Construction & Renovation of Buildings with Fiberboard Sheathing

Q&A on using fierboard insulating sheathing

OK to leave Fiberboard Sheathing in Place when Installing New Siding?

An InspectApedia reader asked us:

My Home was built in 1970 and has 2x4 exterior studs and fiberboard sheathing.

Over that it has original wood siding and vinyl siding over that. I want to install new windows and vinyl siding but I'm unsure how to go about it.

I want to demo all the way down to the fiberboard and add 1" of rigid foam (increase my R-Value), install a vapor barrier, and then vinyl siding.

So my concern/question comes down to the fiberboard, because I know you can't nail to it. Should I leave the existing fiberboard and just nail through to the studs underneath? On 2021-01-22 by Jon

Answer by (mod) - why remove fiberboard sheathing during home re-siding?

Why not leave the fiberboard - it has some insulating value and simplifies the job, and nail to the studs.

The downside turns on measuring the total new vs old wall thickness vs. the window and door trim amount of projection out from the walll; aesthetically if the window or door trim end up receding into the finished wall you'll want to build out the trim.

So if you go ahead and remove the fiberboard and everthing right down to the studs you'll want to either install all plywood exterior CDX sheathing or at least use that or diagonal bracing at all building wall corners.

Other readers, be sure to see

SEALANTS & PAINTS FOR FIBERBOARD SHEATHING - to get flashing tape to adhere to the fiberboard surfce

also see

FIBERBOARD SHEATHING IDENTIFICATION

 

Fiberboard Sheating as Insulation

What to Do with an uninsulated building, no vapor barrier, just fiberboard

Question: How should I improve the insulation value of our home sheathed in fiberboard?

Details:

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? - (Mar 3, 2014) derek terveer

Answer: Housewrap Suggestions for an Existing Home

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

See this InspectApedia article:

VAPOR BARRIERS & AIR SEALING BAND JOISTS

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.

About fiberboard's R-value, fiberboard has an insulation value of about R 2.8 per inch of thickness;

see our detailed R-value table at FIBERBOARD PRODUCT INSULATION VALUES

Reader Follow-up Query: fire code on fire resistance

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 terveer

Reply:

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.

For a more-detailed understanding of this question

see FIBERBOARD SHEATHING FIRE RATINGS 

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.

Reply:

I 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.

 

Insulating Value of Fiberboard Sheathing in an RV

Our reader asked:

We are doing a bus conversion to RV. And we brought the sheathing when store had no foam insulation for our walls.

Now I am concerned this may not be a good idea for insulating walls on a bus. Am I correct? On 2020-11-04 by Dena - converting a bus to an RV

Answer by (mod) - R-value of fiberboard for use on an RV?

Dena

Fiberboard sheathing has some insulating value, though typically less than foam board of the same thickness.

You will want to check with your building department and any other experts you can find about fire-safety; for many insulating boards you're require to cover the board with a fire-resistant material.

A modern approach to insulating RVs = recreational vehicles = uses spray foam insulation: more costly to have installed but perhaps very much more effective at cutting air leaks - the top source of un-wanted heat loss or in the cooling season heat-gain.

see

• Valenzuela, Jose. Environmentally Responsible Nomads: A guideline for the renovation of a recreational vehicle using sustainable principles [PDF] (2016).
  Abstract:
  
  There are people who would like to renovate an old recreational vehicle to create a personalized travelling or living experience. Although the restoration of recreational vehicles can be done in many ways, there are not many guidelines or design books to direct the renovation using sustainable principles.
  
  The general public usually does not know appropriate considerations, processes, systems, and methods for the sustainable redesign of a recreational vehicle.
  
  The goal of this thesis is to provide a set of design guidelines to direct the renovation process of a recreational vehicle in a way that the vehicle is in harmony with its ephemeral environment.
  
  The guidelines include material suggestions, insulation properties, passive solar techniques, heating and cooling, electronic appliances and devices, water and waste management, and photovoltaic energy options. They also provide design principles, used in interior design, that deal with issues of space management and multi-use of furniture.
  
  The guidelines developed and described in this thesis are demonstrated by using an existing recreational vehicle shell and conceptually renovating its interior and exterior.
  
  The demonstration uses hypothetical appliances and equipment due to budget constraints. - retrieved 2020/11/04 original source: etd.auburn.edu/bitstream/handle/10415/5002/Jose%20Valenzuela%20-%20thesis%20-%20final.pdf?sequence=2&isAllowed=y
  
  Available at https://inspectapedia.com/Manufactured_Homes/RV-Converstions-Energy-Efficient-Valenzuela.pdf
  
  

Also see this industry source of RV Standards and Regulations

• rvia.org/standards-regulations/laws-and-regulations

where you may find some useful resources.

What Kind of Flashing to Use with Fiberboard Sheathing?

I am doing a pretty extensive remodel to my 80s house, which has fiberboard sheathing. The sheathing appears to be about 1/2" thick. There is no flashing around the existing windows or doors. I plan to replace the windows and the siding.

The new windows are flanged, just like the originals, but unlike the originals, I want to flash the openings.

I also want to install a house wrap of some kind. But I'm not sure what sort of flashing to use with this sheathing, and I'm not quite sure how to install house wrap over it, as the sheathing has no nail holding ability.

Does anyone know, will self-adhesive flashing tapes adhere to fiberboard sheathing? And how do you install house wrap over it?

Will nailing the house wrap to only the studs be adequate?

I've contemplated removing the sheathing, or sheathing over it with plywood, but I am curious to hear what others think. Thanks for any thoughts. 2019-03-08 by Leo

Fiberboard Sheathing Flashing Tip by (mod)

I have not found good adhesion between adhesive-flashing tapes and fiberboard; heck it doesn't even always stick to plywood nor to housewrap.

Though I like these stick-on flashing tapes, for this case IMO You'll be better served to carefully apply housewrap and metal flashings over those windows.

See details

at PEEL & STICK FLASHING MEMBRANES

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

Reply:

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.

See also

FIBERBOARD SHEATHED WALL BRACING

What Sealant/Paint to Use when Repairing BuildRite & Similar Fiberboard Sheathing

Our reader JP CHarlebois, a builder and remodeler, asked:

I’m looking for a good product to prime or seal the surface of bildrite or other fiberboard sheathings.

When we do repairs or new window/door installs I want a way to help our flashing tape to adhere without easily peeling the top layer of fibers off.- JP Charlebois JP Handyman & Remodeling Mobile 612-363-0398 - MInneapolis & St. Louis Park, Minnesota, Email: jphandyman@mac.com - 6 April 2025

Reply by DF:

That's an interesting question. I remember having the same problem myself. I even tried different flashing tape materials to see which ones would stick best.. I would do some experimenting. Try using a spray lacquer primer sealer like BIN or Enamelac.

If that works it will have the advantage that it dries very quickly. Send me some photos and let me know how that works.

Of course best is to cover the building exterior with housewrap and use flashing tape as we show above, but a nice installation detail is to seal the fiberboard around building openings, then apply flashing tape.

JP followup remark: we use oil based primer on fiberboard

Thank you, we did do some oil based primer and that seems to make a pretty big difference. We also use the zip flashing liquid applied stuff and that seems to stick pretty well to the faces of the blackboard.

The uncoded fiber board piece that was on this particular wall and any of the areas where the black had been scraped off from us cutting the siding off, needed the oil based primer as well, and then the zip flashing stuck really well.

Moderator followup:

Are you also using house wrap on these buildings? My OPINION is that that's preferable whenever siding has been removed to expose bare fiberboard sheathing on a building exterior.

JP answers:

We are installing house wrap now, yes. This property was leaking inside the brand new addition. We also pulled the roofing back and put proper step flashing and proper membrane for the roof to wall connection.

Why do I only see fiberboard insulating sheathing on older homes?

How do I deal with post-fire and smoke odors on a Fiberboard-Sheathed Home?

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 usable on fiberboard

Blaine,

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.

See details

at FIRE & SMOKE ODOR REMOVAL 

Watch out: be sure to also read

FIRE DAMAGED BUILDINGS, ASSESSMENT, REPAIR

and

SOOT HAZARDS in BUILDINGS

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.

Does insulating fiberboard meet the 2009 IRC/UBC requirement?

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? -Dec 10, 2012 Anonymous

Reply:

Anon,

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.

Also be sure to review

FIBERBOARD SHEATHED WALL BRACING to close any question about the strength of your walls sheathed in fiberboard.

Black exterior fiberboard sheathing on 1974 home - thinking I should replace it with OSB?

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. (Mar 27, 2014) Charles

Reply:

Charles,

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.

Watch out: be sure to read

FIBERBOARD PROTECT from WATER

How do I repair a small hole in fiberboard sheathing

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

Reply:

Jerry,

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.

For detailed advice on fixing fiberboard damage

see FIBERBOARD PANEL REPAIR

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.

 

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

Reply:

Philipe,

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).

Watch out: leaks through a brick veneer wall risk more than wet fiberboard sheathing; you also risk costly and unhealthy mold contamination and over longer periods, rot or insect attack. Be sure to see our advice

at BRICK WALL LEAK REPAIRS 

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

Reply:

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.

Also see, FYI, some of the history of Fir-Tex at

• Fir-tex, - see IDENTIFY Fir-Tex Firtex Firetex Insulating board

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.

 

How do I apply stone veneer to fiber board sheathing?

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. 6/19/2014 Amy VonArx

Reply:

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.

Search InspectAPedia for or see these articles (by clicking on the links below)

STONE VENEER WALLS

And

BRICK VENEER WALL LOOSE, BULGED

To see articles discussing the topic. Let us know if that material is unclear.