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Community and Q&A

Roxul batts and Roxul Comfortboard insulation without plywood?

S1laswren | Posted in General Questions on

I’m building a timber frame house in Vermont and was thinking of using Roxul batts in the 2×4 stud walls (studded between the posts). Then putting 2″ of Comfortboard on the exterior, then strapping vertical and horizontal for cedar shingles.

One question I have is that since I’ve made the timber frame so rugged and stiff, (I have hardwood 3×5 bracing going from close to the post tops down to the sill, it is not moving), I decided not to have plywood or rough-cut boarding on the frame. Is there any problems with this with air or vapor passing through?


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  1. GBA Editor
    Martin Holladay | | #1

    Your planned wall assembly is missing a few important layers.

    First of all, your wall needs an air barrier. It might even need two air barriers.

    Second, your wall needs a water-resistive barrier between the siding and the insulation. (It's required by building codes.)

    Third, you should know that it's generally not a good idea to install studs between the posts of timber-frame buildings and to insulate between the studs. Many people did this in the 1970s and 1980s, and regretted it. Differential movement between the timber posts and the stud walls introduced cracks and air leakage.

    You really want all of your wall insulation to be on the exterior side of your timber frame. Moreover, you want a bulletproof air barrier on the interior side of your wall insulation.

    So, from the inside out: timber frame; air barrier; insulation; WRB; rainscreen gap; siding.

    For more information, see:

    All About Water-Resistive Barriers

    Questions and Answers About Air Barriers

  2. S1laswren | | #2

    I left a few details out. The post and beams have grooves routed into them to receive the drywall to help deal with the shrinkage of the timbers. That should help with air movement, I hope. Would it be better if the assembly looked like this. Timber frame, drywall,mineral wool batts,then foil face foam, rain screen then siding. Will the foil face insulation act as a better air barrier the the comfort board?

  3. GBA Editor
    Martin Holladay | | #3

    I'm a little confused about how you intend to support all of these layers. Are you now proposing to build 2x4 or 2x6 walls on the exterior of your timber-frame, in order to have a skeleton on which you can attach your drywall, your mineral wool insulation, and your rigid foam?

  4. S1laswren | | #4

    I apologize for the confusion. There will be a stud wall with MW between the post. The posts have a groove in them to receive the drywall. This will help deal with the gap between the drywall and post as the post shrinks. I ll be able to make a airtight drywall wall. On the exterior of the studs I'll attach 2" of rigid foam, rain screen then siding. The original post mentioned comfortboard on the exterior , I' m thinking of using 2" of polyiso now.

  5. GBA Editor
    Martin Holladay | | #5

    I'm skeptical that your proposed method of installing drywall will be airtight -- especially if your timber frame has diagonal braces. Even if you attempt to caulk the grooves that are designed to receive the drywall, these joints will crack as the timber frame dries out.

    However, if you attempt to create an exterior air barrier, you are likely to be more successful. This could be done with either exterior plywood, exterior OSB, exterior rigid foam, or a durable European membrane from 475 Building Supply or Small Planet Workshop.

    Your plan to use exterior polyiso will work, although there are a couple of caveats. The first caveat is that the polyiso might shrink, making it a dubious choice for an air barrier over the long term. The second caveat concerns whether rigid foam makes sense as a WRB; for more information on this last issue, see Using Rigid Foam As a Water-Resistive Barrier.

  6. wjrobinson | | #6

    4-4 posts between frame posts
    Drywall nailed through backside
    Two layers 2" poly foam taped
    Strap with 2- or 1-

    Done this way for forty years by a local timber builder

  7. Expert Member

    AJ, Why use 4"x4"s?

    This discussion reveals the problem with timber framing. Unless you use SIPS, whatever infill framing you use to hold insulation and finishes is almost always robust enough to have supported the structure without the timber framing.

  8. wjrobinson | | #8

    The timber frames I referred to are fantastic builds... Beautiful... Functional... No problems... Highly insulated... Low ACH.... Loved by all.... Owners.... Guests...

    Malcolm.... I disagree with any negatives you may be postulating. The 4-4s look great and are not a negative. SIPs for this builder are less desirable. Do you build timber frames Malcolm? How many have you designed and built over the the last forty years? My local frame builders are long time pros.

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