Gypsum board wall sheathing (C) Daniel Friedman Fiberboard Building Sheathing Water Protection
Protect Insulating Board Sheathing From Water Damage

  • PROTECT FIBERBOARD from WATER - CONTENTS: importance of protecting Celotex®, Homasote®, Masonite® Nu-Wood® and other insulating fiberboard building sheathing products from water or from getting wet or soaked
  • 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?

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

Protect fiberboard insulating sheathing from water & water damage:

This article explains the importance of protecting fiberboard sheathing and insulating board from getting soaked - water can damage fiberboard products.

We also answer questions about fiberboard water resistance, fiberboard recycling.

This article series includes the definition, ingredients, history, use, fire resistance & insulating properties of fiberboard sheathing. This article series also 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.

Green links show where you are. © Copyright 2017, All Rights Reserved.

Importance of Protecting Fiberboard Insulating Sheathing from Water

Celotex insulating lumber ad

[Click to enlarge any image]

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.


Moldy Homasote insulating board sheathing (C) Daniel Friedman

Watch out: while fiberboard products, especially sealed or waxed products are rather resistant to mold growth, soaking the material can lead to mold contamination problems, as we illustrate at FIBERBOARD SHEATHING MOLD CONTAMINATION


Continue reading at FIBERBOARD CEILING & WALL COVERINGS or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.



Or see this

Article Series Contents

Suggested citation for this web page

PROTECT FIBERBOARD from WATER at - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.


Or use the SEARCH BOX found below to Ask a Question or Search InspectApedia


Frequently Asked Questions (FAQs)

Click to Show or Hide FAQs

Ask a Question or Search InspectApedia

Use the "Click to Show or Hide FAQs" link just above to see recently-posted questions, comments, replies, try the search box just below, or if you prefer, post a question or comment in the Comments box below and we will respond promptly.

Search the InspectApedia website

Comment Box is loading comments...

Technical Reviewers & References

Click to Show or Hide Citations & References

Publisher's Google+ Page by Daniel Friedman