Collapsing building © Daniel FriedmanConstruction & Renovation of Buildings with Fiberboard Sheathing

  • SHEATHING FIBERBOARD CONSTRUCTION - CONTENTS: how to repair, add-on-to or remodel buildings at which an insulating board sheathing product was use on exterior walls or roof or on the building interior walls and ceiligns.
  • POST a QUESTION or READ FAQs about buildings at which an insulating board sheathing product was use on exterior walls or roof or on the building interior walls and ceiligns.

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Building on or remodeling structures where insulating board sheathing products were used.

FAQs about repairing, remodeling, or adding on to buildings where wall or roof sheathing is an Beaverboard, Celotex, Homasote, Insulite.

This article provides questions & answers about procedures to repair, remodel, extend, or add-on to buildings where an insulating board sheathing product was use on exterior walls or roof or on the building interior walls and ceilings.

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Construction & Renovation at Buildings with Fiberboard Insulating Sheathing

Fiberboard sheathing construction sketch - Derek TerveerQuestion: uninsulated building, no vapor barrier, just fiberboard

(Mar 3, 2014) Re-posting without link (image to be added to article & comments above) derek terveer said:

[Click to enlarge any image]

House: 1983 twin home in Minneapolis suburb.

Exterior wall between floors (9" vertical) has no fiberglass insulation or vapor barrier and thus the fiberboard sheathing is exposed to internal air and vapor. Total area is approximately 30 sq ft. We get temps down to -15F in the winter.

See diagram given here.

1) Is this common construction 'technique' from the 80's?

2) I am concerned about water damage from condensing water vapor.

3) I am concerned about heat loss.

4) What is the standard mitigation technique and cost?

5) Is this worth worrying about?


Derek I agree that continuing the housewrap over the band joist is a better practice

See this InspectApedia article:


for details.

In Steve Bliss's Best Practices article series provided at InspectApedia we argue for housewrap over the band joist not because of air penetration through the joist itself (nonsense!) but because of possible leaks at the framing connections & interfaces; I'd prefer to see housewrap over the rim joist or band joist, and indoors we'd insulate on the interior side of that framing member to avoid a cold band of heat loss around the home.

Yes, the method you described was widely used if not the absolute most energy efficient. A thermal scan of the building exterior would get us past arm-waving and on to an actual assessment of the heat loss at your building from this feature and might also point out other heat loss points of greater concern.

Regarding improvements: it'd be a costly deal to R&R siding just to add housewrap over this feature, but insulating from the interior is likely to be worthwhile; if you used foam insulation that'd also address air leakage.

Reader Follow-up Query: fire code on fire resistance

(Apr 18, 2014) derek terveer said:

I see this article states "If the boards are exposed in an occupied space, fire codes will require that a fire resistant finish surface such as drywall be installed."

In my case, I have noticed that there is exposed fiberboard in an empty chimney chase (outside the living space), with a piece of foam over a portion of it (because the original builder didn't insulate the wall covered by the chimney chase).

1) Does the entire exposed area of fiberboard on the wall adjoining the living space inside the (empty) chimney chase have to be covered with a fire resistant surface? There is a lot of it and it would be quite difficult to reach - requiring a ladder built up within the chase itself.

2) Does any kind of fire resistant surface have to be installed over the foam that has been screwed on to the outside of the fiberboard?

The chimney chase has a sheet steel cap.


Derek, thanks for the interesting fiberboard v fire exposure question. As the interior of a chimney chase is not occupied space, different construction guidelines apply.

But it would make sense to check with your local building department about the allowable materials for a chimney chaseway and while you're at it, as you're describing exposed foam therein, mention that as well, and finally, check the requirements for fire blocking in the chimney chase against how yours was constructed.

Reader reply:

The thing that I'm afraid of is that the city may make me firerock the entire inside of the chimney chase, which would be an awful lot of work. It is pretty constricted space inside and I'd probably have to end up building a temporary work platform of some kind inside the narrow chimney chase.

There were a number of things in this house that were not built to code, even back in the 80s (1984), so I don't know if the original builders (long bankrupt) were cheating on this or if it was allowed by code at the time.

As a compromise, I'm thinking of screwing a sheet of decorator drywall (3/8") on top of the foam to retard ignition (165F, I believe) and fire promotion. It should never get that hot in the chimney chase so it is mostly to retard oxygen and promotion.


would test the worry about what the city might make you do against the loss against which the codes are trying to assure. In my experience people who approach code enforcement officials asking for help often find officials cooperative and happy to find an acceptable solution to the concern. Doing a half-baked job that might fool someone later into thinking that the proper installation was made could be very dangerous in several regards.

Start by finding the answer to the question.

Question: Why do I only see fiberboard insulating sheathing on older homes? How do I deal with post-fire and smoke odors?

I have a home that was damaged by smoke and water and has fiberboard wall sheathing. There is an issue as to the affect of smoke and water on this product. It seems to be porous and susceptible to the water and smoke. How would the front side of this product be cleaned and dried out.

Why is this product not as popular as before.? I only see it on homes 30 years plus - Blaine Jelus 11/17/2011

Reply: Standard post-fire odor control sealants


Regarding the shift away from using fiberboard sheathing in homes, I have not yet found an authoritative account but I can offer several probable explanations:

First it should be noted that bagasse (sugar cane) based insulating fiberboard sheathing continues to be produced in the U.S. at least by Celotex Inc., as you will read in CELOTEX HISTORY & PRODUCTS.

It appears that a common contemporary (2012) application of this product is in the construction of mobile homes. Product literature for Blue Ridge Fiberboard describes Celotex SturdyBrace® for use in wood frame construction as well.[2].

A 1955 U.S. FPL report offers the early history of growth in the use of insulating fiberboard sheathing and is quoted in the article above. By 1950 the product was used extensively in some areas of the U.S.

I believe that rising energy costs that came in several waves after the 1950's and especially beginning in the Arab oil crisis in the 1970's led people to focus on much higher levels of building insulation in the attics and walls of typical wood frame residential buildings than the less than R-2 provided by 1/2-inch insulating fiberboard sheathing used alone (as it was at first).

So a combination of plywood (later OSB) structural sheathing and the placement of usually 3 1/2" fiberglas batts in 2x4 walls or 6" batts in 2x6 walls gave both structural stiffness and a much higher level of insulation (R11 to R19 as opposed to about R2-3). There was less incentive then to continue to use fiberboard as the primary building sheathing.

However some producers, including Celotex, have continued to produce newer insulating board products including those made of open or closed cell foam covered by a paper or foil surface, for example Celotex XR-4000 or Celotex CG-5000 foil-faced polyisocyanurate foam board. These products are still lighter than fiberboard and offer still higher R-values.

Blaine: Fiberboard insulating sheathing does not, in my field experience, pose more of an odor or smoke absorption problem than other wood-based building materials or even drywall.

Regarding post-fire deodorizing of a building that used fiberboard wall sheathing, at fire jobs I've inspected, regardless of the building materials used to construct wood framed walls (plywood, OSB, wood studs, or wood fiberboard insulation) in areas of smoke and fire odor problems the contractors often conclude by coating the wall cavity with a paint intended as an odor sealant - this step prevents persistent odors that may linger and annoy building occupants after the finish surfaces have been replaced.

Question: Should I remove Georgia Pacific fiberboard sheathing when re-siding my home?

I am having new vinyl siding put on my home. This GP Sheathing was underneath. Do you recommend leaving it and residing the house or pulling it off and putting newer material up? It was built in 1982. Thank you! - Robert 8/17/11

Reply: Why?

I'm not sure why you'd need to remove old sheathing boards when installing siding, but if you did so you may find that you need to install plywood or OSB or another wall sheathing product in its place. Provided the original sheathing was un-damaged, you're not gaining anything that I can see.

But you may need to install house wrap and use tape-flashing around windows and doors as part of your vinyl siding project. See VINYL SIDING INSTALLATION and HOUSEWRAP AIR & VAPOR BARRIERS for details. And see our housewrap warning note just above.

Question: how do I repair a small hole in fiberboard sheathing

How do I repair a 2 inch hole in celotex? - Jerry 7/30/12



If the wall cavity is open to access you can add insulation of just about any sort: fiberglass, foam, or even a scrap of celotex or similar fiberboard insulation. Frankly, if the wall is insulated and that exact spot in the fiberboard sheathing is not required as a nail base for siding, just repair the exterior siding and you should be fine. You can leave the fiberboard alone.

Question: how do I install flanged replacement windows on a fiberboard sheathed building?

Can flanged replacement windows be installed over 1970's era fiber board sheathing? using the same general installation method that is used if the exterior were sheathed with plywood or OSB? - Walt 9/17/2012


Walt, it should be fine as long as there is framing to nail to on all sides, and flashed against leaks. If not you'll need to frame in or fir out a rough opening for your windows to fit the replacement unit.

Question: fiberboard sheathing used behind brick veneer wall gets wet - is that OK?

I have fiberboard installed behind the brick veener of my house. Both faces are black, but the middle of it has the natural fiberboard color. The fiberboard start at the top of the brick veener wall (1 1/2 inch air gap) and stop at the foundation in front of the sill plate

. If some rain goes behind the brick, does the fiberboard will resist over the long term? I was surprised to realize that at a specific section of the sill plate, there were some signs of past water infiltrations. - Phillipe C. 10/31/12



Fiberboard is generally treated with a moisture repellent and can tolerate a rare wetting event but in my opinion was not designed for frequent wetting. It is common for a brick veneer wall to be constructed with a drainage plane behind the brick. Be sure that the weep openings at the bottom of the veneer wall are open so that you don't accumulate water (inviting insects into the structure).

Question: Can I glue carpet on to Fir-tex carpet board underlayment?

I have a floor underlaymet band name "fir-tex carpet board" can I glue down carpet to it? and if so what is the process? thank you - V. Tedesco 11/8/2012


V. tedesco,

Low density fiberboard will not bond well to carpet adhesive, and some LDF fiberboard coatings using wax may prevent adhesion at all. Harder fiberboard products, uncoated, may accept carpet mastic applied with a notched trowel ok.

Question: does insulating fiberboard meet the 2009 IRC/UBC requirement?

(Dec 10, 2012) Anonymous said:

We have a insul board installed on the exterior walls of our 1 story house. We are addting a second floor additon. Will the insul board meet the 2009 IRC/UBC requirements?



Insulating board sheathing is code-approved provided that the structure has the requisite bracing. Since final construction code approvals are the authority of the local building department, that's whom you should ask - as there may be local codes that pertain.

Question: found black exterior sheathing on 1974 home - thinking I should replace it with OSB?

(Mar 27, 2014) Charles said:

I have recently discovered that my home built in 1974 has black fiberboard exterior sheating, when I pulled off some of the pink fiberglass insulation, I noticed that there was quite a bit of mositure, as I am redoing the whole house, this was the same in the kitchen and bathroom.

So basically the fiberglass insulation facing the fiberboard as well as the fiberboard facing the inside of the house is wet, even frozen in some spots. After removing the insulation overnight, the fiberbaord dried up in the spots that I pulled off the insulation. I have aluminum siding and plan on residing the house with vinyl in the summer or early spring, I am thinking I should replace the fiberboard with OSB? Any thoughts on this issue.



Unless the fiberboard actually absorbed water, can't be dried, and / or is damaged, replacing it is not likely to be cost effective nor necessary. Unless there is a serious mold problem I'd leave the sheathing in place.

THese products were treated to resist moisture uptake, so perhaps the moisture you see is on the surface.

Instead, focus first on where that moisture came from.

If there are leaks in the exterior wall you'll still have a problem.

If the problem was leaks inside the wall - say from plumbing - those need to be repaired.

If the problem was (as is likely) the absence of a working vapor barrier or more likely, air leaks into and out of the wall, those are what need to be addressed when repairing the building.

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.

Reader Question: how do I apply stone veneer to fiber board sheathing?

6/19/2014 Amy VonArx said:

What are the spec's for putting stone veneer on fiber board sheeting? Do I have to cover with OSB or plywood sheeting first? My house was built in 1964 and has this on it. I want to stone veneer the front porch area.


Amy, masonry veneer walls are typically supported by a structural ledger of steel, or by a foundation wall, and are also secured by mechanical connections to the building structural wall. There are also requirements for moisture barriers, weep or drain openings, and other derails.

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[This question appeared originally at FIBERBOARD SHEATHING]


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