United States

Department of

Agriculture

Forest Service

Forest

Products

Laboratory

Research

Paper

FPL 408

Wood-Based

Paneling as

Thermal Barriers
Abstract

Fire penetration tests were conducted to examine how

the thermal protection afforded insulation in fireexposed wall assemblies is influenced by wood-based

panel type and thickness. Small-scale specimens (20-by

20-in.) were fabricated reflecting 1/4- to 3/4-inch

plywood over 1-inch slabs of foam plastics; 5/8-inch

plywood panels with no backup material, over a

3-1/2-inch loose-fill cellulosic insulation, 3-1/2-inch air

gap, 1/2-inch foam plastic, or 3-1/2-inch glass-fiber insulation; and 5/8-inch particleboard, 5/8-inch hardboard

composite, 5/8-inch solid wood, 5/8-inch fire-retardanttreated plywood or 1/2-inch gypsum wallboard over

l-inch slabs of foam plastic. These specimens were

then subjected to the time-temperature fire exposure

given in ANSI/ASTM Standard E 119.

Because this research was conducted only to

characterize material response and not to define the

more complex performance of full-scale assemblies according to all the requirements of ANSI/ASTM Standard

E 119, the information is not directly applicable to

establishing the “finish rating” for a given panel type.

However, the results do increase our understanding of

what panel characteristics influence “finish rating” in

assemblies.

Acknowledgment

The author wishes to acknowledge the contribution of

Ronald Knispel who conducted the fire penetration experiments. The author also thanks Koppers Company

for providing the fire retardant-treated specimens.

Note:

Since completion of the tests reported in this paper,

a small scale horizontal exposure furnace test for

testing thermal barriers over a calcium silicate board

was added to the Uniform Building Code. As a result,

1/2-inch thick gypsum wallboard specimens and 5/8-inch

thick plywood specimens were tested over calcium

silicate board substrates and foam plastic substrates

in the FPL small scale vertical exposure furnace. In

these tests, the differences in the mean values for the

gypsum wallboard and the plywood were not significant. These results will be reported in a report, “Effect

of Calcium Silicate Substrate on Thermal Barrier

Results,” by Robert H. White.
United States

Department of

Agriculture

Forest Service

Forest

Products

Laboratory1

Research

Paper

FPL 408

June 1982

Wood-Based

Paneling as

Thermal Barriers

B y

ROBERT H. WHITE, Forest Products Technologist

Introduction

An interior finish on a wall, floor, or roof assembly provides thermal protection to the rest, of the assembly

when exposed to fire. This thermal protection is usually

utilized to protect the load-bearing elements or to prevent the early involvement of certain combustible

materials in a fire. The amount of protection provided is

sometimes expressed in minutes as the finish rating

(10)

2

 or as the protective membrane performance (1).

Both terms refer to the time at which the surface of the

element being protected reaches an average

temperature rise of 250° F or maximum temperature

rise of 325° F as the assembly is subjected to the fire

exposure specified in ANSI/ASTM E 119 (1). This condition will be called the 250°/325° F temperature criteria

throughout this report.

In recent years, building codes have required foam

plastics to be fully protected from the interior of the

building by a thermal barrier of fire-resistive materials

having a finish rating of not less than 15 minutes. A

1/2-inch Type X gypsum board is the only material

specifically regulated by these model codes for composition and installation that qualifies (8). However,

building codes suggest that standard 1/2-inch gypsum

board can also provide adequate thermal protection to

foam plastic insulation. Because they wish to use a

wood-based interior finish, inquirers have asked the

U.S. Forest Products Laboratory (FPL) to investigate

parameters influencing the thermal barrier performance

of wood.

In response, FPL scientists conducted fire penetration

tests to determine how the thermal protection provided

fire-exposed assemblies varies with type and thickness

of wood-based panels. Two heat transfer analyses were

also used to obtain theoretical estimates of the performance of the wood-based panels. Building code

specific performance requirements for a thermal barrier

require the finish rating to be determined by conditions

specified in the ANSI/ASTM E 119 standard (1) test procedure. Cases not meeting the specific requirements

may be individually accepted based on other diverse

tests approved by the building code official. The acceptability of some of these tests is controversial (3,9).

Determining the protective membrane performance according to ANSI/ASTM E 119 requires testing

100-square foot (ft2) wall assemblies, or 180-ft2 floor and

roof assemblies. Testing of such large-scale

assemblies was not practical for the more limited purposes of this study. Therefore, 2.8-ft2

 specimens were

evaluated only to determine how a panel’s thermal performance is influenced by characteristics of the

material used. For example, no determination was

made as to how long a full-size membrane would stay

in place nor was the performance of joints a factor as

they are in large-scale tests. Because of the differences

in test scale and interpretation, the results in this

1Maintained at Madison, Wis., in cooperation with the University of

Wisconsin.

2

Italicized numbers in parentheses refer to literature cited at the end of

report.
report do not necessarily reflect the finish rating for the

protective membrane of a full-scale assembly.

Research Method

The specimens were tested in the FPL small vertical

furnace (fig. 1) and included panels of different

thickness over 1-inch slabs of foam plastic and 5/8 inch

thick plywood over other types of insulation (table 1).

The specimens were subjected to fire exposure on one

face until the 250 °/325° F temperature criteria of

ANSI/ASTM E 119 were satisfied. Two or three replicates of each type of specimen were tested. For most

of the tests, the specimens were conditioned at 80° F,

30 percent relative humidity (RH), a conditioning that

has traditionally been used for FPL fire tests. The

resulting 6 percent equilibrium moisture content in

wood is the lowest recommended average moisture

content for interior use of wood products in the United

States (12). For most of the country, the recommended

average moisture content for interior use of wood products is 8 percent (12). The ANSI/ASTM E 119 standard

specifies 73° F, 50 percent RH conditioning with an

equilibrium moisture content of 9 percent (1).

Equilibrium moisture content for the 80° F, 65 percent

RH conditioning is 12 percent.

Specimens

The author used three variations of specimen

construction:

(1) Specimens with a panel over a slab of foam plastic

had the 20- by 20-inch panel and the foam plastic

nailed to a wood frame with three upright members

(figs. 1 and 2). (2) Specimens with air gap, glass-fiber, or

cellulosic insulation had no center upright member in

the frame; the frame side not exposed to fire was

covered with 3/8-inch plywood. (3) Specimens with no

insulation had the plywood tested without a wood

frame.

Six thermocouples were located at the fire-exposed,

panel-insulation interface. Five were attached to the

unexposed panel side with 3-inch square pieces of duct

tape. Thermocouples were positioned at the center of

the panel and at the center of each quadrant. The sixth

thermocouple was attached at the center of the fireexposed side of the insulation (or vapor barrier). On the

unexposed side of the assembly, two thermocouples

were placed beneath 3-inch square asbestos pads (fig.

1). All thermocouples were 30-gage iron-constantan.

Materials

The untreated plywood, hardboard, and gypsum

wallboard were obtained from commercial sources as

4- by 8-foot sheets. Starting at one corner, eight 20- by

20-inch specimens were cut from each sheet. Random

numbers were used to select the specimen to be used

in each test. The particleboard and solid wood

specimens were cut from materials left over from

previous studies. The 20- by 20-inch specimens of fireretardant treated (FR) plywood were supplied by a commercial treater.

2

Figure 1.–Test specimens and FPL small

vertical furnace.

(M 147593-9)

Southern pine (SP) plywoods were graded as exterior

BC, species group 1. The 1/4-, 3/8-, 1/2-, 5/8-, and

3/4-inch panels had 3, 3, 4, 5, and 5 plies, respectively,

all which were southern pine (Pinus sp.).

Mixed species (MS) plywoods were graded as exterior

AC, species group 1. The 5/8-inch mixed species

plywood had face plies, two cross plies, and a center

ply of Douglas-fir (Pseudotsuga menziesii), aspen or

cottonwood (Populus sp.), and a western yellow pine

(Pinus sp.), respectively. The 3/4-inch mixed species

plywood had face plies and two cross plies that were

Douglas-fir (Pseudotsuga menziesii) and a center ply of

spruce (Picea sp.).

Particleboard was a commercial floor-underlayment

particleboard made from southern pine (Pinus sp.) and

urea-formaldehyde adhesive.
Table I.–Description of specimens tested in small vertical furnace

Specimen Specimen

Number code1

Type2

Panel

Approximate

thickness Conditioning

Insulation

Type

Approximate

thickness

1 SP1/4- 6-F SP plywood

2 SP3/8- 6-F SP plywood

3 SP1/2- 6-F SP plywood

4 SP5/8- 6-F SP plywood

5 SP3/4- 6-F SP plywood

8

7

8

9

10

11

MS5/8- 6-F

MS3/4- 6-F

PB5/8- 6-F

HB5/8- 6-F

SW5/8- 6-F

FR5/8- 6-F

MS plywood

MS plywood

Particleboard

Hardboard

Solid wood

FR plywood

12 SP3/4- 0-F SP plywood

13 SP3/4- 9-F SP plywood

14 MS3/4- 9-F MS plywood

15 MS5/8- 9-F MS plywood

16 MS5/8- 12-F MS plywood

17 SP5/8- 6-F/2 SP plywood 5/8

18 SP5/8- 6-N SP plywood 5/8

19 SP5/8- 6-A SP plywood 5/8

20 SP5/8- 6-G SP plywood 5/8

21 SP5/8- 6-C SP plywood 5/8

22 GY1/2-30-F Gypsum 1/2

23 GY1/2-65-F Gypsum 1/2

In.

1/4

3/8

1/2

5/8

3/4

5/8

3/4

5/8

5/8

5/8

5/8

3/4

3/4

3/4

5/8

5/8

°F Pct RH

80 30

80 30

80

80

30

80

30

30

80 30

80 30

80 30

80 30

8 0 30

80 30

Foam plastic

Foam plastic

Foam plastic

Foam plastic

Foam plastic

Foam plastic

ovendry Foam plastic

73 50 Foam plastic

73 50 Foam plastic

73 50 Foam plastic

80 85 Foam plastic

80 30

80

80

30

30

80

80 30

30

80

80

30

65

Foam plastic

None

Air gap

Glass-fiber

Cellulosic

Foam plastic

Foam plastic

Foam plastic

Foam plastic

Foam plastic

Foam plastic

Foam plastic

1

1

1

1/2

3-1/2

3-1/2

3-1/2

1

1

1

—

1

1

 Specimen code indicates type and thickness of panel, equilibrium moisture content (relative humidity in the case of

gypsum), and type of insulation.

2

 SP = southern pine, MS = mixed species, FR = fire-retardant treated.

Hardboard was a composite consisting of two 1/4-inch

and one 1/8-inch layers of a commercial tempered hardboard of unknown composition. The layers were glued

together with phenol resorcinol adhesive.

Solid wood specimens were made from 20-inch long

boards cut from 5-inch wide planks of southern pine

(Pinus sp.). The boards were glued together (plain sideto-side grain joints) with phenol-resorcinol adhesive to

make a 20-inch wide specimen. A different plank was

used for each of the three specimens. Ovendry densities of the planks were 42.4, 34.3, and 39.2 Ib/ft3

 for

the first, second, and third specimens tested,

respectively.

Fire-retardant-treated (FR) plywood was graded C-D interior with exterior glue and had been fire-retardant

treated with an Underwriters Laboratories classified

commercial pressure treatment. The density as tested

was about 38 Ib/ft3. The four plies were southern pine

(Pinus sp.).

Gypsum board was a regular gypsum wallboard. The

densities as tested were 43.3 ± 1.5 Ib/ft3 and the actual

thicknesses of the gypsum board specimens were

0.496 ± 0.004 inch.

Glass-fiber insulation had a batt of glass-fiber with

kraft and standard foil facing on the fire-exposed side.

Glass-fiber density was about 0.6 Ib/ft3; the 3.5-inchthick batt had a thermal resistance R-factor of 11°

F·h·ft2/Btu.

Cellulosic insulation was a loose-fill wood fiber derived

from newsprint. The fire-retardant-treated insulation

had an R-factor of 13° F·h·ft2/Btu for the 3.5-inch thickness and settled density of 2.7 Ib/ft3

. A plastic vapor

barrier was installed on the fire-exposed side of the

insulation.

Foam plastic insulation had a glass-reinforced polyisocyanurate foam plastic core with aluminum foil facings.

The 1-inch-thick sheathing had an R-factor of 7.2°

F·h·ft2/Btu and a density of 2 lb/ft3.

Test Procedure

The vertical furnace (fig. 1) has a 20-inch square opening on its side into which the assembly was inserted.

The furnace is equipped with pipe outlets for discharg3
Figure 2.–Specimen with plywood and

plastic foam sheathing nailed to a

wood frame.

(M 147688)

ing natural gas into the furnace. All air for combustion

was admitted by natural draft through vents at the bottom of the furnace with baffling to get proper distribution. Inside the furnace, a single iron-capped thermocouple was located opposite the center of the panel

and 2-inches from the exposed surface of the panel.

This is closer placement than the 6 inches specified in

ANSI/ASTM E 119 (1). The gas supply of the furnace

was regulated by monitoring the iron-capped thermocouple so that the temperature followed the

ANSI/ASTM E 119 time-temperature curve (1).

Temperatures on this curve are 1,000°, 1,300°, and

1,399° F at 5, 10, and 15 minutes, respectively.

Recorders monitored the temperatures indicated by the

various thermocouples during the tests.

Theoretical Analyses

Predictions of times for the 250°/325° F criteria given

in ANSI/ASTM E 119 were made. using two theoretical

procedures. The first procedure was the Aerotherm Corporation Charring Material Thermal Response and Ablation (CMA) computer program (13). The CMA computer

program is an implicit finite-difference computational

procedure for describing the one-dimensional thermal

behavior of a two-sided ovendried slab that can ablate

on one surface and decompose in depth. The surface

temperature and surface recession rate are specified in

the boundary condition option used in this study.

The second procedure was a computer program for the

thermal analysis of the fire endurance of construction

walls (FDCW) (4). The FDCW computer program is a

finite-difference computational procedure for onedimensional heat transfer in which heat transfer occurs

by conduction through solids, and by radiation and convection through air spaces. For each solid layer the program includes phase changes with heat generation or

absorption (e.g. vaporization of water).

The computer programs required the input of a number

of parameters. Selection of values for the input

parameters was based on experimental measurements,

literature data, and previous experience with the computer programs. Details on the input for the CMA computer program are given in a previous publication (1 3).

The property subroutine in FDCW (4) was modified to

account for effect of moisture content on thermal properties at temperatures below 221° F and for char properties at temperatures above 550° F.

Measurements were made to obtain data for thickness,

ovendry wood density, char density, moisture content,

thermal conductivity, and surface recession (table 2) of

the wood-based panels. It is likely that the mass in a

piece of char is from a volume of wood greater than the

volume of the piece of char. Because the CMA program

assumes that the change from wood to char density is

due only to a mass change, a reduction in the

measured char density is necessary. Data for char density were arbitrarily reduced by 50 percent to levels in

agreement with previous experience with the CMA computer program. Data for the different panels of untreated southern pine plywood (specimen Nos. 1-5,

12-13, 17-21) were averaged to obtain the input values

for char density/wood density ratio, thermal conductivity, and surface recession. Single ANSI/ASTM C 518 (2)

thermal conductivity tests were conducted on each of

the different panel materials. For the FDCW analyses,

the heat of absorption was the product of the heat of

vaporization of water 970 Btu/lb, the ovendry wood density, and the fractional moisture content.

Input values for the insulation was based on data in the

literature. The thin foil on the foam plastic insulation or

glass-fiber insulation was not included in the analyses.

For the CMA analyses, the convection heat transfer

coefficient at the unexposed back surface was 0.44

Btu/h·ft2

· °F.

Experimental Results and Discussion

The 250°/325° F temperature increase criteria were

satisfied after 2.6 to 15.8 minutes of ANSI/ASTM E 119

fire exposure (table 3). Results for the different woodbased specimens tested depended on the thickness,

species, density, fire-retardant treatment, and moisture

content of the panel, and on the presence of insulation

behind the panel. The influence of each of these

parameters on the response are discussed in the

following paragraphs.

Panel Thickness

As plywood thickness tested (6 pct moisture content)

was increased, time periods until the temperature

criteria were satisfied increased linearly (fig. 3). Assuming a straight line through the data points (specimen

Nos. 1 to 5), the increase was 20.6 minutes per inch

thickness. The linear correlation coefficient for the data

(fig. 3) was very high (0.995). The average results for the

5/8- and 3/4-inch untreated southern pine plywood

(specimen Nos. 4 and 5) were 10.2 and 12.1 minutes,

respectively.

4
Table 2.–Input parameters of panels for theoretical analysis of assemblies

Specimen

No.

Specimen

code’ Thickness Wood Char Moisture Thermal Surface

density2density3content conductivity4

recession

In. Lbs/ft3Lbs/ft3 Pct Btu. in In./hr

ft2 hr °F

1 SP1/4- 6-F 0.26 33.3 9.6 5.8 0.79 0.73

2 SP3/8- 6-F .36 31.6 9.2 5.6 .79 .73

3 SP1/2- 6-F .49 33.5 9.7 5.6 .79 .73

4 SP5/8- 6-F .62 36.4 10.6 5.2 .79 .73

5 SP3/4- 6-F .73 32.0 9.3 5.4 .79 .73

6 MS5/8- 6-F .61 30.5 8.8 4.4 .62 .43

7

8

MS3/4- 6-F

PB3/8- 6-F

.73

.62

29.3

46.2 12.4

8.5 4.6

6.0

.67

.84

.03

.59

9 HB5/8- 6-F .61 60.0 17.9 4.5 .82 .59

10 SW5/8- 6-F .62 38.6 13.2 6.9 .80 .87

11 FR5/8- 6-F .63 36.0 13.9 6.4 .91 .55

12 SP3/4- 0-F .73 32.2 9.4 0.0 .79 1.20

13 SP3/4- 9-F .74 32.0 5— 7.9 .79 5—

14 MS3/4- 9-F .74 30.0 5— 7.0 .67 5—

15 MS5/8- 9-F .61 29.7 5— 6.0 .62 5—

16 MS5/8-12-F .61 29.2 5— 8.7 .62 5—

17 SP5/8- 6-F12 .61 36.7 10.6 5.6 .79 .73

18 SP5/8- 6-N .62 36.7 10.6 5.2 .79 .73

19 SP5/8- 6-A .61 37.8 10.9 5.0 .79 .73

20 SP5/8- 6-G .61 37.5 10.9 5.0 .79 .73

21 SP5/8- 6-C .61 36.7 10.6 5.2 .79 .73

1

 Specimen code indicates type and thickness of panel, equilibrium moisture content, and type of insulation. (See table 1)

2

 Based on ovendry weight and volume at moisture content of test.

3

 For FDCW analyses, char density was 29 pct of wood density for all specimens.

4

 At temperature of 75° F, temperature correction based on ratio of absolute temperatures.

5

 No CMA analysis performed due to high moisture content.

The 10.0 minutes average result for the 5/8-inch mixed The 2.7 and 10.2 minute average results for the 1/4- and

species plywood (specimen No. 6) was close to the 5/8-inch southern pine plywood (specimen Nos. 1 and 4)

southern pine plywood result (specimen No. 4). This were considerably less than previously reported results

mixed species plywood had thin faces of Douglas-fir (77). These older results were for tests conducted in

but was mostly western yellow pine and aspen/cotton- 1937 in which 1/4- and 5/8-inch phenolic resin, Douglaswood. The average result for the 3/4-inch mixed species fir plywood had times of 6 and 16 minutes, respectively.

plywood (specimen No. 7) was 14.0 minutes or 1.9 In the latest series of tests ignition of the plywood

minutes greater than for the southern pine plywood panel occurred about 0.8 minute after the start of the

(specimen No. 5). A t-test of the data for specimen Nos. test. In the 1937 tests, ignition of the panels occurred

5 and 7 indicated that the difference in the means was about 4.0 minutes after the start of the test. The difsignificant at the 5 percent level. For the specimens ference in the ignition times is probably due to differconditioned at 50 percent RH (specimen Nos. 13 and ences in the testing procedures. In the 1937 tests the

14), the difference of the means for the 3/4-inch furnace was preheated before the beginning of the test.

southern pine plywood (13.8 min) and the 3/4-inch In the latest series of tests, the normal practice of havmixed species plywood (15.6 min) was significant at the ing the furnace at room temperature at the beginning of

10 percent level. The 3/4-inch mixed species plywood the test was followed. Subtraction of the difference in

was Douglas-fir except for a center ply of spruce. Thus, ignition times would reduce the 1937 results to 2.8 and

times for the 250°/325° F criteria may be dependent on 12.8 minutes for the 1/4- and 5/8-inch plywood, respecthe species of the plies. Unfortunately, current species tively. The remaining difference in the results may be

identification of plywood is based only on the face due to the fact that the 1937 plywood specimens were

plies and only the species group is noted in the grade fabricated in the laboratory with only Douglas-fir plies.

marks. All of the plywood tested were graded as Also, the moisture content of the 1937 specimens was

species group 1. not recorded.

5
Table 3.–Time until 250°/325° F temperature criteria were satisfied on exposed side of the panel1

Specimen Specimen

No. code2

Test No. Standard Coefficient

Mean deviation of

1 2 3 variation

--------------------------Min-------------------------- Pct

1 SP1/4- 6-F 2.7 2.7 2.6 2.67 0.06 2.2 2

3

SP3/8- 6-F

SP1/2- 6-F

4.4

7.1

4.4

6.8

4.4

6.7

4.38

6.87

.01

.21

0.2

3.0

4 SP5/8- 6-F 10.1 10.0 10.4 10.17 .21 2.0

5 SP3/4- 6-F 12.0 11.8 12.5 12.10 .36 3.0

6 MS5/8- 6-F 10.0 9.6 10.3 9.97 .35 3.5

7

8

MS3/4- 6-F

PB5/8- 6-F

14.1

14.0

13.6

13.3

14.3

13.9

14.00

13.73

.36

.38

2.6

2.8

9 HB5/8- 6-F 14.2 13.7 14.3 14.07 .32 2.3

10 SW5/8- 6-F 11.3 10.5 11.6 11.13 .57 5.1

11 FR5/8- 6-F 11.3 11.0 — 11.15 .21 1.9

12 SP3/4- 0-F 8.8 8.5 — 8.65 .21 2.4

13 SP3/4- 9-F 14.0 13.7 — 13.85 .21 1.5

14 MS3/4- 9-F 15.8 15.4 — 15.60 .28 1.8

15 MS5/8- 9-F 10.6 10.5 — 10.55 .07 0.7

16 MS5/8-12-F 12.3 12.6 — 12.45 .21 1.7

17 SP5/8- 6-F12 11.0 10.4 10.6 10.67 .31 2.9

18 SP5/8- 6-N 11.2 11.6 11.7 11.50 .26 2.3

19 SP5/8- 6-A 10.4 10.0 10.9 10.43 .45 4.3

20 SP5/8- 6-G 9.6 9.5 10.0 9.70 .26 2.7

21 SP5/8- 6-C 10.1 10.4 10.7 10.40 .30 2.9

22 GY1/2-30-F 12.8 11.6 14.4 12.93 1.40 10.8

23 GY1/2-65-F 14.8 15.4 15.10 .42 2.8

1

The 250° F plus initial limit on average temperature was the criterion except for tests 1 and 2 of specimen Nos. 22 and

23 (gypsum). For all the tests the difference between the times for the two criteria was 1.3 minutes or less.

2

 Specimen code indicates type and thickness of panel, equilibrium moisture content, or relative humidity (gypsum), and

type and thickness of insulation. (See table 1)

The 6.9 minute average result for the 1/2-inch southern

pine plywood is also less than published results for

1/2-inch Douglas-fir plywood. In similar tests using an

electric furnace heated with silicon carbide elements, it

took 11.5, 10.5, and 12.0 minutes for a polystyrene foam

and 10.5, 10.0, and 11.0 minutes for a polyurethane

foam, respectively, to reach an average temperature

rise of 250° F at the interface of plastic foam and

1/2-inch Douglas-fir plywood protection (6). During these

tests, flames were observed in the furnace chamber

about 4 minutes after start when the conditioning prior

to testing was 70° F and 50 percent RH. Again, time

differences appear due to differences in testing procedures (e.g., electric heating elements versus natural

gas) and the difference in conditioning of the plywood.

Panel Density

Results for the 5/8-inch plywood, particleboard, hardboard, and solid wood (specimen Nos. 4, 6, 8, 9; and 10)

involved a range of densities. Based on a linear regression (fig. 4), the data indicate an increase of 0.156

minute to the times for the 250°/325° F criteria per unit

(Ib/ft3) increase in the density. The linear correlation

coefficient for the regression again was high (0.90).

Results obtained for the hardboard composite are probably low for its density level since the char tended to

fall away at the gluelines of the layers.

In previous tests of one-ply specimens of four types of

1/2-inch structural flakeboards (5), the times for the

250°/325° F temperature rise ranged from 10.2 to 13.0

minutes. The average result for the 1/2-inch southern

pine plywood (specimen No. 3) was 6.9 minutes. In the

flakeboards tests there was no insulation behind the

panel but the thermocouples were under asbestos

pads. The flakeboards had densities of 40 to 46 Ib/ft3

and the flakes were Douglas-fir, aspen, or a mixture of

western species.

Fire-Retardant Treatment

The average result for the C-D FR plywood (specimen

No. 11) was 1 minute greater than the result for the untreated B-C plywood (specimen No. 4). A t-test comparison of the data for these specimens indicated that

the difference in the means was significant at the 5

percent level. Differences between the two plywoods

unrelated to the fire-retardant treatment, such as the

grade of the plywood, may have affected the results.

These results showing a slight improvement for the FR

6
Figure 3.–Thicker panels of southern pine

plywood required longer times to

achieve the 250°/325° F criteria.

(M 149891)

Figure 4.–Denser 5/8-inch panels required

more time to reach the 250°/325° F

criteria.

(M 149892)

plywood are consistent with previous test data (7 ). In

1937, five 24- by 24-inch specimens of 3/8-inch three-ply

plywood were tested in a small vertical furnace. The

plywoods were a composite of untreated plies and plies

impregnated by a hot and cold method using a solution

of ammonium sulfate and monoammonium phosphate.

The 250° F temperature rise results ranged from 8.0 to

9.0 minutes, depending upon which of the plies were

treated.

Panel Moisture Content

The 5/8-inch mixed species plywood (specimen Nos. 6,

15, and 16), the 3/4-inch southern pine plywood

(specimen Nos. 5, 12, and 13) and the 3/4-inch mixed

species plywood (specimen Nos. 7 and 14) were tested

after conditioning to a range of moisture contents.

Assuming a linear relationship (fig. 5), the data indicate

that increasing the percentage moisture content by 1

increases the times of the 250°/325° F criteria by an

average 0.63 minute. Linear correlation coefficients of

the regressions were 0.95 or greater. The moisture contents (ovendry method) of the plywoods were lower than

the equilibrium moisture contents normally obtained

with wood at the different conditionings.

Insulation

While the presence of insulation behind the panel had

an effect on the times needed to achieve the 250°/325°

F criteria, there is a considerable overlapping of the 95

percent confidence intervals (table 4). The average 11.5

minute result for the 5/8-inch southern pine plywood

without any insulation (specimen No. 18) was significantly greater (5 pct level) than results for the plywood

with insulation of 9.7 to 10.7 minutes (specimen Nos. 4,

17, 19, 20, and 22). The 11.5 minutes result (specimen

No. 18) is based on thermocouples attached to the unexposed side of the panel with duct tape. In these

tests, four additional thermocouples were under

asbestos pads on the unexposed side. The average

250°/325° F result for these thermocouples was 10.2

Figure 5.–An increase in the moisture

increased the times for three

plywood panels to reach the

250°/325° F criteria.

(M 149893)

7
Table 4.–Confidence intervals (95 pct) for the test of 5/8-inch southern pine plywood over different types of insulation.

(Based on three tests of each specimen)

Insulation

type

Specimen Mean

No. (Min)

95 Percent confidence

interval (Min)

9 10 11 12 13

None 18 11.5

1/2-in. Foam Plastic 17 10.7

3-1/2-in. Air Gap 19 10.4

3-1/2-in. Cellulosic 21 10.4

1/2-in. Foam Plastic 4 10.2

3-1/2-in. Glass-Fiber 20 9.7

minutes, significantly different (5 pct level) than the

result for the thermocouples under the duct tape and in

general agreement with the results for the panels with

insulation. The use of asbestos pads on the unexposed

side of test specimens is specified in ANSI/ASTM E-119

(1). These tests illustrate the influence of such pads on

heat accumulation at the surface of the panels.

A t-test comparison among the specimens with insulation indicates that the mean result for the glass-fiber

(specimen No. 20) is significantly less than the mean

result for the 1/2-inch foam plastic (specimen No. 17) at

the 5 percent level. Differences between the means for

the 1-inch foam plastic, 1/2-inch foam plastic, air gap,

and cellulosic specimens (specimen Nos. 4, 17, 19, and

21) were not significant at the 5 percent level. Correlations of the results with insulation values (R-factors)

were inconclusive.

In the cellulosic insulation tests, water vapor from the

plywood may have been trapped by the plastic vapor

barrier. The presence of a gap between the vapor barrier and the plywood was indicated by the large difference in times recorded for the thermocouple on the

cool side of the plywood and times recorded for the

thermocouple on the hot side of the insulation. This difference for the cellulosic tests was 0.5 to 1.8 minutes

versus 0.6 minute or less for the other specimens. The

gap may have increased the 250°/325° F times for the

cellulosic insulation-backed specimens.

Gypsum Wallboard

Average results for the 1/2-inch regular gypsum board

were 12.9 minutes for those conditioned at 80° F and

30 percent RH (specimen No. 22) and 15.1 minutes for

those conditioned at 80° F and 65 percent RH

(specimen No. 23). The gypsum board normally failed

due to the transfer of heat through cracks that

developed in the panels during the fire exposure. The

difference of 2.2 minutes in the means for the two conditionings was not significant at the 10 percent level.

8

Statistical comparison of times for the 250°/325° F

criteria and densities of the panels as tested indicated

that the variation in times was mainly due to differences in the densities. Differences in the

thicknesses were minimal and not factors in the

results. Variation in the total density could have been

due to differences in the dry mass, the chemically combined water, and the absorbed water components of

the density.

Due to the higher variability of the regular gypsum

board (conditioned at 80° F, 30 pct RH), the results for

a number of the wood-based panels (conditioned at 27°

C, 30 pct RH) were within the 95 percent confidence interval of the gypsum board (table 5). In t-test comparisons of all 5/8- and 3/4-inch wood-based panel

results (specimen Nos. 4 to 11) with the gypsum board

results (specimen No. 22) the differences in the means

were not significant at the 5 percent level. However, the

means for both of the 5/8-inch untreated plywoods

(specimen Nos. 4 or 6) were significantly less, by about

3.0 minutes, than that for the gypsum board at the 10

percent confidence level. In a similar fashion, a difference of 2.7 minutes was significant at the 10 percent

level for the 5/8-inch mixed species plywood (specimen

No. 16) and the gypsum board (specimen No. 23) conditioned at 80° F and 65 percent RH.

In similar tests using an electric furnace heated with

silicon carbide elements, it took 16.0, 15.0, and 16.5

minutes for polystyrene foam and 14.5, 15.0, and 14.5

minutes for polyurethane foam, respectively, to reach

an average temperature rise of 250° F at the interface

of plastic foam and 1/2-inch regular gypsum board protection (6). Conditioning before tests was 70° F and 50

percent RH.

Theoretical Prediction Results and Discussion

The theoretical predictions were substantially different

than the experimental results (table 6). Except for the
Table 5.–Comparison of 95 percent confidence intervals for the 5/8-inch and 3/4 inch wood-based panels with the interval

for the 1/2-inch regular gypsum board (80° F, 30 pct RH conditioning). (Based on two tests of specimen No. 11,

and three tests of rest of specimens.)

Panel type Specimen Mean

No. (Min)

95 percent confidence

interval (Min)

9 11 13 15 17

5/8-in. Hardboard Composite 9 14.1

3/4-in. MS Plywood 7 14.0

5/8-in. Particleboard 8 13.7

1/2-in. Gypsum Board 22 12.9

3/4-in. SP Plywood 5 12.1

5/8-in. FR Plywood 11 11.2

5/8-in. Solid Wood 10 11.1

5/8-in. SP Plywood 4 10.2

5/8-in. MS Plywood 6 10.0

Table 8.–Predictions of the times to reach the 250°/325° F temperature criteria for the wood-based panels

Specimen

No.

Specimen

code1

Average

experimental

result

CMA2

prediction Error FDCW3

prediction Error

Min Min Pct Min Pct

1 SP1/4- 6-F 2.67 2.95 + 10.5 5.14 + 92.5

2 SP3/8- 6-F 4.38 3.95 - 9.8 6.97 + 59.1

3 SP1/2- 6-F 6.87 5.50 - 19.9 9.92 + 44.4

4 SP5/8- 6-F 10.17 7.70 - 24.3 13.37 + 31.5

5 SP3/4- 6-F 12.10 8.81 - 27.2 15.28 + 26.3

6 MS5/8- 6-F 9.97 7.66 - 23.2 12.77 + 28.1

7 MS3/4- 6-F 14.00 9.08 - 35.1 15.11 + 7.9

8 PB5/8- 6-F 13.73 7.95 - 42.1 16.66 + 21.3

9 HB5/8- 6-F 14.07 10.32 - 26.6 18.54 + 31.8

10 SW5/8- 6-F 11.13 8.47 - 23.9 15.40 + 38.4

11 FR5/8- 6-F 11.15 8.04 - 27.9 14.61 + 31.0

12 SP3/4- 0-F 8.65 8.61 -0.5 11.59 + 34.0

13 SP3/4- 9-F 13.85 4

8.81 -36.4 18.20 + 31.4

14 MS3/4- 9-F 15.60 5

9.08 -41.8 18.31 + 17.4

15 MS5/8- 9-F 10.55 6

7.66 -27.4 13.69 + 29.8

16 MS5/8-12-F 12.45 6

7.66 -38.5 16.01 + 28.6

17 SP5/8- 6-F12 10.67 7.48 - 29.9 14.48 + 35.7

18 SP5/8- 6-N 11.50 8.82 - 23.3 16.36 + 42.3

19 SP5/8- 6-A 10.43 7.91 - 24.2 16.16 + 54.9

20 SP5/8- 6-G 9.70 7.54 - 22.2 14.99 + 54.5

21 SP5/8- 6-C 10.40 7.62 - 26.7 16.96 + 63.1

1 Specimen code indicates type and thickness of panel, equilibrium moisture content, and type and thickness of insulation

2 Aerotherm Corporation Charring Material Thermal Response and Ablation computer program (73).

3 Computer program for the thermal analysis of the fire endurance of construction wails (4).

4

 Same as specimen No. 5.

5

 Same as specimen No. 7.

6

 Same as specimen No. 6.

9
1/4-inch plywood, the CMA predictions were 0.5 to 42

percent lower than the experimental results and the

FDCW predictions were 8 to 63 percent higher than the

experimental results. For the 1/4-inch plywood

(specimen No. 1), both procedures overestimated the

resistance times.

Linear regression was used to evaluate the variation of

the results listed in table 4 with the changes in the

three parameters of thickness, density, and moisture

content (table 7). For a unit increase in either

thickness, density, or moisture content of the plywood,

the increase in times for the 250°/325° F criteria based

on the theoretical predictions were in general agreement with the experimental results. Neither the CMA or

FDCW results indicated the 1.9 minute difference between the two 3/4-inch plywoods (specimen Nos. 5 and

7). The rankings of the 5/8-inch plywood specimens with

different insulation types (specimen Nos. 4, 17- 21) by

the experimental results and the two sets of predicted

results were inconsistent.

CMA predictions were low primarily because the CMA

procedure does not consider the influence of phase

change or transfer of moisture in the material. During

the tests, the temperature rise in the panels was

delayed due to the heat absorption associated with

moisture vaporization. For the ovendry plywood

(specimen No. 12), the CMA prediction was within 1 percent of the average experimental result.

The FDCW predictions were high probably because the

FDCW procedure does not consider the recession of

the char at the surface. In the case of the 3/4-inch

southern pine plywood (specimen No. 5), the

thicknesses of the panels after about 13 minutes were

25 percent less than the initial thickness. The error for

the FDCW prediction was 26 percent for specimen No.

5. Very little surface recession was recorded in the

tests of the 3/4-inch mixed species plywood (specimen

No. 7). The error for the FDCW prediction for specimen

No. 7 was an acceptable 8 percent. Heat generation/absorption associated with thermal decomposition may

also be responsible for some of the FDCW error.

The results for both procedures are dependent upon the

values used to define the boundary conditions and the

material properties. The FDCW predictions probably

would have been different if the published property subroutine (4) had been used. In using these programs to

predict the behavior of thermal barriers, there is likely

to be a range of reasonable values for the input

parameters.

Summary

The thermal barrier performance of a wood-based

paneling will be dependent upon a number of panel

material characteristics. Increasing density, moisture

content, and thickness of a panel were found to

significantly increase the times to achieve the

250°/325° F rise in the temperature on the unexposed

face of the panel. The change was linear for the panel

properties range used. The species of the wood probably had an effect as well on the resistance times. A

slight increase in the times was observed in the tests

of fire-retardant-treated plywood when compared to unTable 7.–Variation of the times to reach the 250°/315° F temperature criteria with changes in three parameters1

Increase in times until 250°/325° F

temperature criteria per unit increase

Parameter Units Specimen

Nos.

in parameter

Experimental CMA2 FDCW3

results predictions predictions

------------------------------- Min -------------------------------

Thickness in 1-5 20.5 13.0

Density lbs/ft3

4,6,8-10 .16 .08

Moisture content Pct 4

5,12,13 .655 NA7

5

7,14 .667 NA7

6

6,15,16 .590 NA7

1

 Based on linear regression of results listed in table 4, and data in table 2.

2

 Aerotherm Corporation Charring Material Termal Response and Ablation computer program (13).

3

 Computer program for the thermal analysis of the fire endurance of construction walls (4).

4

 3/4-inch southern pine plywood.

5

 3/4-inch mixed species plywood.

6

 5/8-inch mixed species plywood.

7

 Not applicable.

22.4

.19

.812

1.333

.764

10
treated material. The presence of insulation behind the

panel reduced the times to achieve a 250°/325° F rise

in temperature.

Since the fire resistance of some materials is very

dependent upon the conditioning (temperature, relative

humidity) of the material, the evaluation of materials

depends upon what conditioning is employed. The

response of a majority of panels was evaluated after

80° F, 30 percent RH conditioning. This is a level in

wood base panels more conservative (a lower moisture

content) than may be expected in many regions of the

United States.

Due to the greater variability of the regular gypsum

board results, the 95 percent confidence intervals derived from three tests of each of the wood-based panels

were mostly or completely within the confidence interval of the 1/2-inch regular gypsum board. The performances of the 5/8-inch hardboard composite, the

3/4-inch mixed species plywood, the 5/8-inch particleboard, and the 3/4-inch southern pine plywood were

comparable to the performance of the 1/2-inch regular

gypsum board. The performances of the two 5/8-inch

untreated plywoods were less than the 1/2-inch gypsum

board. The performances of the 5/8-inch FR plywood

and 5/8-inch solid wood specimens were between the

1/2-inch gypsum board and the untreated 5/8-inch

plywoods.

Continued research is needed to improve methods

available to predict analytically the thermal barrier performance of wood-based products. Inability to accurately predict response with the procedures used was

due to inadequacies in the theoretical models and an

insufficient data base for proper selection of input

material properties.

Literature Cited

1. American Society for Testing and Materials. Standard methods of fire tests of building construction and materials. ANSI/ASTM E 119- 79. ASTM,

Philadelphia, Pa.; 1979.

2. American Society for Testing and Materials. Standard test method for steady-state thermal

transmission properties by means of the heat

flow meter. ANSI/ASTM D 518- 76. ASTM, Philadelphia, Pa.; 1976.

3. Anonymous. Using foam plastic insulation safetya rebuttal. Fire J. 73(5):62- 66; 1979.

4. Fung, Francis C. W. A computer program for the

‘thermal analysis of the fire endurance of construction walls. NBSIR 77- 1260. Natl. Bur. of

Standards, Washington, DC.; 1977.

5. Holmes, C. A.; Eickner, H.W.; Brenden, J. J.; White,

R. H. Fire performance of structural flakeboard

from forest residues. USDA For. Serv. Res. Pap.

FPL 315, For. Prod. Lab., Madison, Wis.; 1979.

6. Lie, T. T. Contributions of protected plastic foams

to fire growth. Fire Study No. 37. DW. Bldg. Res.,

Natl. Res. Council of Canada, Ottawa; 1975.

7. McNaughton, G. C.; Harrison, C.A. Fire resistance

studies on plywood wall panels filled with various

insulation materials. USDA For. Serv. For. Prod.

Lab. unpubl. rep., Madison, Wis.; 1938.

8. Nosse, John H. Enforcement of foam plastic regulations. Special Combined Issue Building Safety

Mar./Apr. p. 14- 20. (Special combined issue of

The Building Official and Code Administrator,

Building Standards, and Southern Building); 1979.

9. Stahl, John A. Using foam plastic insulation safely.

Fire J. 72(5):43- 47, 118; 1978.

10. Underwriters Laboratories, Inc. Fire Resistance Directory. Northbrook, III.; 1980.

11. U.S. Forest Products Laboratory. Fire resistance

tests of plywood-covered wall panels. FPL Rep.

No. 1257, Madison, Wis.; 1961.

12. U.S. Forest Products Laboratory. Wood handbook:

Wood as an engineering material. USDA Agric.

Handb. No. 72, Madison, Wis.; 1974.

13. White, Robert H.; Schaffer, E. L. Application of CMA

program to wood charring. Fire Technol.

14(4):279-290, 296; 1978.

2.0-12-6/82

11
U.S. Forest Products Laboratory

Wood-Based Paneling as Thermal Barriers,

by Robert H. White, Res. Pap. FPL 408, FPL,

For. Serv., USDA. 12 p. Madison, Wis.

Fire penetration tests were conducted to

examine how the thermal protection afforded

insulation in fire-exposed wall assemblies is

influenced by wood-based panel type and thickness. This research was conducted only to

characterize material response and not to

define the more complex performance of fullscale assemblies, but the results do increase

our understanding of what panel characteristics

influence "finish rating" in assemblies.
U.S. GOVERNMENT PRINTING OFFICE: 1982-554-030/ 2