Celotex old insulating board (C) Daniel Friedman Fiberboard Building Wall & Roof Sheathing, Insulating Board Sheathing, Sound Insulation Board Product Guide

  • SHEATHING, FIBERBOARD - CONTENTS: How to recognize/identify Celotex®, Homasote®, Masonite® and other insulating fiberboard building sheathing products used as exterior wall sheathing and as interior wall & ceiling surfaces. Fiberboardf was used indoors as both a finish-surface to be painted or as a base to which plaster and then paint were applied.
  • 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: definition, ingredients, history, use. This article 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.

Here we provide fiberboard product names and we describe the components, properties, and applications of various fiberboard, hardboard, and insulating board or sound deadening board products. We also answer questions such as do Celotex or Homasote or other fiberboard and insulating board products contain asbestos? fiberboard water resistance, fiberboard recycling.

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Fiberboard Building Sheathing: Black board, grayboard, buffaloboard exterior sheathing

Fiberboard sheathing (C) Daniel Friedman

Article Contents

What is Fiberboard Insulating Sheathing?

Fiberboard insulating sheathing board was used and continues in use as a structural wall sheathing board 15/32-inches thick (one board was 1/2") and with R-value of about 1.5.

Fiberboard insulating sheathing was and continues to be made of plant cellulose such as wood fibers, combined with a binder, a water-resistive coating or component (such as paraffin and/or asphalt), and other treatments that we detail below. Structural properties and moisture resistance were confirmed by US FPL testing. Moisture uptake did not exceed 2.2%.[15]

Readers should also see Sheathing, Gypsum board, and SHEATHING, OSB as well as Sheathing, Plywood for a discussion of these common building roof and wall sheathing products.

At DRYWALL, PLASTER, BEAVERBOARD we discuss other interior sheathing boards that were used on interior walls and ceilings.

At SIDING HARDBOARD we discuss hardboard exterior building siding such as sold under the Abatibi and Boise Cascade brands.

At MOLD on FIBERBOARD INSULATING SHEATHING we discuss mold growth on or in fiberboard sheathing.

A History of Fiberboard Insulating Sheathing

In addition to plywood, OSB, and gypsum board, impregnated fiberboard produced in 4 ft. widths and varying lengths up to 12-feet has been used as exterior building insulating sheathing in North America since at least 1909 (see our discussion of Homasote™, below and see Masonite™ and other hardboard Sheet and Siding Building Materials).

Actually hardboard is older than that. According to the Boston Museum of Fine Arts, fiberboard was first patented in 1858 and was produced by a number of manufacturers (listed below) and made of a variety of plant fibers (including bagasse from sugar cane) or wood chips, wood byproducts, and by one company waste paper and by another company flax shives [12][15] . Indeed it was Lyman's 1858 invention of a method for separating the fibers of wood, probably born from other work on improving the cotton gin, felting hat bodies, and making paper, that made these products possible.[13]

Fiberboard wall sheathing (an example fiberboard product photograph is shown at above left), when intended for use on a building exterior is installed by nailing directly to the wall studs, most often with let-in diagonal bracing or plywood panel bracing at the building corners to assure building rigidity.

A 1955 U.S. FPL report offers the early history of growth in the use of insulating fiberboard sheathing.

Insulating fiberboard sheathing is used extensively in house construction. In 1920 insulating board accounted for only about 4 percent of the sheathing used for new residential construction. In 1940 its use had risen to 20 percent and by 1950 to 30 percent. It is more popular in certain regions of the country than others. In 1950 about 50 percent of the wall sheathing used in the Middle West [of the U.S.] was of fiberboard, while in the Pacific Northwest where lumber is more easily obtained, insulating fiberboard was used in only about 10 percent of the new houses erected. [15]

In earliest use, fiberboard sheathing material was sold as a means of insulating the home at little additional cost since it was used to replace the horizontal or diagonal tongue-and-groove board sheathing that was in wide practice up to the 1940's or 50's; but despite early claims (later dropped) that wood fiber sheathing was a structural material, in homes where fiberboard sheathing was used, additional framing was commonly constructed at the building corners as diagonal bracing to stiffen the building walls.

Modern product literature for fiberboard insulation[2] and standards for products such SturdyBrace® Structural Fiberboard Sheathing [14] make clear that the product is produced and used as structural sheathing. Quoting:

STURDYBRACE® improves the structural integrity of homes and light commercial buildings and eliminates the need for corner bracing. The product meets codes for wind shear and seismic conditions [2]

Synonyms for Fiberboard Insulating Sheathing

Synonyms for fiberboard include: brown board or brownoard, insulating board, Celotex, Homasote, Insulite, "fibre-board (Br.); fibreboard (Br.); carton-fibre (Fr.); carton fort (Fr.); aglomerado de madeira (Port.); particle board; composition board; wallboard; hardboard; fiber board; high-density fiberboard (HDF); medium-density fiberboard (MDF); low-density fiberboard (LDF)[11] Additional fiberboard insulating sheathing product names appear throughout this article.

Fiberboard Manufacturers & Product Brand Names

  • Beaver Board;
  • Blue Ridge
    • Sturdy-Brace
  • Celotex or Knight Celotex [11]
    • Celotex Blue Ridge™ Fiberboard
    • Celotex SturdyBrace®
    • Premium Fiberboard Insulating Sheathing
    • StructoDek®
  • Cornell Board;
  • C-X Board;
  • Duron® [IPI];
  • Feltex;
  • Fir-Tex;
  • Homasote®;
    • 440 SoundBarrier
    • Firestall®
    • EasyPly® Roof Deck
    • Thermasote® Nailbase Roof Insulation
    • N.C.F.R.® Thermasote® Nailbase Roof Insulation
  • Insulite
    • Bildrite Sheathing (exterior building sheathing)
    • Fiberite (interior finish)
    • Graylite (interior finish)
    • Graylite Lok-Joint Lath
    • Insulite (see below)
    • Insulite Lok-Joint Lath
    • Ins-Lite Lok-Joint Lath
    • Satincote
    • Smoothcote
  • Masonite® [IPI];
  • Medex; Medite;
  • Nu-Wood;
  • Presdwood®;
  • Quartrboard;
  • Upson & "Upsonboard"

In the article below we provide additional details about fiberboard product names and we describe the components, properties, and applications of various fiberboard, hardboard, and insulating board or sound deadening board products. These product names include

What is Fiberboard Insulating Sheathing Made Of?

Fiberboard wall sheathing post flooding (C) D FriedmanThe range of materials used to produce plant-based boards and "lumber" used in construction is broad, including bagasse (sugar cane fibers), bark, flax, grass, hemp, jute, peanut shells, reeds, sawdust, straw, and wood pulp tailings or byproducts. In 1955 the US FPL reported:

Wood fiber is the most common material used in the manufacture of insulating fiberboard. Two large companies use bagasse, while another company's board is composed mostly of waste paper. Flax shives are used to some extent by one manufacturer.[15]

There has been a variety of techniques to produce, bond, and give desirable properties (waterproofing, vermin proofing, rigidity, structural strength, sound and heat insulating properties) to fiberboard products, in general the boards are made from a mixture of ingredients that are pressed or rolled, and bonded using asphalt, clay, decxtrin, paraffin wax, plaster, urea formaldehyde resin, or other binders. Carbon black is used by some manufacturers in very small quantities (about 1%).

Our photo (above left, provided by a reader) illustrates use of fiberboard sheathing beneath a brick veneer wall. The demolition was performed during building renovations.

Fiberboard sheets or lumber have been produced in three densities for different applications:

  1. Low density (soft and relatively thick (e.g. 7/16")) used for insulating sheathing or soundproofing
  2. Medium density (Medex, Medite)
  3. High density (Masonite, Upson board, Marinite, and some Homasote products)

There both non-structural and structural fiberboard panels that did not require this additional bracing have been produced. Some fiberboard sheathing products can claim adequate structural shear strength, particularly if the proper nails and nail pattern are used.

Homasote roof insulating board (C) D Leen and D Friedman Other contemporary producers of fiberboard building sheathing besides Homasote™, and Masonite™ include International Bildrite (Bildrite structural), Georgia Pacific (Stedi-R & Stedi-R-structural), Knight Celotex (Celotex premium insulating), and Temple Inland (Temple fiber brace).

Fiberboard sheathing, also called black board, gray board, or buffalo board sheathing in some areas, is a fibrous material impregnated by (or in some cases coated with) a stabilizer and water repellant - asphalt on early versions of this material that we have found.

While it's not easy to find and identify this material on a building wall unless indoor or outdoor demolition is being performed, you can spot the product in building attics on the gable-end walls.

The R-value of fiberboard sheathing is higher than plywood, gypsum board, etc, and is rated at about R 2.4 per inch (or about R 1.2 in more typical half-inch thickness with which it is applied. The board also reduces sound transmission into buildings. r framing in North America continued until about 1920. (CF Reference due: Age of Barns, op.cit.).

Importance of Protecting Fiberboard Insulating Sheathing from Water

Question: can you seal Celotex board in the attic? will sealants harm it?

Can you seal the celotex board in the attic, does it have a negative affect on the Celotex - Steve 3/16/2012

Reply: sealing insulating fiberboard is not necessary - it is moisture resistant - but insulating fiberboard should be kept dry during construction and protected by a moisture barrier between wall siding and the sheathing

Steve, if you are asking about painting Celotex or similar fiberboard products, you can do so, but with the caveat that unless you use a suitable paint, perhaps a lacquer primer/sealer such as BIN, you may get brown bleed-through of the material. Paint won't injure the board nor harm its insulating properties, though it might slightly affect its sound insulating properties.

If you are sealing to try to reduce moisture uptake I'll add that these products have usually been built or treated to make them water and moisture resistant. Paraffin and asphalt are common ingredients that add water resistance to fiberboard insulation, and according to the US FPL, the commercial standard for insulating fiberboard placed a limitation of 10% on water absorption for sheathing purposes, and 7 to 10% when the board was used as lath for plaster interior walls, roof boards, or as an interior finish surface. The FPL found that

None of the [fiberboard insulating] boards investigated ... exceeded a moisture absorption of 2.2%[15]

From this and from our warning just below you will see that seal-coating an insulating fiberboard as a move to reduce moisture uptake is unnecessary in normal use as long as the material has been protected from soaking during construction and is installed with a proper wall-moisture barrier [housewrap] between the sheathing and finish wall siding.

If your insulating board products are newer foil or kraft-faced foam board products, there is no reason to apply a moisture sealer to them, and where foil was used, I doubt that it would adhere well anyway. If the boards are exposed in an occupied space, fire codes will require that a fire resistant finish surface such as drywall be installed.

Watch out: Keep insulating fiberboard dry during construction and protect it from wetting after installation. During building construction, insulating fiberboard sheathing should be protected from water (rain, melting snow) on the jobsite during construction before it is applied to the structure itself. The US FPL found that

Soaking [fiberboard insulating sheathing] for 6 hours before test reduced the modulus of rupture and lateral nail resistance of the fiberboard to about 75 percent of the value for dry material. The moisture content was about 16 percent. When the material was soaked for 48 hours before test, the modulus of rupture and lateral nail resistance were reduced to about 40 percent of the values for dry material, and the moisture content was about 45 percent. [15]

The results of tests of the several properties investigated indicated that all 14 [insulating fiber] boards meet the commercial standard. The reductions in strength caused by wetting indicate that the insulating fiberboards should be kept dry during erection. Of even more importance is the necessity of keeping sheathing dry during service by the installation of good moisture barriers in those houses erected in parts of the country where moisture condensation is a problem.

Fiberboard Roof Sheathing

Fiberboard products were also used for roof sheathing produced by several manufacturers.

Watch out: OPINION-DF: where fiberboard roof sheathing was used alone to support roof shingles or other roof coverings, and noting that fiberboard products and their performance varies by manufacturer, application, and installation details, some fiberboard products may become fragile with age, traffic, or leaks, risking roof shingle blow-off, or worker fall injuries. On a roof replacement job one of our workers [DF] stepped onto an area where the roof decking had been damaged by leaks, and broke through to the attic below. Any significant or chronic water leakage in a roof whose shingles are supported by fiberboard insulating sheathing risks a roof surface collapse.

According to Homasote[1], at least two important clarifications are in order:

Our roof application is a structural roof deck (2’ X 8’) product which in  the 46 years I’ve been associated with Homasote has never failed if installed properly. Our deck will wick out any water if installed properly.

One should not refer to fiberboards as if all fiberboards were the same. Our [Homasote®] products are much denser then the other fragile ones manufactured in the U.S. and are thus considered superior by the industry.

Homasote® roofing products include

  • Firestall®
  • EasyPly® Roof Deck
  • Thermasote® Nailbase Roof Insulation
  • N.C.F.R.® Thermasote® Nailbase Roof Insulation

When stripping existing roof shingles to perform a shingle tear-off for re roofing, Homasote® and other fiberboard roof sheathed roofs require special precautions to avoid damaging the roof sheathing during shingle tear-off. Homasote® provides the following advice: [Quoting from "Roof Shingle Tear-Off Procedure for Homasote Products" available from Homasote ].

The removal of existing shingles to re-roof Homasote roof deck or nailbase roof insulation requires a change from the conventional tear-off method used to re-roof wood surfaces.

To strip existing shingles from a Homasote roofing product, the following must be done:

  1. Instead of getting under shingles with a “shoveling motion” and then scraping forward to get the nails out you must use a stripping tool or roof shovel to pry up shingles away from the deck, by pulling the nails straight out.
  2. Care must be taken so that the roof shovel does not damage the surface of the deck.
    Inspect the condition of the deck as the tear-off continues. Any stubborn nails remaining after the shingles are removed should be pulled out using a crowbar or claw hammer along with a wood block as a fulcrum to prevent damage to the deck.
  3. Inspect the stripped deck for any signs of deterioration or physical damage. Any damaged areas should be replaced with the same Homasote product.
  4. Prepare deck for the new finish roof installation in accordance with standard roofing practice and the roofing materials manufacturer’s installation instructions. The Homasote Company requires the use of a ring-shank roofing nail for all shingle installations.

Installation instructions, general requirements and the most up-to-date information on Homasote roofing products are available from Homasote.®.

Fiberboard Sound Insulation

Fiberboard products are also used for sound insulation, such as Homasote's 440 SoundBarrier used on walls and over subflooring or in ceilings. According to Homasote this system is recognized in UL L500 Series Floor/Ceiling assemblies.

Guide for Identifying Photographs of Homasote®, Celotex®, Insulite & Similar Fiberboard & Insulating Sheathing Board & Plasterboard Products

Celotex insulating lumber ad Insulating building sheathing made by Homasote® is produced by the Homasote Company, a manufacturer in the U.S. in New Jersey, and similar fiber sheathing products have been used both as a sound barrier and for exterior sheathing on buildings. Insulating board sheathing has been widely used on building exterior walls, under roofs, and against masonry foundations in finished basements.

Homasote Co., the oldest manufacturer of building products from recycled materials in the United States, was founded by Eugenius Harvey Outerbridge as Agasote Millboard Company, and has been producing this material since 1909. In 1936 the company changed its name to its best known product, Homasote.

Originally, Homasote® produced sanded "agasote" sheets used in the roofs of passenger railroad cars, moving, in 1915, to automobile roofs, and in 1916 to construction products. Homasote was widely used for military barracks in both WWI and WWII and is still promoted for sound resistant sheathing and other applications.

Celotex®, Homasote®, Thermafiber®, and similar insulating building sheathing board products are still sold as a lower cost alternative to plywood or OSB for building sheathing. The product is used as structural paneling, insulation, concrete pouring forms, and expansion joints.

Reader Question: Is it OK to re-side a home with Homasote building sheathing on its walls?

My home was built in the early 1940s and it was re-sided around 1994 (possibly a bit later but that's all the info we have). There are what we thought was sheetrock on the outside, but someone suggested it may be Homasote boards from when the house was originally built. Two questions, if they are these boards from 50 years ago (at the time the house was re-sided), was that ok to do based on housing codes? Could they contain asbestos? Thanks. - K 12/2/2011

Reply: Yes

I've not found any building code issue constraining the procedure of re-siding a home because of the type of exterior sheathing used on the structure. The asbestos question is addressed in earlier FAQs addressed above. Wood or plant fibers are not asbestos materials.

Watch out: fiberboard sheathing should be protected from water that penetrates some building siding systems (such as aluminum or vinyl siding) by installing a water or moisture barrier - housewrap. See HOUSEWRAP AIR & VAPOR BARRIERS

How to Identify Celotex® Insulating Board and Fiberboard Products

Our photographs below show Celotex® insulating board with an older Celotex fiberboard building sheathing board at left and a more recent Celotex insulating board product shown at below-right. Also see this closeup of an older Celotex insulating sheathing board product.

Celotex old insulating board (C) Daniel Friedman Celotex tuff-R insulating board (C) Daniel Friedman

Components & Properties of Celotex Insulating Lumber

Celotex described their Celotex Insulating Lumber as an exterior sheathing product intended for use as a base beneath plaster or beneath a stucco building exterior as well as for roof insulation. Celotex insulating lumber was sold in 7/16" thicknesses (and possibly other thicknesses), in 4 foot width boards at lengths from 8 ft. to 12 ft. and weighing about 60 pounds per square (100 sq.ft.).

Celotex insulating lumber ad Celotex insulating lumber (today we call it insulating board or insulating sheathing) was sawed "like ordinary lumber" and nailed directly to the building framing to support stucco, brick veneer,m or other finishes.

Celotex Insulating Lumber was a cellulose fiber board made from bagasse or sugar cane fiber using a felting process, and produced in Celotex's New Orleans LA plant.

These fibers "each of which contains thousands of sealed air cells", were fabricated into "building lumber" using a patented press and bonding process. Several patents listed in the mid 1920's addressed the production of insulating, structural, and sound-absorbing board products for walls, roofs, and ceilings produced by Celotex.[10]

In wood frame construction Celotex insulating lumber was used as a structural sheathing to replace horizontal or diagonal 3/4" thick board sheathing while adding insulating and sound-deadening properties.

The company described the insulating value of this new product as

"... equal, as insulation, to 3 1/3 inches of solid wood, 12 inches of solid plaster, 12 inches of solid brick, or 24 inches of solid concrete".

We estimate, based on the wood comparison, that the R-value of this 7/16" thick board was about R-3.

According to Celotex this insulating lumber product was waterproof, could be painted, and could be used itself as an exterior finish as well, though we have not seen any surviving examples of that application.

Celotex's insulating lumber was also advertised for use as an interior finish, left natural, stained, painted, or stenciled. Celotex recommended its use also as a base for plaster walls or ceilings. From our own field inspections, we believe that Celotex insulating lumber or similar products were indeed left exposed as an interior finish most commonly in summer camps, cottages, and in commercial or farm buildings.

According to one source the material was also used to construct insulated shipping boxes. [8]

By 1925 Celotex had published "Celotex insulating Lumber Specifications and Details for Standard Building Board" and also offered "Your Home" a plan book of twenty-five ideal small homes.

A review of the patents and product description for Celotex insulating lumber products shows that asbestos was not among the product's ingredients.

Celotex Regular Insulation Sheathing Continues in Production as Blue Ridge™ Fiberboard & SturdyBrace® Sheathing

In 1955 there were at least fourteen different insulation fiberboards examined by the US FPL. Today Celotex continues as a major producer of the product. [15]

As of 2012 Celotex continued to produce a wide range of insulation products including PIR polyisocyanurate foam boards in various designs and for various applications. The company continues to produce Regular Insulation Sheathing as a 1/2-inch thick insulating board with an R-value of 1.2. Celotex Regular Insulation Sheathing is described in contemporary product literature as:

Produced from cellulosic interlaced fiber bond with natural binders. Manufactured from Sugar cane by-product called bagasse, recovered wood chips and consumer newsprint. Available in 4' width in standard lengths of 8' and 9'.

Designed for use as an insulating sheathing for frame walls under exterior finishes such as lap and panel sidings, stucco, masonry veneer and shake shingle products.[2]

Celotex Blue Ridge™ Fiberboard, also referred to in its product literature as SturdyBrace® is a wood fiber product that is described by the company as:

SturdyBrace® meets national codes for structural integrity. The insulating exterior wall sheathing is competitively priced. You may bid with confidence year round. SturdyBrace® improves the structural integrity of homes and light commercial buildings and eliminates the need for corner bracing. SturdyBrace® meets codes for wind shear and seismic conditions. Check with code officials in your area.

Trapped moisture in exterior walls is caused by condensation and water leaks. This moisture can lead to mold growth and other water related problems. SturdyBrace® “breathes” permitting the moisture to escape into the outside air. Mold experts recommend use of wall sheathing with a minimum ASTM permeability rating of 5. SturdyBrace® rates more than 20.0 perms while OSB and plywood have permeability ratings of 2 and less than one respectively.

SturdyBrace® is an earth-friendly fiberboard made of recovered wood fibers interlaced and bonded with asphaltic binders for strength. It is easy to install and cuts with a knife, saving you time and money. SturdyBrace® is available coated on one or six sides. SturdyBrace® saves energy costs at a low cost per R-value. Our wall sheathing provides R-value of 1.3 per 1/2”, more than doubling the R-Values of OSB and gypsum sheathing.

*Building codes may require use of weather resistant barrier. [2]

Watch out:

Blue Ridge Fiberboard products must not be used in close proximity to chimneys, heater units, fireplaces, steam pipes or other surfaces which could provide long term exposure to excessive heat (maximum 212*F) without adequate thermal protection. [2] - guidespec

Identifying Homasote® Brand Fiberboard

According to Homasote®,

[Homasote brand fiberboard products in] cross section would not show layers of fiber since our products, unlike other fiberboards, are not layered.

With aging our products normally have a yellowish brown tint otherwise they are gray.

In all cases, unsanded Homasote Products all have very visible patterns on the face and back side surfaces. As far as I can tell, none of the samples shown below [at Unidentified Fiberboard Products] have our mold patterns.

Each fiber is coated with a wax emulsion thus making the panel weather resistant. In a vertical application, should the panel get wet, the water will wick out the bottom as long as it’s installed properly and elevated off the bottom surface. In a horizontal application it will react like plywood and the water should not de allowed to pond on it especially after it stops raining.

Where structural shear strength is needed by using the company's recommended ring-shanked nails in a specified nailing pattern.

Insulite, Another Cellulose Fiber Board Product useful for plasterboard

Insulite plasterboard from patent app.Insulite was a cellulose-based (all wood fiber) insulating board or sheathing material that, unlike Celotex, was made from wood pulp byproduct or tailings fibers.

The Insulite board was treated to "... resist moisture, vermin and rodents" and also was sold as a "sound deadener"[10] and in some applications the product was installed in the air space between gypsum board partitions to improve sound isolation between building areas.

Insulite was described as having stronger structural properties than Celotex, the latter being superior for insulation and sound insulation while Insulite offered greater strength for other applications. Insulite was

... composed of large sliver-like particles often 1/16" to 1/32" in width and say one half an inch long. These sliver like fibers give great porosity to the mass but they render the binding together of the particles more difficult. [10]

Insulite as a plaster board contained rabbeted grooves or "joints" in its surface to which plaster or other material could be applied. Insulite's name for this product was Lok-Joint Lath. (The same engineers later developed "Bildrite Sheathing" that was used to replace horizontal wood bracing in wood frame construction.) The product cost was as low as 5 cents per sq.ft.

Insulite board advertisementIn "Insulite Co. vs. Reserve Supply Co", a 1932 lawsuit, relevant patents and ingredients are described, including a composition of plaster of paris, cement, or other like substance, combined with hair, wood fiber, sawdust, wool, wood shavings, excelsior, straw, or similar substances. (Asbestos was not cited in the product description. )[11]

Treadway B. Munroe, from Forest Glen, Maryland, was a prolific inventor who patented a variety of cellulosic board products assigned to companies including Dahlberg (St. Paul MN and Celotex, Chicago IL). One of his early patents U.S. No. 1,333,628, described a plaster-board of fibrous material intended to provide a less costly base for plaster walls and ceilings.

This was the earliest citation of "Insulite" that we could find. It improved on the original "insulite" construction by including additional long fibers for strength combined with more short fibers to serve as filler for the mass, developing a board that was light weight, had adequate strength, and included entrained air for improved insulation.

This invention, instead of impregnating the insulating board with a waterproofing compound, simply coated its surface. The result was a product [intended and claimed to be] well suited for use as plasterboard.

Sound absorbing board for walls and ceilings", Patent No. 1,554,180, issued to W.S. Trader, September 15,1925, first disclosed a wallboard constructed from "Celotex", a felted mass of strong bagasse fibers, so compacted as to be capable of use as an artificial lumber in that it can be sawed and nailed, and has sufficient strength in many cases to be substituted for lumber.

That same patent mentions "Insulite", a building board made from wood pulp tailings and which likewise has a porous fibrous body portion and which is possessed of considerable strength so that the same can be nailed, etc.

Celotex was preferred as an insulating material because its internal cells produce a sound-deadening insulating effect.

Also see Fiberboard insulating sheathing or board sheathing products and see Masonite® hardboard siding products.

Other Insulite Product Names

We found references to Insulite mastic as early as 1913.

By 1940 we find the additional sheathing product names associated with Insulite, a Minneapolis MN company.

  • Bildrite Sheathing (exterior building sheathing)
  • Fiberite (interior finish)
  • Graylite (interior finish)
  • Graylite Lok-Joint Lath
  • Insulite (see below)
  • Insulite Lok-Joint Lath
  • Ins-Lite Lok-Joint Lath
  • Satincote
  • Smoothcote

Masonite™ and other hardboard Sheet and Siding Building Materials

Masonite hardboard (C) Daniel Friedman

(History, more photos, & dates in process, CONTACT us, contributions invited)

Our photo (left) shows the back side of an early hardboard interior-use product labeled "Genuine4 Masonite Quartrboard".

More about hardboard sheet products used on building interiors is found at DRYWALL, PLASTER, BEAVERBOARD

Unidentified Fiberboard Products

Fiberboard sheathing like Homasote (C) Daniel Friedman

At left is an insulating fiberboard product that is not the Homasote™ brand.

Homasote Chairman and CEO Warren L. Flicker has generously added these comments that assist in distinguishing among fiberboard product brands and manufacturers:

The pictures at left and below show a brown side and a black side [and show layering when broken to expose the material in cross-section]. They are not Homasote®.

Homasote type insulating sheathing board (C) Daniel Friedman Homasote type insulating sheathing board (C) Daniel Friedman

Our photos (above left and right) show close ups of fiberboard insulating building sheathing board products that are not Homasote™ brand, including a torn cross section showing the layered fibrous character of this material.

Homasote roof insulating board (C) D Leen and D Friedman

Our photo (left) shows pieces of fiberboard roof insulating fiberboard removed from a building by reader/contributor Doug Leen.

The varying colors of the two sides of the material are visible - the darker side of this insulating board may have been that exposed to light and air during its life in the building. The material looks like a Celotex product.

According to Thermafiber® it is not their product.

According to Homasote® this is not their product.

Reader Question: From a 1944 Minnesota home, can you tell me what this soft brown fiberboard material is?

Hi, I was wondering if you would be able to identify the following pictures. They are receptacles cut outs from an interior wall. House was built in 1944, in the state of Minnesota. It looks like a fiberboard material is the backer. But not really sure what finished side is. It's almost like a cement board that's glued directly to the fiberboard. It's very strong. Anyway any info you could give me would be greatly appreciated. - M.S.T. 3/26/14

Fiberboard insulating wallboard from a 1944 Minnesota home (C) InspectApedia Fiberboard insulating wallboard from a 1944 Minnesota home (C) InspectApedia


Your photos look like a wood product fiberboard sheet that has been coated or painted presumably on the room side. Depending on the age you should be alert for possible lead paint hazards.

This material was used both as an insulating sheathing on building exteriors (under siding) and as a finish interior wall and ceiling board such as in the case you describe.

Take a look through the fiberboard examples above for some close comparisons to make an educated guess about the exact material. To be more precise we'd need to make a lab comparison with known samples in our library.

Fiberboard insulating wallboard from a 1944 Minnesota home (C) InspectApedia Fiberboard insulating wallboard from a 1944 Minnesota home (C) InspectApedia

Reader follow-up:

I was wondering if you could identify the concrete/slash stucco covering that is glued to the fiberboard. It is 1/2" thick and is uniform in thickness. It has many small rocks and sand mixed in it. The top 1/8" of it is different color than the bottom. Once again any info you can give me would be appreciated.


Wall test cut shows construction (C) D FriedmanWe're looking at fiberboard backer used as a base on which a plaster coating was applied. We see the rough coat - the thicker white layer - also known as a "scratch coat" and then the thinner tan layer is most likely the smooth coat or top coat of plaster, perhaps on which there was also paint finally applied.

I've encountered this construction before in some older homes from the 1920s to late 40's in the U.S. and I imagine it was used elsewhere.

Watch out: some plaster from that era contained asbestos.

At CEILINGS & WALLS, PLASTER TYPES you can see a photo similar to yours but as your image is better I'll want to add it there.

At above left we see my photo from that article. It shows that the very first or original layer of wall was fiberboard over which plaster was applied. My pen point is indicating the plaster. The brown fiberboard backer is to the left of the pen.

Question: can we identify this fiberboard sheathing from a 1975 home in New England?

Fiberboard sheathing, unidentified, ca 1975 (C) InspectAPedia & Mike Fiberboard sheathing, unidentified, ca 1975 (C) InspectAPedia & Mike

I am trying to identify this wall sheathing material. It appears to be an asphalt impregnated fiberboard sheathing, but does not have a brand name. This photo was in the attic at the gable end behind the chimney. The entire garage also uses this product. The home is a two story colonial build in 1975 in New England. Thanks for any help. - Mike - home inspector - 11/09/2012


Mike, I agree that in your photos the sheathing material looks like fiberboard sheathing, and that branding can be difficult. Take a look at our article [above] on this material for some colors and other properties.

Sorry I can't say more from just photos. But in any case fiberboard sheathing was not impregnated with asphalt but rather typically with a wax for water resistance. Some brands of fiberboard using a dark surface coating might fool you into thinking it was asphalt, but as you'll read in the common ingredients listed above, that's not so likely.

Because the properties of these various products are similar across brands (varying in density, coatings, water resistance, nail-holding power) I'm not sure we need to know the brand to evaluate the material shown in your photos - it's water damaged, meaning that we ought to be looking for related insect damage or rot or mold on nearby building materials, and we ought to be finding and fixing the leaks.

Thanks for the excellent photos - I'll post them here at InspectApedia to permit other professionals to comment.

Reader Follow-Up:

My client was very concerned about the water damaged materials. It appears that all of the viewable sheathing was in poor condition. There was evidence of a prior WDI treatment on the front porch where I noticed a series of drilled holes through the concrete slab. The basement is finished limiting the scope of my home inspection. - M.Q.


Thanks for the follow-up M. Indeed, to my surprise (as the materials were originally treated and sometimes even smelled waxy or like petroleum products) I've on occasion found insect infestation right in the fiberboard sheathing.

IAs you are a professional home inspector I imagine that we'd probably agree that the underlying questions are:

  1. Do we need to address an active insect infestation
  2. If there was prior treatment, was it performed properly and safely
  3. Is there accompanying structural damage that needs repair
  4. Is the sheathing serving as a nail base for siding - if so and if it is extensively damaged such that removal and replacement of siding become necessary, significant costs would be involved.
  5. Have the causing leaks been fixed

It is my OPINION that if there is no active infestation and no actionable structural damage, (you'd have pointed out obvious movement and warned about possible hidden damge not discoverable without demolition), no ongoing leaks, and if siding is nailed to studs not sheathing, then if there is no longer ongoing leakage, the impact of leaving the materials in place is minimal.

The tough question is whether there is enough visible damage (or discovered damage by perhaps some gentle invasive investigation) to justify more extensive exploration for significant problems.

Often, such as in some mold or IAQ investigations, we reach agreement with the client to stop cutting into the building by noting the absence of visible evidence of structural movement, damage, or infestation sufficient to justify, in our joint opinion, furher invasive inspection. That does not promise that there are no hidden concerns, but rather that we can't together find reason to justify continued hacking apart of the building.

If the client wants further investigation, I'd pick the "most suspect" areas for hidden damage and start there, reasoning that if the most-suspect areas don't reveal actionable trouble, it would be reasonable to quit exploring.


Celotex insulating lumber Fiberboard Sheathing Health & Environmental Questions

ASBESTOS: Is there Asbestos Content in Insulating Board Products such as Celotex, Homasote, Insulite?

Asbestos is not an ingredient in fiberboard insulating sheathing.


and at FIBERBOARD SHEATHING MSDS found just below.

OFF-GASSING: Question: off-gassing hazards from low density fiberboard roof sheathing vs MDF Sheathing

5/9/2014 Anonymous said:

Are there any cases of individuals suffering health issues from exposure to a fiberboard roof? I"m thinking of off gassing due to excessive exterior heat conditions.


Offgassing from fibergoard roof? Most likely you're asking about fiberboard roof sheathing, right?

I'm doubtful you'll be able measure significant offgassing from low density roof sheathing products in service such as soft density or low density fiberboard.

However more recently there are indeed MDF products sold for use as structural panels including roof sheathing. Details are at Definition & Characteristics of MDF Medium-Density Fiberboard

The article above on this page describes low density fiberboard products and explains that describes how that sheathing material is produced describes wood products and waxes. So to start we'll want to get clear what sort of roof sheathing you are actually describing.

Research on VOC Emissions & Building Materials

Here are some helpful citations that describe the ingredients, manufacturing process, and research of several types fiberboard products. You will find citations (including some I give below) of outgassing of MDF fiberboard products used in other applications (not the soft "Homasote®" type roof sheathing board):

  • Baer, Norbert S., and Paul N. Banks. "Conservation notes: Microenvironments." (1987): 301-305.
  • Corneau, Diane. "Effectiveness of barriers to minimize VOC emissions including formaldehyde." Forest products journal 56, no. 9 (2006).
  • Craddock, Ann Brooke. "Construction materials for museum storage." (1988).
  • Cane, Simon. "Museum density fibre board." Conservation news (United Kingdom Institute for Conservation of Historic and Artistic Works) 41 (1990): 13.
  • DeStefano, Jim, P. E., and David S. Gromala. "By Dirk M. Kestner, PE; Jennifer Goupil, PE; and Emily Lorenz, PE."
  • Gilbert, Nicolas L., Mireille Guay, Denis Gauvin, Russell N. Dietz, Cecilia C. Chan, and Benoît Lévesque. "Air change rate and concentration of formaldehyde in residential indoor air." Atmospheric Environment 42, no. 10 (2008): 2424-2428.
  • Gilbert, Nicolas L., Denis Gauvin, Mireille Guay, Marie-Ève Héroux, Geneviève Dupuis, Michel Legris, Cecilia C. Chan, Russell N. Dietz, and Benoît Lévesque. "Housing characteristics and indoor concentrations of nitrogen dioxide and formaldehyde in Quebec City, Canada." Environmental Research 102, no. 1 (2006): 1-8.
  • Love, Thomas. "Pressed ceramic fiber board and method of manufacture." (patent) US20020098336 A1 (2002)
  • Zhang, Luoping, Craig Steinmaus, David A. Eastmond, Xianjun K. Xin, and Martyn T. Smith. "Formaldehyde exposure and leukemia: a new meta-analysis and potential mechanisms." Mutation Research/Reviews in Mutation Research 681, no. 2 (2009): 150-16

DISPOSAL: Question: can we dispose of fiberboard by using it as mulch?

Is Celotex recyclable? We just removed some and I wondered if it could just be broken up on the ground like mulch or does it have chemicals in it. - Karen Bradshaw 7/25/11

Reply: fiberboard sheathing or insulating boards are not recommended as yard mulch

Karen the recycle-ability of fiberboard sheathing products like Celotex or Homasote is an interesting one. These products that are made principally of wood fibers or other plant fibers and a binder and are usually disposed of as construction debris. The properties of insulating fiberboard sheathing were thoroughly described by the U.S. Forest Products Laboratory in a 1955 report[15] as well as in original and current manufacturer's product literature and MSDS sheets.

But watch out: trying to break up any fiberboard product into small mulch like fragments risks creating an irritating or problematic dust hazard for eyes and respiration.

I'm unsure how well the binder or coating chemicals are bonded to the material (some products used paraffin), but I wouldn't use this product for mulch in any case. Some newer insulating boards may contain plastics and some older ones appear to contain bituminous coatings or binders.

You will find that the treatments used to make these insulating boards moisture resistant and to impart stiffness also mean that they will not break down or bio-degrade as a yard mulch.

MSDS data for fiberboard insulating sheathing products

In sum these are benign products for the most part, though wood dust particles from any wood material can be a potential hazard. For the specifics of your fiberboard siding you'll want to consult the MSDS of the particular product. For example:

  • Homasote 440 - quoting: Cellulose based material containing 1 to 2% paraffin wax, CAS 8002-74-2, and less than 0.1% copper base pesticide, CAS 39290-85-2. The product contains no known hazardous or carcinogenic components.
  • Knight-Celotex Fiberboard™ MSDS (Marrero Plant), MSDS No. 00040-85F, 4/19/2002, trade name Premium Fiberboard Insulating Sheathing, Manufactured Home USB, Coated 1 & 6 Sided, lists the following ingredients: Cellulose (<= 96%), Starch (<= 10%), Paraffin Wax (<= 2%), Carbon Black (<= 0.5%), Clay (<= 2%), and Lamination Adhesive (<= 3.5%). Original source: anoziraworks.com/uploads/Celotex_Fiber_board.pdf
  • StructoDek® high density roof insulation, laminated board & coated 1 & 6-sided, MSDS: 001-86F, 4/12/2005, Knight-Celotex LLC, Northfield IL Tel: 800-596-9699 lists these ingredients: Cellulose (<95%), Starch (<9%), clay (1%), Carbon Black (1%), Wax (1%), Laminating Adhesive (< 3.5%). Original source: usply.com/downloads/other/msds-structodek.pdf
  • Sturdy-Brace High Density Fiberboard, produced by Blue Ridge™ Fiberboard - MSDS: "Hazardous Components": Wood Fiber*. [In other words, Asbestos is not present].
    * *: Wood dust is listed by the IARC as a human carcinogen (Group 1). Structodek High Density Fiberboard Roof Insulation is defined by OSHA (29 CFR Part 1910) as an "Article". A manufactured item which is formed to a specific shape or design during manufacture which does not release or otherwise result in exposure to a hazardous chemical under normal conditions of use.

Mold Growth & Wood Boring Insect Susceptibility of Fiberboard Building Insulating Sheathing Products

We do not usually find mold growth on fiberboard building insulating sheathing nor insect damage to this material. Possibly the resin binder and coating is unattractive to insects and the moisture resistance of some coatings also reduce the ease of mold growth on this material.

Moldy Homasote insulating board sheathing (C) Daniel Friedman

However in sufficiently challenging conditions such as very wet conditions or prolonged exposure to water and moisture or insects, we have found both extensive mold growth on Homasote type insulating board (photo, below left, in a wet basement against a masonry wall) and evidence of insect damage to an interior wall fiberboard sheathing product, probably Beaver board or Upson board (in the attic of a leaky building, below right).

See MOLD on FIBERBOARD INSULATING SHEATHING our full article on mold growth on or in fiberboard sheathing.

At DRYWALL, PLASTER, BEAVERBOARD we provide the history of Beaver board and Upson board, and we discuss other non-structural interior sheathing boards that were used on building interior walls and ceilings.

INSECTS: Reader comment: insect attack through fiber-type flooring

Moldy Homasote insulating board sheathing (C) Daniel Friedman(Feb 27, 2013) Teresa Mietus said:

I live in a 50's home.

Termites literally built an entire empire between the top and bottom layers of a fiber type floor board.

If not for the mud tunnels, the floor would have fallen through

! What you don't see - can hurt you.

Question: Question about Homasote product from 1940s. Mold problems on gypsum board exterior building sheathing? Asbestos in exterior gypsum board sheathing products?

Gypsum board wall sheathing (C) Daniel FriedmanWas there a Homasote product used in the early 1940s on the exterior of houses that looks like sheet rock? How long does this last for?

If it is still on the house, could it contain asbestos and/or contain mold due to lack of sunlight?

Was there "code" at some point that would have forced individuals who were to replace vinyl siding on the house over these boards to replace with proper products after a certain date? Thank you, K.B.C.

Reply: Properties of Homasote® type fiberboard compared with gypsum-based exterior wall & roof sheathing boards

A competent onsite inspection by an expert usually finds additional clues that help accurately diagnose a problem with sheathing, leaks, and mold or asbestos sources in buildings - the concerns you expressed. That said, here are some things to consider:

Homasote® fiberboard sheathing is a wood fiber product, not a gypsum or plasterboard product. However there were indeed gypsum-based sheathing board products used on buildings both as wall sheathing (under siding and over studs) as well as roof sheathing.

Having inspected quite a few buildings that used this material, my OPINION is that it has proven surprisingly durable so long as it was kept dry. Wet the material can become soft, and also one might find mold growth on the paper backing of the gypsum board.

We describe these two different product types


at FIBERBOARD SHEATHING - Sheathing Celotex Homasote & Other (this article)

as well as an introduction in FRAMING MATERIALS, Age, Types where we describe the history of building framing and sheathing materials.

Some Gypsum board and plaster board products indeed contain asbestos as does some joint compound

Indeed some gypsum--based drywall products did contain asbestos into the 1980's. I have not, however, tested nor seen test results specifically for exterior wall sheathing using that material. I suggest sending a small sample, a square inch would be plenty, to a certified asbestos testing lab - the cost should be less than $50. Do let me know what you find as the results will be helpful to other readers.

Even when gypsum board or plaster board did not itself contain asbestos, some joint compounds did contain that material right up into the 1980's. But used as an exterior sheathing, at the buildings I've seen, there was no top coating of joint compound and tape on this type of sheathing board (as there would be on drywall used for interior wall coverings).

Mold growth on gypsum board building sheathing?

About mold growth: the simple absence of light is not sufficient to cause problematic mold growth in building cavities.

See details about mold growth on fiberboard sheathing

See MOLD EXPERT, WHEN TO HIRE for help in deciding if in your particular case hiring a competent professional to inspect and test the building is justified.

Requirements to Remove Old Siding & Sheathing When Re-Siding a Building?

Finally, I've not found any national building code that requires a homeowner to replace one existing siding or wall sheathing material with another. The decision on siding-over existing surfaces vs. doing a (more expensive) tear-off depends on at least these considerations:

  • The condition of the structure beneath the siding and sheathing. Siding and sheathing may need to be removed for structural repairs, for example.
  • The condition of the siding and sheathing itself: very rough siding in poor condition forms a bad base over which to install new siding materials. The installer might simply nail furring strips over the existing walls, then install the new siding to the furring, or s/he might recommend a tear-off.
  • Aesthetic/cosmetic concerns: when you build out a building's exterior walls with furring and new siding, the thicker wall will usually extend past existing window and door trim - giving the windows and doors a "sunken" look that you might not like. A solution is to build out the building trim at the same time.

    Some builders install wood or foam backer over the existing trim and wrap the new opening in aluminum so that it projects out past the new siding.

Watch out: be careful not to add multiple vapor barriers to a building wall. Installing an air and moisture resistant vapor-permeable housewrap should be fine however, and is required by some building codes and some product manufacturers.


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