Celotex old insulating board (C) Daniel FriedmanFiberboard Sheathing Fire Ratings
Insulating Board Sheathing Fire Resistance

  • FIBERBOARD SHEATHING FIRE RATINGS - CONTENTS: fire ratings of insulating fiberboard building sheathing products used as exterior wall sheathing and as interior wall & ceiling surfaces.
  • POST a QUESTION or READ FAQs about fiberboard building sheathing: how to identify fiberboard products, fiberboard uses, fiberboard, Celotex, Homasote, Insulite & other brands, fiberboard ingredients, does fiberboard contain asbestos?
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Fiberboard insulating sheathing fire ratings:

Fire resistance of fiberboard sheathing used on roofs, exterior walls, interior walls & ceilings.

This articleseries describes and provides photographs that aid in identifying various insulating board sheathing materials used on building walls and roofs, such as Homasote, Celotex, Insulite, and Masonite insulating board sheathing products.

We describe the components, properties, and applications of various fiberboard, hardboard, and insulating board or sound deadening board products. W

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Fire Ratings & Fire Resistance of Fiberboard Sheathing

Celotex insulating lumber ad 9 September 2015 Andre said:

Is older homasote board a fire-rated wall covering? I'm doing some renovation in a mid-19th century house. One attic room was converted to a bedroom by putting homasote panels over the approximately 1x6 planks that covered the interior attic walls.

I don't know when this took place, but guess it was at least a couple decades ago, if not considerably longer.

Is homasote of that age fire-rated? Would the total assembly (homasote over 1x planks) be fire-rated? If not, can I get to a fire-rated assembly by applying 1/2" drywall directly over the homasote?

Reply: here will provide fiberboard fire rating tables & data


While there are some modern fiberboard products that are fire rated you cannot safely assume that a new much less an antique fiberboard sheathing is fire-rated nor that it has good fire resistance.

In fact some,perhaps many fiberboard sheathing products were treated with a wax or other substances for water-resistance, increasing the combustability of the material. Fire experts cite building sheathing, including fiberboard, as an important fuel contributing to fire spread on buildings.

The exterior fire has layers of fuel that contribute to the fire: vinyl siding and trim, flammable insulation (foam or fiber board), and flammable sheathing (plywood or OSB). All contribute to a large body of fire outside the structure that is providing a flow path up to the eaves or soffits that provide little or no resistance to the vertical spread.

The exterior fire is fueled not only by the siding but also by the building insulation (foam or fiber board) that is directly under the siding (photo 8). Plastic house wrap may be under the insulation and, of course, the sheathing of oriented strand board (OSB) is under that.

The OSB contains large amounts of glue to hold the strands together to form a usable board. All of these combustible components contribute to creating a large body of exterior fire. - Fire Engineering, "Rapid Fire Spread at Private Dwelling Fires", (Vol. 166, October 2013), retrieved 14 September 2015, original source:

HIlado (1998) cites the flammability of fiberboard sheathing products and other sources such as some testing agency and U.S. state agencies cite the flame spread ratings as follows:

Flame Spread Classification of some Common Building Materials

Material or wood species Flame Spread Rating Flame-Spread Class
Hardboard siding panels < 200 III
PA Wood Structural Panels (includes APA 303 Sidings such as T1-11) 76-200 III
Brick 0 I
Cedar, Western Red 69 II
Fiberboard, Medium Density 167 III
Gypsum Wallboard 10-15 I
Fiber Cement exterior materials 0 I
Inorganic reinforced cement board (such as asbestos cement millboard) 0 I
Masonite <200 III
Particle Board 116-178 III
Plywood, pine 120-140 III
T-111 76-200 III


Source: Louisiana State Government, "Information on Construction Requirements, Flame-Spread Ratings", retrieved 14 Sept 2015, original source:

Hilado, Carlos J. Flammability handbook for plastics. CRC Press, 1998. Excerpting, this handbook notes in Table 3.2 Ignitability Characteristics of Some Materials as Measured by the USF Ignitiability Test [excerpts, p. 69]

  • At a heat flux W/cm2 for three heat flux levels of 5.8, 8.1, and 10.5,
  • Cellulose fiberboard, 1/2 -inch has an ignition time of 11 seconds, 5 seconds, and 3 seconds
  • Fiberboard soundstop, 1/2-inch has an ignition time of 15 seconds, 6 seconds, and 4 seconds
  • Fiberboard sheathing, 1/2-inch has an ignition time of 17 seconds, 7 seconds, and 5 seconds
  • Medium density fiberboard, 1/2-inch has an ignition time of 34 seconds, 21 seconds, and 14 seconds
  • Hardboard, 3/8 inch, has an ignition time of 77, 26, and 17 seconds respectively
  • Hardboard, unfinished, 1/4-inch has an ignition time of 222 seconds, 22 seconds, and 12 seconds
  • Cotton fabric has an ignition time of 10 seconds, 6 seconds, and 3 seconds

Flame-Spread Classification Flame-Spread Rating or Index for the three classes

Class I (or A) 0 - 25

Class II (or B) 26 - 75

Class III (or C) 76 - 200

From the first reference above:

The most widely accepted flame-spread classification system appears in the National Fire Protection Association Life Safety Code, NFPA No. 101. This Code groups the following classes in accordance with their flame-spread and smoke development:

Class A - Flame-spread 0-25, smoke developed 0-450.

Class B - Flame-spread 26-75, smoke developed 0-450.

Class C - Flame-spread 76-200, smoke developed 0-450.

NFPA 101 primarily applies this classification to interior wall and ceiling finish materials. Roof coverings must meet a different set of criteria.

Watch out: Unless you know that your fiberboard is a fire-rated product the safer assumption can be inferred from this note from Georgia Pacific about its "regular fiberboard sheathing”:

GP Sheathing products are marked with a hazard label because they may be a potential fire hazard if exposed to flame, such as plumbing torches. Caution must be taken that these products do not come in contact with open flame or with temperatures high enough to ignite them or cause smoldering combustion. Regular Fiberboard Sheathing products should not be used under stucco.

Homasote currently offers a medium density fiberboard boards designed for use in residential and commercial buildings that require a Class A (25 flame spread) rating. That does not by any means claim that all fiberboard sheathing of all brands and ages carries a Class A flame spread rating.

Early efforts to make fiberboard fire RESISTANT show up in patent disclosures and fiberboard sheathing product modern fire ratings are discussed by the various manufacturers of fiberbboard such as Georgia Pacific and Homasote.


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