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Interior rigid foam installed vertically?

Mark Morley | Posted in Green Building Techniques on

My home in Santa Rosa, CA (zone 3c), has a stucco exterior and uninsulated 2×4 walls. I was hoping to make gradual improvements to the envelope as my budget allows. However, a termite problem has accelerated my schedule, at least in the bathroom, which I have now stripped down to the studs, inside and out. I would like the exterior finish to stay uniform, so I thought a good option for increasing insulation would be to use some kind of batt in the walls with 2″ or so of rigid polyiso on the interior. I saw somewhere that much of the additional insulation value is lost to thermal bridging of the plates in this instance. What I’m wondering is if the rigid insulation can be aligned vertically on 24″ centers with the 2×2 battens offset from the 2×4 studs and attached to the top and bottom plates and the drywall blocking that, in this wall anyway, is located halfway up the wall?

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Mark,
    Q. "What I'm wondering is, if the rigid insulation can be aligned vertically on 24-inch centers?"

    A. Yes, you can install interior rigid insulation, either horizontally or vertically. I'm not sure what you mean by "on 24-inch centers." Do you intend to rip the foam into strips and insert it into the stud bays? Or do you intend to install a continuous layer of rigid foam on the interior side of the studs?

    Q. "... with the 2x2 battens offset from the 2x4 studs and attached to the top and bottom plates and the drywall blocking that, in this wall anyway, is located halfway up the wall?"

    A. I'm a little lost here. If these 2x2 battens are furring strips to simplify drywall installation, and if the "battens" are installed on the interior side of a continuous layer of rigid foam, then I would advise you to use 1x3s or 1x4s, not 2x2s. On a conventionally framed wall, these 1x3 or 1x4 furring strips would usually be installed horizontally, either 16 inches on center or 24 inches on center.

  2. Mark Morley | | #2

    Thanks for the reply, Martin. I think my terminology was incorrect. What I was imagining was, essentially, a second stud wall, 2" thick, with rigid foam board ripped to fit between the 2x2 studs, which would be on 24" centers. There would be no top or bottom plates on this 2" thick wall; the studs wouls be attached directly to the interior face of the top and bottom plates and intermediate blocking of the existing 2x4 wall. I am not wedded to this approach but would like to achieve a significant increase in insulation value without losing more than about 2" of floor space from a fairly small bathroom. Thanks again for your comments.

  3. GBA Editor
    Martin Holladay | | #3

    Mark,
    The rigid foam insulation will be much more effective if you install it in a continuous layer rather than between your proposed 2x2 studs.

    Many people install gypsum drywall directly on top of interior rigid foam, with no battens or furring strips. Just attach the drywall with long screws that reach through the foam to the underlying studs.

  4. Mark Morley | | #4

    Great. I'll give it a go. Thanks, Martin. I really appreciate this site and your advice.

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