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Fiberboard sheathing © Daniel Friedman at InspectApedia.comFiberboard Sheathing Ingredients
What is Fiberboard Insulating Board Sheathing Made-of?

  • POST a QUESTION or COMMENT about fiberboard building sheathing: how to identify fiberboard products, fiberboard uses, fiberboard, Celotex, Homasote, Insulite, Temple, & other brands, fiberboard ingredients, does fiberboard contain asbestos?

What are the ingredients in fiberboard insulating sheathing or wall, ceiling, or roof sheathing used on or in buildings:

Ingredients of fiberboard sheathing.

This article series 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.

InspectAPedia tolerates no conflicts of interest. We have no relationship with advertisers, products, or services discussed at this website.

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What is Fiberboard Insulating Sheathing Made Of?

Fiberboard wall sheathing post flooding © D Friedman at InspectApedia.com

While wood pulp is and was the most frequently-used raw material in the production of fiberboard in rigid sheets as a building material, the range of materials used to produce plant-based boards and "lumber" used in construction is broad, including

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]

[Click to enlarge any image]

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

As we explain at FIBERBOARD SHEATHING, the home page for this topic, 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.

This fiberboard sheathing product is used as structural paneling, insulation, concrete pouring forms, and expansion joints.

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

    Also see the definition at LDF LOW DENSITY FIBERBOARD
  2. Medium density (Medex, Medite)

    Also see the definition at MDF Medium-Density Fiberboard
  3. High density (Masonite, Upson board, Marinite, and some Homasote products)

    Also see the definition at HDF HIGH DENSITY FIBERBOARD

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, also discussed at SHEATHING, FIBERBOARD where you will find a complete list of fiberboard manufacturers, include:

Fiberboard sheathing, also called black board, black fiberboard, 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.

Keep in mind that fiberboard products vary in color, thickness, density, texture, and coatings. Black fiberboard will often be coated in an asphalt water-resistant layer but other fiberboard, especially for indoor use may be tan, gray, brown, etc.

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.

Ingredients & 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.

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

In "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]

For example:

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Reader Comments, Questions & Answers About The Article Above

Below you will find questions and answers previously posted on this page at its page bottom reader comment box.

Reader Q&A - also see RECOMMENDED ARTICLES & FAQs

Question: is NuWood insulating sheathing hazardous?

2020/04/01 Craig Ungerman said:

We own a house that was built in 1939 that Nuwood waterproof insulated sheathing was used.

Can you tell me what materials was used in it specifically is there anything toxic or cancer producing that I should be aware of?

This Q&A were posted originally at IDENTIFY NU-WOOD PANELS

Reply: Ingredients in NuWood and similar fiberboard products

Hi Craig

My OPINION is that the principal hazards from un-disturbed, intact NuWood or similar fiberboard insulating sheathing on homes are below the limits of detection. The material is principally wood fiber and in older products, waxes or coatings, located where it is not exposed to contact, nor even to weather: on the exterior of building framing and covered by a siding material.

However if you are demolishing or renovating a home with fiberboard or NuWood sheathing, there could be wood-dust hazards (see our IARC citation above), with perhaps lower levels of airborne plastics or coating materials and, depending on the age, condition, and history of the building, even more-hazardous chemicals such as now discontinued pesticides that might have been applied in some areas.

In general, as you can read in more detail above, wood-based "fiberboard" is a product made of wood fibers bonded together with a resin adhesive and, depending on the process, water, heat, and/or pressure.

A reason that fiberboard products came into production in the first place was to make use of wood byproducts such as sawdust, shavings, bark, and cutoffs that are residue from the production of lumber and other products.

Additional ingredients include paraffiin (wax) as a component to prevent the fibers from clumping during production of fiberboards, and in some products also as a water-resistant component, along with adhesives such as UF - Urea Formaldehyde (interior fiberboard products) or Phenol formaldehyde (PF) (exterior-use fiberboard products) in modern fiberboard and hardboard products.

Some fiberboard products, particularly hardboard in modern products may also be coated with a clear protective resin film, often polyethylene orother polyolefins, possibly polypropylene or other copolymers, and occasinally by PVC (polyvinyl chloride) or even nylon either alone or in combination to provide a protective surface. Coatings are applied as a laminate to the hardboard surface.

Specialty fiberboard or hardboard products are also produced incorporating a covering or layer of aluminum foil, paper, and polyethylene plastic.

Two different processes are used: wet process using water to distribute the wood fibers as they are formed into a mat, and a dry process that is similar but omits the water, often using heat and pressure.

Please take a look at IDENTIFY NU-WOOD PANELS where we describe NuWood and its ingredients. Don’t hesitate to ask follow-up questions.

You'll also find some NuWood PDF files and scholarly articles that describe the product in more detail.

Our patent citations throughout this article series also often add details about the production process and ingredients in fiberboard products including NuWood.

Additional references on construction & components of fiberboard are given above.


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