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Rigid insulation under subfloor, over joists

Spencer_True_Story | Posted in General Questions on

We’re renovating our 1940s timber framed house here in Ontario, and I’m trying to solve both an insulation and framing issue at the floor. The floor is about 6” out of level across various dips and humps that will be addressed structurally by new footings and posts in the 4’ crawlspace. I plan on leveling the floor with tapered sleepers and a new layer of 3/4” ply on top. My idea is to lay a layer of 1” ridgid insulation down on the existing subfloor and then apply the sleepers and ply over it, screwing down to the joists through the insulation. This would provide a solid air seal between the house and unconditioned crawl space, and provide as easy plane to tie in the MemBrain I’m using on the interior. Running full sheets of ridgid insulation saves tons of time over the cut and cobble approach under the joists and seems like a more fool proof method. Has anyone every tried this? The flooring will be nail down hardwood, if that matters.


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  1. GBA Editor
    Martin Holladay | | #1

    Your suggested method won't work, because your tapered sleepers will dig into the rigid foam -- they will dig in gradually, as you load the floor above with furniture and people walking around. As the sleepers dig into the rigid foam, your wall won't be level anymore.

    Don't try to do too many things at once. Start by leveling your floor -- either by your method (sans foam), or by demolishing the existing subfloor and sistering new level joists to each existing joist.

    Once you have your new level subfloor installed, you can decide how to insulate your floor assembly.

    1. Spencer_True_Story | | #3

      Martin, thanks for the reply. As there are 4 rooms I want to level together, sistering seems like it would be the better option. Thanks for the input!

  2. Expert Member
    Peter Yost | | #2

    What about leveling the floor as you add the new footings and posts?

    And getting air barrier continuity across a floor plane is all about what penetrations you have to seal and linking your floor air barrier to your wall air barrier.

    My experience is mostly in cold climates and we just have not had very good results trying to vent crawls by insulating and air sealing at the first floor level. And you will need a sort of "mechanical column" for whatever connections (plumbing) you need to protect from freezing.



    1. Spencer_True_Story | | #4

      I will be doing the post and beam work at the same time as the leveling, it just strikes me as being easier to treat them separately. The existing joists are old round timbers and I mainly just want to support them mid span.

      There will most likely be a mechanical room that is built out and insulated in the center of the crawl space which is about 4’ tall. It’s the only way in the house to centrally locate all the pumps and furnace, and to your point, will be a PITA.

      Do you generally recommend spray foam on the basement/ crawl space walls? We have a field stone foundation and it seems like that approach would be significantly more costly.

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