Moldy Homasote insulating board sheathing (C) Daniel Friedman

Mold Growth on Fiberboard Building Sheathing & Insulating Board Sheathing Products in Flooded Homes

  • MOLD on FIBERBOARD INSULATING SHEATHING - CONTENTS: Mold contaminated fiberboard sheathing: how much insulating board should be removed from a home after flooding?Does insulating board or fiberboard support mold growth?How to recognize Celotex®, Homasote®, Masonite® and other insulating board building sheathing products. Repairing or replacing "blackboard" fiber insulating board on homes
  • POST a QUESTION or READ FAQs about mold on fiberboard building products

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Fiberboard mold: occurrence & detection of mold contamination on fiberboard & wood-fiber-based insulating board sheathing.

This article discusses flood damage and mold damage on fiberboard insulating products and building sheathing products such as Homasote, Celotex, and Masonite insulating board sheathing.

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

Moldy Homasote insulating board sheathing (C) Daniel FriedmanWe 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.

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, 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).

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.



Reader Question: Question about Homasote product from 1940s & Mold problems

Was 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?

Gypsum board wall sheathing (C) Daniel FriedmanIf 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 plaster-board 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 and resistant to mold and rot 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)

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. Water or high moisture would be a requirement for nearly all indoor mold contamination. And indeed I have found mold growth on paper backer on plaster-board exterior wall sheathing, in the wall cavity in buildings where there had been leaks into those spaces.

So if your home's walls were leaky (from ice dam leaks at the eaves, from leaks around windows or doors, from wind-blown rain penetrating damaged siding, etc) then there might be problematic mold growth on those surfaces - in the previously or currently wet areas. Whether or not this problem deserves investigation and remediation is not something one can decide without more detailed information.

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.

Question About Mold on Celotex Building Insulating Board Following Hurricane-Caused Flooding


We are survivors (not victims) of the Tennessee flood that came on May 1st 2010. Water was in our home for 34 hours including the 1st level (basement) and 3 feet on the main level.

We have taken everything out of the building, removing interior materials down to the studs (walls) and joists (ceilings and floors). Our Insurance is ready to make their first offer.

They asked about the sheathing saying if it was chip board it would need to be removed. Or plywood should be OK if ventilated correctly. Well, it's nether, occurring to your website it's Celotex™, the back is smooth while the inside is fibrous. I cut a chunk out and it's wet. Should that come out? I say yes. The home was built. 1970s

Insulating board in Tennesse flooded home (C) Daniel Friedman Insulating board in Tennesse flooded home (C) Daniel Friedman

How do you replace wet fiberboard insulation? Remove outside brick? Remove the stud walls. Surely not!

Also FEMA says the basement is the 1st level of the home, because it has a door to enter & exit to the outside. Insurance saying it's a basement. (which covers nothing). If you are unable to do this Pro-Bono, we understand, and we thank you for your web site. - T.


OPINION: Since we have certainly seen that Celotex and other fiberboard insulation products can sometimes support mold growth, the wet insulating board should be removed. A photograph of moldy insulating board in a flooded home is shown in our photo at left and as discussed also
at MOLD GROWTH & WOOD BORING INSECT Susceptibility of Fiberboard Building Insulating Sheathing.

Also see INSULATION MOLD CONTAMINATION TEST where we describe non-visible but significant mold contamination found in fiberglass insulation that has been wet or exposed to high moisture or high mold levels.)

Watch Out: In this case someone recognizing the great cost of redoing the brick veneer may want to try installing a very very secure barrier between the celotex and building interior - it might work but this questionable approach risks leaving a mold problem in place - something that could later be a problem for building occupants, especially people at extra risk such as asthmatics, elderly, infants, immune people.

Watch Out: Leaving wet insulating board in place and then re-building over it creates a very high risk of later hidden mold in building walls and ceilings. It is tricky to decide that the insulating board is really dry throughout as its dryout can take weeks, even months where the insulation is covered by a brick exterior wall, by contacting wood framing, or by other materials.

Watch Out: The right hand photo shown above tells us that interior drywall or plaster were removed only for the first four feet of the flooded wall.

This is about 12" above the high water level in the building. There is a good chance that water wicked higher into building materials including the drywall, insulation, and fiberboard insulation in these walls. Leaving wet, possibly moldy insulation in place also leaves a future property resale difficulty as the home may be "stigmatized" as not having been thoroughly cleaned and repaired following a major flood.

How is Wet Insulating Board Removed Where a Masonry Veneer is Installed Outside?

Temporary support wallUnfortunately to completely remove the wet insulating board from this building will be a costly and destructive task. You probably don't have the option of "removing the studs from inside" since the brick veneer is typically tied to and thus supported by the wood framing.


The illustration at left, contributed by a reader's contractor or insurance provider, illustrates the placement of a temporary support wall to allow repairs to the building's exterior wall.

It is important that the brick ties are re-connected securely to the new or replacement framing.

If necessary, retrofit veneer wall ties can also be installed from outside of a building by drilling and inserting patented anchors through the veneer wall and into the supporting structure behind it.

The contractor and other parties involved as well as local building officials need to agree on a safe procedure for the particular structure involved. But here are some general comments about how insulating board is removed from a building where an exterior veneer of brick or stone are installed.

A temporary support wall is constructed inside the building parallel to the wall where the insulating board must be removed. The support wall involves a top header, support posts, and often a sill plate that distributes its load safely across the floor below or down to a basement slab or footings.

We have repaired a brick veneer building from the inside (for example in a non-flood case of extreme termite damage) by working carefully on the interior side of the veneer wall, in small sections so that there was not a risk of wall collapse.

We removed and replaced damaged wallboard, insulation, and studs. New studs and sill and top plates were installed and metal ties were reconnected to the studs to continue to secure the brick veneer wall. It was technically not difficult, but it was a labor intensive procedure.

The temporary support wall allows the contractor to remove sections of exterior wall studs (and the insulating board between the wall studs and the exterior masonry veneer) without risking the ceiling above collapsing. The support wall is spaced far enough in from the original exterior wall to give access to the original studs and wall, but close enough to the outside wall to safely support the floor above - often just a couple of feet.

The support wall is usually constructed using a heavy beam or header rather than a simple flat 2x4 top plate. The beam allows the temporary wall's supporting posts to be placed 4 feet or more apart, giving plenty of working space.

How Much Insulating Board to Remove?

You might know better just how much of the insulating board needed to be replaced above whatever has been soaked if you have a very thorough inspection and testing for mold, including test cuts to check the hidden side of the insulating board (such as Celotex™ material) in the most-suspect locations where moisture may have been present.

Where Does Mold Occur on Insulating Board?

Where we have found insulating board products such as Homasote or Celotex to be moldy, it has been in areas of flooding that wet the board.

If none of the dry, un-damaged insulating board is found to be moldy, where it is left in place, the seal up approach or use of a fungicidal sealant on dry surfaces might be fine. If you can you send us photos of the home inside and out we may be able to comment further on where to look for hidden problems. But the best approach is to bring in an experienced inspector who knows where to look and how to test building materials for wetness, dampness, or mold.

What Are the Structural Differences Between Chipboard, OSB, and Insulating Board?

You won't find "chipboard" used as building sheathing. "Chipboard" is compressed sawdust-like material, often used for shelving. But you might find oriented strand-board or OSB sheathing in place, depending on when your home was built. That is probably what your insurance company meant by "chipboard" sheathing. Ask your Insurance company if their issue with OSB sheathing that has been soaked is a structural concern or a mold concern.

Insulating board products such as Celotex™ or Homasote™ are not structural. Where those products were used as wall sheathing you'll typically find either diagonal wood or metal bracing at the building corners, or structural plywood used in those locations.

Bottom Line: Don't Leave Wet or Moldy Materials In Place After Building Flooding

But in any case don't leave a mold reservoir in place in a building following flooding. Doing so simply risks having to do the whole job over again later.

Bottom line, we expect that in your case, since you described soaking wet insulating board materials, the Celotex™ needs to come out because of the mold concern. Any attempt to reconstruct the building while leaving soaking wet material in place will very likely produce a new wall or ceiling cavity mold problem and the whole job would end up needing to be done again.

Finally, your insurance company may define the lowest floor of your home as "basement" in order to reduce their loss coverage expense. But if the lowest floor opens to grade on at least one side, and was finished as living space, especially supported by FEMA's own description, you may be able to satisfy your insurance company that this was code-approved living space not "basement" (a non-living space area).

There are no fees to consult with us regarding natural disasters such as area flooding.


Continue reading at MOLD / ENVIRONMENTAL EXPERT, HIRE ? for more about these questions and their answers.



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