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Tape exterior XPS insulation and housewrap?

elyk22 | Posted in Green Building Techniques on


Long time follower first time poster.. I have learned so much from this site and appreciate all the knowledge you people share.

We are building a tight house in zone 6 Ontario. The wall stack from inside is as follows:

2×3 uninsulated service wall
2×8 stud wall filled with dense pack cellulose
Cross bracing for lateral strength
2″ XPS on exterior
Tyvek WRB
Furring Strips (airgap)
Hardiboard Siding

Having the Membrain will help wall dry to the inside.. but wondering on taped XPS on outside, not being so permeable, will trap any moisture from drying to exterior?

I have read a bit of conflicting ideas of leaving the XPS seams open in this case to let it dry better to exterior if needed. As long as the tyvek is taped in good detail with quality tape that lasts.

I have read using the XPS as the barrier itself.. but do not want to miss any detailing on that method and then have issues. The house wrap is a belt and suspenders approach that I feel safer with.

Also I know I’m a bit shy on exterior XPS (dew point) for a 2×8 wall.. but $$ to go any thicker with exterior foam is not likely to happen.

What would you guys do?



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  1. GBA Editor
    Martin Holladay | | #1

    First of all, with 2" of exterior XPS, don't expect any drying to the exterior. Walls with an exterior layer of rigid foam are designed to dry to the interior, not the exterior.

    Second, tape the XPS seams. Airtightness is always good; it never makes sense to make a layer of your wall deliberately leaky.

    Third, this isn't a very good wall design. You broke the rules. Here is a link to an article that explains the important design principles to this type of wall: Calculating the Minimum Thickness of Rigid Foam Sheathing.

    Your studs are too deep (creating too deep a layer of air-permeable insulation) and your rigid foam layer is too thin (not thick enough to keep the interior surface of the rigid foam above the dew point in winter). The MemBrain helps a little bit to bring your risky wall in the direction of less risk -- but it would have been much better if you used 2x4 or 2x6 studs, and thicker rigid foam.

  2. user-4524083 | | #2

    KYLE - Consider this: Drywall ; 2x4service wall insulated with Roxul(or DP cellulose) ; 2" space insulated with Roxul comfort board (or cellulose blown when the 2x4 wall is blown ; Plywood or OSB sheathing , on inside of 2x6 wall as an air barrier with this bearing wall insulated with cellulose or more Roxul ; tyvek ; strapping ; siding. Avoids foam, allows wall to release vapor in both directions, same thickness (more or less) as your wall. The primary air barrier is in the middle of the wall. A variation of this would be to have Membrain on inside of 2x6 or outside of 2x4, and sheathing outside. If there is a second floor, it could be supported by the inner 2x4 wall or "hung" off the outer wall (using taller studs) so that the full insulation of the outer wall is on the outside of the "rim" joist. This is a good site to float ideas. Best of luck to you with this project.

  3. GBA Editor
    Martin Holladay | | #3

    One other possible solution to your dilemma: Instead of exterior rigid foam, just install exterior semi-rigid mineral wool insulation. That change will allow some outward drying.

  4. elyk22 | | #4

    Thanks Martin and Kevin,

    There is still time to make changes.. I will look into your ideas more and post back later. Thanks for your quick response.


  5. user-1135248 | | #5

    With the caveat that you can't tape seams on rockwool and it's
    not going to be much of an air barrier anyways, right? What's
    best practice there, just get it all butted up snugly and make sure
    the air barrier behind it is good?


  6. GBA Editor
    Martin Holladay | | #6

    Good point. If Kyle makes the switch from rigid foam to exterior mineral wool, it would be advisable to include a layer of exterior sheathing (for example, plywood) with taped seams, followed by a WRB like housewrap, under the mineral wool.

  7. elyk22 | | #7

    I'm still waiting to see if we could go with 2x6 walls and 3 or 4inch XPS at this point.. or go the comfortboard route which I like from an environmental perspective. Question: Would one more inch of XPS (total 3 inches) be enough to move this risky wall into a safer zone? Taking ideas from here: Comment #14 by MJ makes me think our stack could work ok.. but of course it could be better. Also a note: we have very dry winters and quite humid summers. If that factors in on choices please let me know. Thanks so much.. I'll continue my research and post back soon.

  8. elyk22 | | #8

    Hello again,

    Location update: I'm about an hour north of Orangeville, so near the warmer zone 6.

    I know I'd still have some moisture in walls with this.. but would this stack be 'less' risky.

    2x3 uninsulated service wall
    2x8 stud wall filled with dense pack cellulose (borate treated)
    Cross bracing for lateral strength (no sheathing)

    2" Type 2 EPS (PlastiSpan® HD) -- OR -- 2" IKO Ener-Air
    (both having greater perm.. dry to exterior potential)

    Tyvek WRB
    Furring Strips (airgap)
    Hardiboard Siding

    This wall is to be air-tight as possible - interior and exterior tape detail, relative humidity kept in check in winter (bathroom exhaust fan... dry winters here as well), HRV in house

    Thanks so much. I'd love to design the perfect wall.. but at this stage cost and time is a factor.

    Also I know this is science and I don't want any great risk.. this will be a lifetime home.



  9. GBA Editor
    Martin Holladay | | #9

    The difference in permeance between 2 inches of XPS and 2 inches of EPS isn't enough to make a big difference here. The issue really isn't the rate of drying to the exterior; the issue is keeping the interior-facing side of the rigid foam warm enough to prevent condensation during the winter.

    To improve this wall, you need thicker foam or shallower studs -- ideally both.

    The 2x8s are a mistake. Either 2x4s or 2x6s are preferable. And thicker foam will keep you out of trouble. In your climate zone, 2x6 studs and 3 inches of EPS would work well.

  10. elyk22 | | #10

    ok thanks again for quick reply Martin - I appreciate your advice.

  11. elyk22 | | #11

    We have changed our wall to 2x6 and 3.5" EPS insulation. We are detailing the EPS as our exterior air barrier - blower door test to take place before fluffy insulation.

    Now that we are above dew point and using the exterior sheathing as our air barrier, the service wall becomes less important correct? We could run services through 2x6's (dense packed with mesh) > Membrain > Drywall > High perm latex paint. Save some build costs on 2 x 3 service wall (although it provides a place to add a bit more insulation).

    I'm a bit confused if an interior air barrier is even needed? Would Membrain still be useful here.. taped and sealed?

    I read the One barrier or two article.. but did not come to a clear conclusion.

    Thanks so much


  12. Expert Member
    Dana Dorsett | | #12

    Detailing the gypsum to be air-tight is sufficient when the cavity fill is dense-packed.

    With 3.5" of EPS on the exterior you have plenty of dew point margin at the sheathing, and the moisture buffering capacity of the cellulose gives it even more protection. A layer of MemBrain doesn't hurt (and may be rated a "like" from inspectors accustomed to seeing polyethylene vapor barriers detailed as air barriers with vapor hats over the electric boxes, etc), but it isn't really necessary with that stack up.

  13. elyk22 | | #13

    Thank-you Dana for your quick response.

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