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Community and Q&A

Solving thermal bridging.

Dana St.Ours | Posted in Energy Efficiency and Durability on

I am building a wood frame home in southern Maine. I am using a combination of 2″ spray foam and unfaced fiberglass batt in the exterior 2×6 walls for a total of R25. Would installing 3/4″ rigid foam board insulation between the exterior 1/2″ sheathing and the cement board siding create any moisture issues in the sheating, given there is spray foam on one side and rigid foam on the other? I am concerened about thermal bridging.

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Replies

  1. User avatar GBA Editor
    Martin Holladay | | #1

    Dana,
    Your question comes up often. You're right that it's not a good idea to sandwich OSB or plywood sheathing between two layers of foam. So I tell people: you can put foam on the inside of the sheathing or the outside of the sheathing, but it's better if you don't put foam on both sides.

    So choose.

    If you are worried about thermal bridging, you should install rigid foam insulation on the exterior of your OSB or plywood, and skip the spray foam. Just be sure that your rigid foam is thick enough. For more information, see Calculating the Minimum Thickness of Rigid Foam Sheathing.

  2. Roger Lin | | #2

    So, does that mean it is not a good idea to put EPS foam on the exterior of a SIPs panel? Thanks

  3. User avatar GBA Editor
    Martin Holladay | | #3

    Roger,
    That's a good question. Ideally, you really don't want to sandwich any wood product between two layers of foam.

    If the foam is a relatively thin layer of EPS, some drying can occur to the exterior. Not much, but a little.

    I'm hesitating here, because I don't like the thermal bridging and air leaks that often occur at SIP seams, so I see the advantages of a layer of exterior foam. But it's not ideal. And frankly, SIPs aren't ideal, for a variety of reasons -- the most important being the vulnerability of the OSB facing to rot, and the problems that come from trying to air seal all those seams.

  4. Roger Lin | | #4

    Can't say I disagree regarding the drying potential of Foam on SIPs.

    When you say "the problems that come from trying to air seal all those seams." Are you referring to moisture being trapped between the seams or difficulty sealing the SIPs joints? In terms air-sealing, I think SIPs are already relatively tight and if you apply the appropriate tape, it doesn't seem more difficult that stick framed home.

  5. User avatar GBA Editor
    Martin Holladay | | #5

    Roger,
    Some people use SIP tape on the interior; others just depend on spray foam.

    I agree with you: if you use the tape, the seams are tight. However, if you just use spray foam, these seams can leak.

  6. Roger Lin | | #6

    YES! Going with tape is the way to go.

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