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Raising and insulating an addition floor

ohioandy | Posted in General Questions on

I need to insulate and raise the floor of an addition by about 6″, and am looking for suggestions.

It’s a very old house with full basement, but the addition is 1970’s-vintage on an uninsulated, vented block crawlspace.  The floor structure is presently 2×10 joists, with no insulation.  Zone 5.  For some reason, the addition floor is 6″ below the main house floor.  Was thinking I could kill two birds with one stone, using some combination of reclaimed foam and sleepers to both raise the floor AND insulate, without modifying the existing structure.  Since I’ve gotta go up 6″, I figured I could avoid the proper but expensive approach of encapsulating the crawlspace.

(FWIW, the stud walls will be gutted, so I’ve got the opportunity to address perimeter airsealing.)

GBA search turns up mostly threads about strategies for insulating a floor BELOW the existing subfloor.  If someone remembers this issue being treated before, would appreciate links.  Thanks!

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  1. GBA Editor
    Martin Holladay | | #1

    Assuming (a) the ceiling height is generous, and (b) the foundation details and floor framing details are solid, your plan will work.

    Install 5 inches or 5.5 inches of reclaimed rigid foam, purchased from a recycler, and then install your plywood or OSB subfloor. Secure the new subfloor to the old subfloor with long screws.

  2. Expert Member
    Dana Dorsett | | #2

    In case you haven't already found one, in NW OH there are multiple foam reclaimers within driving distance often advertising here:

    Be sure to air-seal the ends of your sleeper bays. It may be worth installing a 2x4 resting on the foam to the studs right at the ends of the bays to provide something to air-seal the foam & subfloor to, and to provide something to fasten the wallboard & kickboard trim tightly to at the bottom. Going that route 1.5" polyiso on the interior, co-planar with that 2x4 would bring a 2x4/R15 wall to near current IRC code minimum performance (= 2x4/R13 + R10 continuous insulation).

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