This report has been prepares

for-mf-efmation and record

-purposes, and is not to be referenced

in any publication;

NATIONAL BUREAU OF STANDARDS REPORT

9455

FIRE PERFORMANCE CHARACTERISTICS OF EXTERIOR WALLS

by

D. Gross

to

Federal Housing Administration

U. S. Department of Housing and Urban Development

Washington. D. C.

U.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
THE NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards is a principal focal point in the Federal Government for assur- ing maximum application of the physical and engineering sciences to the advancement of technology

in industry and commerce. Its responsibilities include development and maintenance of the national

standards of measurement, and the provisions of means for making measurements consistent with

those standards; determination of physical constants and properties of materials; development of methods for testing materials, mechanisms, and structures, and making such tests as may be necessary, particularly for government agencies; cooperation in the establishment of standard practices

for incorporation in codes and specifications; advisory service to government agencies on scientific and technical problems; invention and development of devices to serve special needs of the Govern- ment; assistance to industry, business, and consumers in the development and acceptance of commercial standards and simplified trade practice recommendations; administration of programs in cooperation with United States business groups and standards organizations for the development

of international standards of practice; and maintenance of a clearinghouse for the collection and

dissemination of scientific, technical, and engineering information. The scope of the Bureau’s

activities is suggested in the following listing of its three Institutes and their organizational units. Institute for Basic Standards. Applied Mathematics. Electricity. Metrology. Mechanics. Heat. Atomic Physics. Physical Chemistry. Laboratory Astrophysics.* Radiation Physics. Radio Standards

Laboratory:* Radio Standards Physics; Radio Standards Engineering. Office of Standard Reference

Data. Institute for Materials Research. Analytical Chemistry. Polymers. Metallurgy. Inorganic Materials. Reactor Radiations. Cryogenics.* Materials Evaluation Laboratory. Office of Standard Refer- ence Materials. Institute for Applied Technology. Building Research. Information Technology. Performance

Test Development. Electronic Instrumentation. Textile and Apparel Technology Center. Technical

Analysis. Office of Weights and Measures. Office of Engineering Standards. Office of Invention and

Innovation. Office of Technical Resources. Clearinghouse for Federal Scientific and Technical

Information.**

‘Located at Boulder, Colorado, 80301. “Located at 5285 Port Royal Road, Springfield, Virginia, 22171.
NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT

421 2502 January 18, 1967

NBS REPORT

9455

FIRE PERFORMANCE CHARACTERISTICS OF EXTERIOR WALLS

by

D. Gross

to

Federal Housing Administration

U. S. Department of Housing and Urban Development

Washington, D. C.

NATIONAL BUREAU OF ST

for use within the Government, and review. For this reason, th whole or in part, is not autho Bureau of Standards, Washingti

the Report has been specifically

IMPORTANT NOTICE Approved for public release by the

Director of the National Institute of

Standards and Technology (NIST) on October 9, 2015

ss accounting documents intended

subjected to additional evaluation

listing of this Report, either in

e Office of the Director, National

jy the Government agency for wtoch

opies tor its own use. <Sb|>

U.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
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FIRE PERFORMANCE CHARACTERISTICS OF EXTERIOR WALLS

by

D. GROSS

ABSTRACT

Test results are presented of surface flammability, smoke accumulation, and the penetration of

flame on both surfaces of typical sections of selected

exterior wall assemblies. The test methods used

are described, and properties are suggested which

could serve as performance criteria for fire hazard

evaluation of exterior walls.

1. Introduction

The overall objectives of this project are to adapt or develop test

methods and techniques for the measurement of the performance of

exterior walls, and to determine these properties for a selected group

of representative walls. From the standpoint of fire, the objectives

were to consider those characteristics of exterior walls which are of

importance to life safety, and to compare the performance of walls of

various types as a means for assisting the sponsor in establishing

levels of acceptability.

One and two-family residences have traditionally been considered less

of a fire problem compared to apartments or office buildings, principally

because rapid escape by a relatively small number of occupants is generally possible. Thus the provisions of the NFFA Building Exits Code pertaining to interior finishes in one and two-family dwellings are seldom

applied or enforced. Nevertheless, nearly one-half the fire deaths

(which number approximately 12,000 annually) occur in one and two- family

dwellings, and, of the more than 500,000 fires in such dwellings annually,

there are nearly 200 multiple-death fires [1]

Fire authorities consider asphyxiation due to the toxic decomposition

products in smoke to be the most serious source of fire fatalities. This

is followed, in order, by rapidly-developing fires which cause panic and

prevent rescue or escape, and by combustible interior finish which con- tributes to the rapidity of fire spread. While it is acknowledged that

interior finishes in residences are not the only or major fire hazard, it

is clearly recognized as a known contributor to fire deaths, and one which can be controlled.

1
• ..

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Of the characteristics of walls which are directly related to life

safety, the following should be considered most important:

oxic products of combustion

Smoke (visual obscuration) , Surface flammability

Flame penetration

Heat of combustion

While it is not possible to relate any of these characteristics

directly and quantitatively to a life hazard under all use conditions,

comparative test methods now exist or can be developed or adapted for

the measurement of all of the characteristics listed except the first

one. Considerably more laboratory work seems to be necessary before the

measurement of the type and quantity of toxic combustion products can be

determined with ease and precision.

It should be noted that, from the standpoint of fire safety, it is

primarily the interior surfaces of a dwelling which are important,

whether these be part of exterior wall assemblies, or of partitions,

ceilings, floors or doors „ However, in some cases, the fire properties

of the outside surfaces of exterior walls, for example smoke production,

rapidity of flame spread, the ease of flame penetration, and internal

fire propagation, may also be important.

In this study, measurements were made of surface flammability, smoke

accumulation, and the penetration of flame on both interior and exterior

surfaces of selected exterior wall assemblies. Time and monetary limitations did not permit the measurement of toxic combustion products or of

the heat of combustion. Studies to evaluate the spread of fire externally through windows, to adjacent structures by firebrands or radiation,

or internally within the wall structure core, were also beyond the scope

of this investigation.

2. Test Methods

2 . 1 Surface Flammability

These measurements were made using the radiant panel flame-spread

test apparatus (see Fig. 1). The detailed test procedure is outlined

elsewhere [2, 3]. In brief, the test requires a 6 by 18 in. specimen,

facing and inclined 30 degrees to a vertically-mounted, gas-fired radiant

panel. The energy output of the panel was controlled to be the same as that from a blackbody of the same dimensions operating at a temperature

of 670 9C (1238°F). Ignition was initiated at the upper edge of the test

specimen and observations were made of the progress of the flame front

down the specimen surface, as well as the temperature rise of the thermocouples in a stack supported above the test specimen. The test duration

was 15 min. , or until sustained flame propagated down the entire 18 in.

length of specimen, whichever time was less. The flame-spread index, I ,

2

was computed as the product of the flamespread factor F

evolution Q, or I = F Q, where

s s

F = 1 +

s t — t

6 3 +

t_-t,

9 6

+

t 12" t 9

+

--t 15- t

12

and the heat

The symbols t 0 to t.

; ^ correspond to times in minutes from specimen

exposure until arrival of the flame front at a position 3 to 15 in.

respectively, along the length of the specimen. The heat evolution Q

is proportional to the observed maximum temperature rise of the stack

thermocouples

All specimens were conditioned by pre-drying at 140®F and then

permitted to reach equilibrium with an ambient maintained at 73+5 °F

and 50+5 percent relative humidity.

2 . 2 Smoke Accumulation

The smoke produced during the burning of the test specimens was collected and measured photometrically, employing a laboratory test

method developed for the purpose [4]. As shown in Fig. 2, the test

utilizes a closed chamber of 18 cu. ft. volume containing an electricallyheated furnace which provides an irradianace of 2.5 w per sq cm (2.2 Btu/

sec sq ft) on the surface of a. nominal 3-in. square specimen. The method

assumes the applicability of Bouguer's law to the attenuation of light

by smoke, and smoke quantity is therefore reported in terms of optical

density rather than light absorptance. Optical density is the single

measurement most characteristic of a "quantity of smoke" with regard

to visual obscuration. To take into account the optical path length L,

the volume of the chamber V, and the specimen surface area producing

smoke A, a specific optical density is defined as D = V/LA (log 100/T),

where T is the percent light transmittance. Thus, for a selected expo- sure in the test chamber, and within certain limitations, a single test

permits rough extrapolation to surface areas and to chamber volumes of

other size.

2 . 3 Flame Penetration

Each test specimen measuring 36 in. square was subjected to heat

and flame impingement from a vertically oriented gas burner centrally

located below the horizontally-mounted specimen (see Fig. 3). The test

apparatus is fully described in Paragraph 4.3.3 "Flame-resistance tests"

of Federal Specification SS-A-118 b, "Acoustical Units, Prefabricated."

The flame was controlled to duplicate the prescribed time- temperature

exposure in standard fire endurance tests [5] (10Q0 eF at 5 min.,

1300°F at 10 min., 1550°F at 30 min.), as indicated by a thermocouple

located 1 in. below the lower surface of the specimen. The available

equipment was utilized as a convenient and economical means for evaluating resistance to flame penetration from a fairly severe but localized

flame. It is not to be considered as equivalent or comparable to the

standard fire endurance test in which the exposing fire is applied to an entire wall surface measuring not less than 100 sq £e in area.

- 3 -

Thermocouples were placed at several locations within the wall

structure and on the unexposed upper surface to indicate the progressive

temperature increase. The unexposed surface thermocouple was placed

centrally and covered with an asbestos pad conforming to the specif ications prescribed in the standard fire endurance test [5]. Visual observations were also made such as the time of occurrence and extent of

flaming in the core.

Separate tests were made on the exterior and interior surfaces of

each wall. In addition, alternate specimens were arranged so that

where possible the effects of flame impingement on an exterior joint

could be compared to that of the unjointed wall section, and the heat

penetration between studs could be compared to that through the studs.

3. Test Results

3 . 1 Surface Flammability

Individual and average flame spread index values are listed in

Table 1.

The average flame spread index values ranged from 0 to 155 for the

exterior wall surfaces. Flames spread to approximately 13 in. on the

painted wood siding, avg I = 49, and along the full 18 in. length on the

unpainted cedar plywood, S avg I = 155. The latter value, which is

typical for uncoated plywood, would be reduced significantly by the application of an adequate coating of a conventional paint, such as of the

alkyd of latex types [6]. The stressed skin plywood sandwich panel with

oil-base exterior paint had an average flame spread index, of 107. Flames

did not propagate along the aluminum lap siding or aluminum sandwich

panel surfaces, for which the flame spread index values were zero.

For the interior wall surfaces, the average flame spread index

values ranged from 0 to 105. There was only a slow, limited spread of

flame along the painted gypsum board (I =8), which formed the interior

surface of three wall systems. Flames propagated to 15 in. on the plywood sandwich panel, which received two coats of latex base paint, and

the average flame spread index was 105.

It was also found that the reverse or cavity sides of two exterior

wall layers propagated flame3 more rapidly when tested as exposed sur- faces. The measured flame spread index value for the fiberboard sheathing was 242 and that of the building pap®r /polyurethane foam/aluminum

lap siding combination was 568. The possible rapid flame propagation

within the core or cavity of a wall system needs to be considered, especially when the surface layers can be quickly melted or burned through from

a severe localized flame source.

- 4 -

3 . 2 Smoke Accumulation

Smoke accumulation tests were performed under both flaming and

non-flaming (smoldering) conditions and the results are presented in

Table 2, in terms of three measured values:

(1) total maximum smoke accumulation. Dm

(2) maximum rate of smoke accumulation over a 2 min. period , R

(3) the time period to reach a "critical' 1 specific optical

density of 16 under the test condition.

If it is assumed that a surface area of 10 sq ft is producing smoke

which distributes itself uniformly within a closed room of 2000 cu ft

volume, and that a light transmittance of 16 percent over a viewing

distance of 10 feet would represent a critical obscuration of vision,

then this smoke level would correspond to a specific optical density

of 16. Provided the generation, agglomeration and other characteristics

of the smoke are similar, then the laboratory chamber may be used to

measure the time period to reach this "critical" specific optical density

under the given test conditions.

From Table 2, it can be seen that, under flaming exposure, smoke from

the painted wood drop siding attained a maximum specific optical density

level, 130, higher than from any of the other exterior surfaces tested. The

unpainted cedar plywood only developed a maximum smoke level of = 94,

but produced smoke more rapidly than any of the other surfaces. The aluminum

lap siding developed the lowest maximum smoke, Dm = 36, and at the lowest

rate of all the exterior surfaces.

Under non- flaming exposure, relatively high maximum smoke levels were

noted for the painted wood drop siding, Dro = 387, the unpainted cedar plywood,

Dm = 366, and the painted plywood, = 309. These values are comparable

to those previously measured for typical cellulosic materials (fiberboard,

plywood, hardboard, red oak), and illustrates that the smoke produced from

smoldering cellulosic materials far exceeds that produced when these same materials are actively flaming. The painted aluminum surfaces, on the other

hand, produced less smoke under the non-flaming exposure.

Of the interior wall surfaces, the painted gypsum board developed low

smoke accumulation levels under both flaming (D^ = 15) and non-flaming

Dm = 32) exposures. This board was used as the interior surface of three

*5all systems. The interior painted plywood surface of the stressed skin

sandwich panel produced smoke accumulation levels similar to those obtained

for the exterior plywood surfaces.

5
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Smoke accumulation tests were also performed on the reverse surfaces

of the exterior and interior layer materials where these formed the interior surfaces of the cavity. Of these surfaces, there was no smoke

accumulation (Dm = 0) for the aluminum foil surfaced gypsum board under

either flaming or non- flaming exposure, a moderate amount for the building

paper/polyurethane foam assembly beneath the aluminum lap siding, and a considerable amount from the fiberboard sheathing beneath the wood drop

siding. The latter material reached higher smoke accumulation levels for

both flaming (Dji, = 180) and non-flaming (Dm = 514) exposures than any

other surface tested, exterior, interior or cavity.

3.3 Flame Penetration

Test results are presented in Table 3, in terms of three measured

events which may be considered "critical" in terms of heat and flame

penetration:

(1) flamethrough of the exposed surface or layer,

(2) a 250 deg F temperature rise in the core or stud space,

(3) a 250 deg F temperature rise on the unexposed surface.

The results indicate that a 250 deg F temperature rise usually

occurs in the core or stud space prior to actual flamethrough of the

exposed surface layer. With flame exposure on the exterior surface,

the times to this critical temperature rise in the core or stud space

ranged from about 6 min. for the aluminum sandwich wall to about 26 min.

for the wood drop siding. The time for actual flamethrough varied from

10 min. for the fir plywood to 26 min. for the wood drop siding.

With flame exposure on the interior surface, the gypsum wallboard

served to limit the time for the critical temperature rise in the stud

space to the range of 22 to 28 min. and the time for flamethrough ranged

between 26 and 38 min. Flamethrough occurred at about 9 min. for the

plywood sandwich panel and at 15 min. for the aluminum sandwich panel.

The time to reach a 250 deg F temperature rise on the unexposed

surface always occurred later than flamethrough of the exposed surface

of the wall assembly, and ranged from as low as 10 to 14 min. for the

plywood sandwich panel to approximately 36 min. for the wood drop siding

wall with exterior surface f ire-exposed , and 65 min. for the same wall

with the interior surface exposed. This temperature rise was reached in

only 8 min. at the vinyl plastic joint of aluminum sandwich wall.

A comparison of the results for between-stud versus through-stud

fire exposure on the wood drop siding wall indicates no significantly

different behavior. It was noted, however, that the vinyl plastic joint

of the aluminum sandwich wall appeared to offer appreciably less resistance

to flame penetration than the unjointed section.

Fig. 4 shows several of the representative wall assemblies after

tests in which flame exposure was applied to the exterior wall surface.

- 6 -
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4. Discussion

Wuh respect to interior lining surfaces, there is clear and ample ev-i ence at surface flame propagation is a characteristic which con- tributes directly to fire hazard. This is probably not true to the same extent for the exterior surfaces of exterior walls.

^ 1 The radiant P anel test method has been used for characterizing the flame spread properties of all types of materials over the past ten years. Although the scope of the ASTM version [3] only recommends it for research and development work, it appears to be suited for performance eva -uation of walls. Other flame-spread test methods are also available .01 comparative f lame - spread measurements, such as the 25-ft "tunnel" test [7] and the 8-ft "tunnel" test [8], On the basis of flame-spread index values previously measured and reported [9, 10], as well as those measured for the representative walls included in this study, it should e possible to establish levels of performance against which to iudge wans 01 various types. It seems reasonable to establish flame-spread index limits for the interior wall surface, the exterior wall surface,

ana che reverse side of the interior and exterior layers where not astened directly to a noncombustible backing. However, the choice of acceptable limits remains arbitrary until such time as the functional relationship between flame spread index and fire hazard is known more definitely.

The production of smoke from the surface of material undergoing

decomposition in the presence of a fire is most conveniently gaged by

its accumulation within a closed chamber. Because of the complexity

of the problem, only the visual obscuration characteristic of the smoke

was considered here, and no consideration was given to the type and quantity of toxic combustion products, to the irritating and harmful effects

of inhaling hot smoke particles, or to human psychological reactions

leading to hysteria, etc„

Of several meaningful measures of smoke production (in terms of optical density), the following three were considered most appropriate: total

smoke accumulation, maximum rate of accumulation, and time to reach a "critical" accumulation level. In actual rooms, not enough is known about

how smoke accumulates and moves in response to natural and forced ventilation with typical door and window openings, for these values to be considered absolute except for closed compartments under identical conditions.

For all other conditions, and particularly when considering exterior sur- faces normally exposed to an unlimited atmospheric volume, the test values

are only useful as comparative measures of smoke production. Data on a wide variety of typical materials used as interior finishes have been

compiled in ref. [4], for both flaming and smoldering exposures.

7

On the basis of smoke, accumulation values previously measured and

reported, as well as those measured for the representative walls included

in this study, it seems reasonable that maximum smoke accumulation levels can be established for the interior wall surface, the exterior wall surface, and the reverse side of the interior and exterior layers where not

fastened directly to noncombustible backing. Although the interior wall

surface is the most critical one, the establishment of higher levels for

the other surfaces would serve to exclude the very heavy smoke-producing

materials. Maximum smoke accumulation levels should be based on the

higher value obtained from the flaming or the nonflaming (smoldering)

test exposures. As with flame-spread index values, such levels must be

set arbitrarily, rather than in terms of verified life hazard relationships.

The ability of an exterior wall structure to resist flame penetration has probably not been too great a factor in life safety in the past.

Nevertheless, this factor must be considered in fire performance evaluation of walls. The method described employs simple test equipment for

providing a severe, localized flame exposure. The specimen is mounted

horizontally, rather than vertically. The occurrence of flame and heat

penetration may be readily determined by visual observation and by means

of thermal indicators, such as thermocouples.

The time to flamethrough is a measure of the wall's resistance to

fire penetration, which could lead to subsequent flame propagation vertically in the stud space or core. The time to a 250 degree F temperature

rise on the unexposed surface is a measure of the wall's ability to insulate

against the transmission of heat which could ignite combustible material

placed against the surface away from the fire. This is meaningful primarily in terms of an external fire. Both types of measurements were

found to be reasonably reproducible in duplicate tests.

On the basis of the flame and heat penetration times measured for

the representative walls included in this study, it seems reasonable that

limiting flamethrough times can be established for both the exterior and

interior wall surfaces, and a limiting temperature rise time can be

established for exposure of the exterior wall surface. In addition to

the typical section of the wall assembly, joints, seams or other details

of questionable fire resistance, should be tested separately and in a manner likely to produce the poorest performance.

Although not included in the test result tabulations, common face

brick would be rated as zero for both flame spread index and maximum

smoke accumulation, and would also provide the highest resistance to

flame penetration.

8
- «« 5: ,i
SUMMARY

Measurements were made of surface flammability, smoke accumulation,

and the penetration of flame on both surfaces of typical sections of

selected exterior wall assemblies. Consideration was also given to

joints and seams, and to the reverse sides of interior and exterior layers

which might become exposed to flame in the event of flamethrough of the

layer. Use was made of available test methods and equipment for the

flame spread and flame penetration studies, while smoke accumulation was evaluated using a test method developed for the purpose.

The following table suggests properties which could serve as performance

criteria for the evaluation of exterior walls:

Property

Surface Exposed

Flame

Spread

Index

Maximum

Smoke

Accumulation

Time to

Flamethrough

Exposed Layer

Time to

250 deg F j Temp Rise !

jlnterior wall surface

! X X X

:

i 1(Exterior wall surface

1 X X X X

j ^Reverse (cavity )side of

j interior layer where

! not fastened directly

| to noncombustible

backing X X

«

Reverse(cavity )side of

exterior layer where

not fastened directly

to noncombustible

j backing X X

iExterior joint or seam X X X

3

•: *

1

In the absence of verified functional relationships between laboratory test values and actual fire hazard, acceptable performance levels

must be set arbitrarily.

_ 9 _

REFERENCES

[1] ''Multiple Death Fires of 1963", NFPA Quarterly, 57_ t p. 332, 1964.

[2] Interim Federal Standard 00136 b (COM-NBS) , Dec. 26, 1962.

[ 3] "Method of Test for Surface Flammability of Materials Using a Radiant Heat Energy Source", A3TM Designation E 162-66 T, 1966.

[ 4] Gross, D., Loftus, J . J . , and Robertson, A.F., "A Method for

Measuring Smoke from Burning Materials", ASTM Paper No. 200,

presented at ASTM Annual Meeting, June 1966.

[ 5 ] "Standard Methods of Fire Tests of Building Construction and

Materials", ASTM Designation E 119-61, 1961.

L6] Gross, D., and Loftus, J.J., "Surface Flammability of Fire Retardant and Conventional Paint Assemblies", Fire Research Abstracts

and Reviews, _3 , 151-8, 1961.

[?] Robertson, A.F., Gross, D., and Loftus, J.J., "A Method for

Measuring Surface Flammability of Materials Using a Radiant Energy

Source", ASTM Proc . 56, 1437-53, 1956.

[8] Gro ss, D. and Loftus, J.J. "Flame Spread Properties of Building

Finish Materials", ASTM Bulletin No. 230, 56-60, May 1958.

[ 9 ] "Standard Method of Test for Surface Burning Characteristics of

Building Materials", ASTM Designation E 84-61, 1961.

Liol "Method of Test for Surface Flammability of Building Materials

Using an 8-Foot Tunnel Furnace", ASTM Designation E 286-65 T,

1965.

- 10 -

USC OMM-NBS -DC

TABLE 1 -- Flame Spread Index Values for Representative Walls

Wall

Number Type

Flame Spread Index

lg

1 Wood drop siding, painted Exterior 49

}

2 Aluminum lap siding Exterior 0

- - r i

3 Aluminum sandwich wall

Exterior

and

Interior

0

i

4 Cedar plywood, unpainted

' ' '

Exterior

'

155

5 Plywood, painted Exterior

.... - -

107

1,2 Gypsum board, painted Interior

- - -

8

1

- -

5 Plywood, painted Interior 105

1 Fiberboard sheathing

Exterior

Reverse

(Cavity)

f

jr 242

l

1,2 Gypsum board, unpainted

Interior

Reverse

(Cavity)

13 |

2 Building paper/polyurethane

foam/ aluminum lap siding

Exter ior

Reverse

(Cavity)

568*

4

1

Gypsum board, aluminum

foil surface

Interior

Reverse

(Cavity)

i 0*

r * Average of two tests; all others

I average of four tests.

TABLE

2.

Smoke

Accumulation

Test

Results

for

Representative

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Fig. 2. Smoke Test Chamber

Fig. 3. Apparatus for Flame Penetration Tests

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Fig. 4. Representative Wall Assemblies after

Flame Penetration Test.

Exterior Surface Flame Exposure


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