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Interior insulation retrofit

user-6962319 | Posted in GBA Pro Help on

I want to add to the interior insulation in a single story room on the north side of my house in Minnesota. I realize this has been asked before, and would appreciate it if someone can point me to articles that apply, or respond anew. The room in 18×28 and has lots of windows. It was built as an addition 30 years ago, with the construction practices of the time, 2×4 studs, osb, tar paper outer wrap, and oiled redwood lap siding. Bays are filled with rolled fiberglass, covered with sheet plastic, and drywall.  I now know that the plastic was not a good idea, but I don’t want to take off the existing drywall, or even consider doing it from the outside. So, are there any big problems with this plan—horizontal strapping screwed through the existing drywall into the studs, 2″ polyiso, tape all seams and outlets, drywall screwed to the strapping, then paneling. If this is workable, how far apart should the strapping be, to be a decent support for the drywall and paneling? I realize there is the issue of thermal bridging from the strapping, and was wondering if there is an insulated product, or if I should just use the lightest cedar 2x4s I can find, or perhaps the issue is negligible in the big picture.

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Replies

  1. Expert Member
    Dana Dorsett | | #1

    Either 16" o.c. or 24" o.c. strapping works with 4x8 drywall. The 16" o.c. spacing usually results in a slightly flatter wall. Since your description was open to some ambiguous interpretation, the polyiso should be tight to the old drywall, the strapping should be between the polyiso and the new drywall.

    The 3/4" air gap between the drywall & foam has almost the same R value as 3/4" wood, so the strapping not really much of a thermal bridge.

  2. Expert Member
    Peter Yost | | #2

    Hi Userxxxx -

    Your post addresses the importance of this new assembly being airtight, particularly since one impact of interior insulation is to make everything to its exterior colder (during the winter). But a colder assembly means less drying potential so just be sure that your management of bulk water--exterior liquid water management--is as good as your air control ayer.

    Peter

  3. user-6962319 | | #3

    You're right Dana, about the need for disambiguation. As a DIYer I was confused about the difference between furring and strapping, if there really is one. Your description has the polyiso panels held against the wall by strapping placed over it and screwed through to the studs. I was imagining screwing the strapping onto the sheetrock and then fitting the panels between them and flush with the inner aspect of the thick 2x4 strapping, screwed to the studs with those big wafer like washers. Then drywall over that. That was where my question about thermal bridging came in. I assume that you thought I was going to use the strapping like furring strips between the panels and old drywall. The method you describe avoids any thermal bridging of course. I get it.

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