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Rigid foam between double walls?

user-5873106 | Posted in General Questions on

We are going to be building a small retirement home in New Brunswick Canada and want it to be as energy efficient as we can afford. We’ve ruled out the idea of 2 x 6 walls with exterior rigid foam because we don’t want to risk having problems with ants getting into the foam. We’ve also ruled out exterior rigid rock wool because the cost here in NB is too high. Double stud walls seems to be our best bet. One wall system proposed by a builder we approached involved a 2 x 6 ( 24″ oc) outer structural wall with “blown in blanket” fiberglass in the cavities between the studs (no installers do dense packed cellulose in our area). Then 2 inches of XPS. Then a poly vapor barrier. Then a 2 x 4 (16″ oc) inner wall with fiberglass batt in the cavities between the studs. We have not decided on this setup and we realize that the 2″ of rigid foam between the walls will not provide as good a thermal break as a larger cavity with blown in. I’ve tried many searches for opinions regarding the idea of putting nothing but XPS between the double walls but have not found anything. I’m hoping for guidance regarding that idea in particular – rigid foam between the walls?

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  1. GBA Editor
    Martin Holladay | | #1

    I think the wall will be safe. It will also have a high R-value (R-43).

    It has a few quirks, however, and costs more to build than cheaper options that would perform as well.

    First of all, green builders generally avoid using XPS because XPS is manufactured with a blowing agent that has a high global warming potential. For more information on this issue, see Choosing Rigid Foam. If you want rigid foam, you should specify EPS or polyiso.

    Second, the polyethylene layer is entirely unnecessary. If you include 2 inches of rigid foam in your wall, the rigid foam is a perfectly adequate vapor retarder. (I imagine that the polyethylene is there to satisfy your local code enforcement official; Canadian code officials tend to insist on polyethylene even in wall assemblies where the polyethylene is unnecessary.)

    As with any wall, you want to pay attention to airtightness. A good interior air barrier is more important than the polyethylene.

    One way to save money is to skip the 2-inch rigid foam and simply leave a 2-inch gap between the 2x6 wall and the 2x4 wall. Then insulate the entire cavity with blown-in insulation.

    For more information on these issues, see How to Design a Wall.

  2. user-5873106 | | #2


    Thank you for the help. Builders and inspectors here in our area have some experience with double walls but not a lot so any guidance we can get is greatly appreciated. In the next couple of weeks, as my wife and I discuss this with different builders, we'll be trying to sort out the details of the wall assembly we will choose; your advice re the poly vapor barrier and using EPS instead of XPS will be a major part of that.

    Martin or anyone else:

    At the moment, it appears that we'll be choosing one of the following wall assemblies:

    #1: Vinyl Siding; Tyvek or Typar Housewrap; 2 x 6 outer structural wall with blown-in blanket fiberglass; 2" EPS Rigid Foam; Poly Vapor Barrier; 2 x 4 inner wall with fiberglass batt; sheetrock

    #2: Vinyl Siding; Tyvek or Typar Housewrap; 2 x 4 outer structural wall; 3" cavity between walls; 2 x 4 inner wall; Poly Vapor Barrier; sheetrock (all "Blown-in Blanket" fiberglass)

    In #2, I've located the poly vapor barrier on the interior side of the inner wall in an attempt to keep the processes of blowing in the insulation and installing the poly as simple as possible.

    Apart from the fact that our decision may be determined by the cost or what the builders are comfortable with, each assembly seems to have an advantage and disadvantage:

    #1 provides an effective vapor retarder that is not perforated by electrical BUT the thermal break where framing of the walls coincide is only R7.5.
    #2 provides a more effective thermal break (R 12.5) BUT maintaining the integrity of the poly vapor barrier will be more difficult.

    I guess what I'm hoping for now is any final advice you can give regarding whether my thoughts about these two wall assemblies are sound and any thoughts you might have about which wall assembly is preferable (assuming the cost is fairly similar).

  3. STEPHEN SHEEHY | | #3

    David: For our double (2x4) stud wall, we installed an air barrier (Siga Majpel) on the outside of the inner wall. The inner wall was built on the floor and we hung the membrane over the top plate and extended it at each end by about a foot. That allowed us to install a continuous air barrier.

    Once the wall was stood up, we sliced the membrane to allow blown in cellulose insulation and taped the holes using Siga tape. Then, after wires and pipes were installed, we used fiberglass batts inside the inner wall. It was pretty simple. Our contractor had never done a double stud wall, but he had no trouble at all. You can pick whatever space between the two walls you want, but if course more space between them allows for more insulation. Our total wall depth was 12 1/2" . We're in Maine, zone 6.

  4. Expert Member
    Dana Dorsett | | #4

    Using foil faced polyiso instead of EPS means the foil facers can be vapour barrier (no polyethylene needed), and is easily air sealed with purpose made aluminum tape.

    At 2" unfaced Type-II (1.5lbs per cubic foot) EPS is about 1.5 US perms- the NBC definition of vapour barrier is just a bit more than 1 US perm, so even without the seams taped it doesn't allow much moisture to pass. Installing polyethylene sheeting next to the gyprock means the 2x4 wall has nowhere to dry toward quickly. EPS with facers are even lower permeance. Installing 2" of polyiso instead of EPS and using the interior side foil facer as the vapour barrier puts the R12 of the polyiso plus R22 of fiberglass (R34+ total) outside the vapour barrier, and R15 or less in the 2x4 wall on the interior side of the barrier., R49 total.

    That makes the interior side R about 30% of the total, and the average temperature at the vapour barrier will be 30% of the average temperature difference between indoors & outdoors. With a presumptive 5C dew point for the conditioned space air, and an interior temperature of 20C, there won't be condensation on foil facer until temperatures drop below -30C outdoors. Even the 99% outside design temperatures in NB are all ABOVE that -30C point, which makes it absolutely safe to skip the interior side polyethylene, avoiding the potential moisture-trap problem..

    (For quick reference -25C = -13F, -30C = -22F )

  5. user-5873106 | | #5

    Thank you neighbour! Hope all is going well in Maine. Crazy winter!

    Your account of how you got the vapour retarder on the exterior side of the inner wall helps a lot. This next question really emphasizes how little I know. Is the cellulose or fiberglass blown into both the outer wall and the cavity between the walls at the same time? I'm having trouble picturing how the insulation can be directed successfully to fill both the cavity between the walls AND the stud cavities of the outer wall unless the cavity is netted off into sections corresponding with the stud cavities in the outer wall (which sounds like a huge job).


    Thank you. I'm trying to determine if I can get 2" thick Polyisocyanurate around here and what the price would be. So far it looks like 1.5" would be the thickest available locally; the price for that is $29.79 so I assume 2" would be $60 or more if it has to be special ordered. I can get 2" thick Amvic Silverboard (foil faced EPS) for $49.99. Would the fact that both the polysio and the EPS are faced on both sides (2 vapor barriers?) be a problem in my wall?

    I'm sure it's just my limited understanding of all this but when you said "installing polyethelene sheeting next to the gyproc means the 2x4 wall has nowhere to dry toward quickly" I'm confused because you seem to be talking about the wall assembly with the rigid foam in it and that assembly does not have the poly next to the gyproc. It's on the other side of that inner wall. Again I know this is probably just me not understanding but I'm wondering if, in that assembly, there is a problem with the poly on the exterior side of the inner wall against the rigid foam (whatever type that might be). I know the poly is not necessary but is it going to cause problems in the wall if I include it to keep the inspector happy?

    Thanks again. I really appreciate all the help you have given me.

  6. Expert Member


    Your building code allows the rigid foam in option A to replace the poly and act as a vapour barrier, but as you say it may be easier to include it to keep your inspector happy. You will no doubt be using it on your ceilings, and sticking with one air-sealing material and details makes things a lot simpler.

    I'd prefer living in a house with option A for a couple of reasons. The air/ vapour barriers are well protected by being further into the wall and have few penetrations. An inner batt-insulated service wall makes any renovations a lot easier than disturbing blown insulation. In a ceiling you just sweep it aside and smooth it out when done. Adding some wiring or a drain in a wall means somehow re-dense packing the area with the material which is now probably lying under-foot.

  7. user-5873106 | | #7


    Thanks. That makes a lot of sense to me. Much appreciated!

  8. STEPHEN SHEEHY | | #8

    David: there is a five inch space between the inner stud wall and the exterior stud wall. The insulation installer just blew cellulose through slits in the membrane covering the exterior side of the inner stud wall. It easily filled The 8 1/2" space. The Siga Majpel membrane is very tough stuff, so there was no concern about blowing it out.
    after the cellulose was installed, electric and plumbing were roughed in and then we installed fiberglass batts in the inner stud cavities.

  9. etting | | #9


    I'm building a double wall with strips of foam between the abutting studs and Roxul batts providing the insulation. You may find the discussion here useful:

  10. SpyingOnMyKeystrokes | | #10

    I realize this thread is pretty old, but maybe the project has been finished and we can get some feedback on the outcome. I'm curious about the reason for using 2 stud walls that have structural capacity as opposed to a larsen truss system with 2x2 or 2x3 non-structural studs on the outside to create space for insulation?

    I was thinking of covering the pressure treated joist with felt.

    Along this topic (and I may add this as another post) I'm wondering if there are any issues with applying (gluing) rigid foam insulation directly to the side of a 2x8 ledger, as an example. Does the rigid foam create issues with trapped water and cause rotting?

    I'm looking to continue a rigid foam insulation course on an insulated raised slab up and around a 2x ledger epoxy bolted to the edge face of the raised slab, fit between joists to help block any thermal bridging where the ledger and joists on either face or top flange hangers are set against a raised concrete slab. It seems like a bit of overkill, but I'd hate to leave a section of slab edge uncovered and end up with cracks or something.

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