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Air sealing exterior Celotex fiberboard sheathing

dhollm | Posted in Energy Efficiency and Durability on


We are desiging the residing of our house located in central New York State. The house was built in 1973 with some later additions done by prior owners. We are going to take the opportunity to add air sealing, water management, rigid insulation, a rainscreen, some new windows, then fiber cement lap siding. Planning to DIY it.

My particular question concerns the air sealing. I want to get the house as tight as possible while we have access.

Originally I was intending to seal the seams of the sheathing using tape and/or caulk. However I’ve discovered that a lot of our sheathing is a fiber board material (Celotex), not plywood or OSB. The pattern seems to be that plywood us used in specific places like corners or under windows, and fiberboard everywhere else. (For whatever reason ).

Anyway, I don’t know how to air seal this fiberboard. I guess my options are:

1. Seal the seams as intended
2. Replace all the fiberboard with OSB and seal as intended, or even better as I can seal to the studs somehow (glue / caulk / gaskets?)
3. Cover the fiberboard with 1/8″ plywood and seal as intended ($$$?)
4. Use a single membrane to provide both air and water management
5. Tape the rigid insulation making it the air / water barrier.

Regarding #1, would the fiberboard allow air to permeate? I don’t know what would bond to the material well. I think the surface is some kind of asphalt coating. It also is friable and I fear would disintegrate under adhesive stress.

Regarding #4, my impression was that housewraps eg Tyvek are not really good as air barriers.

Regarding #5, our plans have been to do the air management, water management, and windows all in the plane of the sheathing with insulation on top of that; this would void that idea.

Thanks for any advice!


From what I can tell, the way the house was sided has a lot of flaws. No tar paper or similar material at all; part of the siding is dense cardboard-ish fake stucco boards which are rotting where reverse shingled “Tudor” details have trapped water, etc.

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  1. GBA Editor
    Martin Holladay | | #1

    As with any air sealing project, there are different ways to proceed depending on your goal.

    I would abandon the idea of sealing the Celotex seams with tape. I don't think that will work.

    If I were you, I would install housewrap (perhaps one of the European housewraps) over the sheathing, and would tape the housewrap carefully, aiming for a good air sealing job. This will never be perfect, but it will help. Then I would also tape the seams of the rigid foam.

    If you take these steps, and if you do a careful air sealing job at all penetrations, I think you'll be satisfied with the reduction in air leakage.

    For more information, see How to Install Rigid Foam Sheathing.

  2. Cjb80 | | #2

    Martin (or anyone),

    Can you elaborate on a topic brought up by the original poster: Are there WRBs that could be used as an air barrier or even an air barrier to some degree? If so, are there some that are better than others? Let's assume that the taping job is equal in quality and there are no tears among all alternatives. The only thing that I can think of is that a more expensive WRB may be less prone to tearing. Furthermore, it's not typical to run the WRB over the roof to create a continuous air barrier.

    Are there issues with WRBs allowing air through? Any other considerations that I didn't list?



  3. GBA Editor
    Martin Holladay | | #3

    Q. "Are there WRBs that could be used as an air barrier?"

    A. Yes. If price is no object, one of the first choices in this category would be a liquid-applied air WRB. For more information on these products, see Housewrap in a Can: Liquid-Applied WRBs.

    Other possibilities include:

    - Rigid foam. For more information, see Using Rigid Foam As a Water-Resistive Barrier.

    - VaproShield WrapShield SA (a vapor-permeable peel-and-stick WRB).

    - Henry Company Blueskin VP (a vapor-permeable peel-and-stick WRB).

    - Pro Clima Solitex Mento

    - Siga Majvest

    The European membranes tend to be tougher than most North American housewraps. Builders who try to install Tyvek or Typar as an air barrier are usually disappointed, because it is quite easy to end up with holes or tears in these products.

  4. user-1061844 | | #4

    Celotex itself would not make a good airbarrier - it is only windtight, not completely airtight and also not stable/durable enough to be taped.

    When selecting an exterior airbarrier - check the reported ASTM E2178 - the number should be very low (practically beyond the capabilities of the lab) - this is the case with vaproshield and the SOLITEX MENTO we import with 475 and have been used for some very airtight projects (far below 0.6ACH50) that have used the ProClima airtight system (including TESCON tapes and gaskets/adhesives). Like this project in Maryland.

  5. albertrooks | | #5


    I like the European membranes quite a bit but an issues is that they are not self sealing to the siding fasteners. If your detail calls for strapping and a rain-screen, then the siding fasteners will only hit the furring strips and will seal well. If you are not planning on furring strips, then I like Martins suggestion of a liquid applied system since they will typically self seal penetrations.

    In any case, do use a good membrane. It's worth it. Also, SIGA Wigluv will stick to Celotex with Dockskin primer:

  6. dhollm | | #6

    Thanks for the feedback and useful project and product suggestions. This has confirmed my concerns with trying to incorporate the Celotex itself into the air barrier construction. The ASTM reference is a good thing to know about in particular.

    I'll have to do some addition but its possible for us that the cost of OSB + tape could be less than one of the better housewraps, especially the self adhered type. But is taped OSB / plywood sheathing generally regarded as a very "tight" air barrier? (We're willing to pay more for a better result.)

    Regarding the self-adhered membranes, could we use something like a roofing membrane (eg ice & wate shield) for the air + water barrier, instead of the more expensive housewraps? I guess the difference may be vapor permeability, but with a rainscreen and sheathing which can dry to the interior, would we really have a problem? The 2" of polyiso we will add to the outside should be more than enough to prevent interior wall condensation in our climate.

    Thanks again & happy hollidays!

  7. GBA Editor
    Martin Holladay | | #7

    Q. "Is taped OSB / plywood sheathing generally regarded as a very tight air barrier?"

    A. Yes. For more information, see Airtight Wall and Roof Sheathing.

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