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Fiberboard sheathing (C) Daniel FriedmanFiberboard Sheathing Ingredients
What is Fiberboard Insulating Board Sheathing Made-of?

  • FIBERBOARD SHEATHING INGREDIENTS- CONTENTS: what are the constituents of fiberboard sheathing board used on building exteriors and in building interiors
  • 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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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.



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What is Fiberboard Insulating Sheathing Made Of?

Fiberboard wall sheathing post flooding (C) D Friedman

The 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]

[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 left, 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
  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.

MSDS data for fiberboard insulating sheathing products gives ingredients

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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