 percent greater than

those obtained at 90-percent relative humidity.

Boards conditioned in air at 25 percent, then

at 90- or 100-percent relative humidity, showed

a definite shrinkage when again dried at 25-

percent relative humidity.

Painting fiber insulating boards as described,

materially decreased the rate of expansion but

did not eliminate expansion attending an in- crease in relative humidity or by wetting with

water. In many cases the ultimate expansions

and contractions of the painted boards were as great as those of the impainted boards.

Boards attached rigidly along their edges to

rigid steel frames, 4 feet square, gave greater

buckling across studs spaced 16 inches than

when the boards were less firmly attached.

Wlien firmly attached along the outside, the

buckling (rise in inches across a 16-inch span)

of the boards ranged from 0.05 inch for the

gypsum wall board to 0.54 inch for one of the

fiber insidating boards when the relative humidity was increased from 35 to 90 percent.

After drying the boards again at 35-percent

relative humidity, some of the buclde remained. ; Consequently, the buckling resulting from the [ second cycle of change in relative humidity j was greater than that of the first cycle.

Insulating lath were attached to rigid steel frames, 4 feet square, so that the middle course gave a vertical joint over a stud. Methods of

j successful application of plaster on fiber insulatj ing lath were determined by experiments in I which the thickness, sand content, strength, !

[16]
and time of set of plasters were varied independently. It was found that:

(1) Three-coat plastering is to be preferred

to two-coat plastering, especially with slowsetting plaster.

(2) Quick-setting plaster is much better than

slow-setting plaster. With three-coat plastering unevenness of the scratch coat resulting

from any buckling of the lath as the plaster

sets could be straightened with the brown coat. Quick-setting plaster was particularly essential

with two-coat plastering. With slow-setting

plasters the buckling of the lath resulted in uneven surfaces and cracks along the joints even with strong plasters.

(3) Weak, oversanded plasters were very unsatisfactory even when % inch thick.

(4) Strong, quick-setting plasters % inch thick were satisfactory, but when the thickness was

only % inch, cracks developed in some instances

along the horizontal joints of the latli iiftci- the

panels had been dried. It is recommended, therefore, that only

quick-setting, strong plaster be used for plastering over fiber insulating lath and that the thick- ness of the plaster be not less than K inch.

With thi-ee-coat plastering the scratch coat (which should be the heavy coat) should be

composed of 1 part of strong gypsum plaster

to 2 parts of sand, by weight, and the brown

coat should be in the proportions 1:3. With

two-coat plastering the plaster should be 1:2,

by weight.

We acknowledge, with thanks, the assistance

of various members of the staff in this work,

particularly the aid given by Joseph Watstein

in taking many of the measurements.

Washington, February 15, 1938.

[17]
\

J
The National Bureau of Standards was established by act of Congress, approved

March 3, 1901, continuing the duties of the old Ofl&ce of Standard Weights and Measures

of the United States Coast and Geodetic Survey. In addition, new scientific functions were assigned to the new Bureau. Originally under the Treasury Department, the

Bureau was transferred in 1903 to the Department of Commerce and Labor (now the

United States Department of Commerce). It is charged with the development, construction, custody, and maintenance of reference and working standards, and their intercomparison, improvement, and appUcation in science, engineering, industry, and commerce. SUBJECTS OF BUREAU ACTIVITIES

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Clay and Silicate Products Whiteware

Glass

Refractories Enameled Metals

Heavy Clay Products

Cement and Concreting

Materials

Masonry Construction

Lime and Gypsum

Stone

Simplified Practice Wood, Textiles, and Paper

Metal Products and Construction Materials Containers and Miscellaneous

Products

Materials-Handling Equipment and Ceramics

Trade Standards Wood, Wood Products, Paper,

Leather, and Rubber

Metal Products

Textiles Apparel

Petroleum, Chemical and Miscellaneous Products

Codes and Specifications

Safety Codes

Building Codes

Building Practice and Specifications Producer Contacts and Certification Consumer Contacts and Labeling

Office Finance

Personnel Purchase and Stores Property and Transportation

Mail and Files Library

Information

Shops

Instrument Woodworking

Glassblowing

Construction Stores and Tool

Room

Operation of Plant Power Plant

Electrical Piping

Grounds

Construction

Guard

Janitorial
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