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Community and Q&A

New floor and joist

Klmanns69 | Posted in General Questions on

Had to replace all floor joist and want to add two inch isolated board can I put it betw÷n floor joist and sub floor

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  1. Expert Member
    Michael Maines | | #1

    Putting foam at that location would only work if the foam was very dense--denser than off-the-shelf polyiso--and if the subfloor was not needed for lateral stability (i.e., bracing the structure.) Even then, you would risk having the fasteners squeak due to the small amount of vertical movement when walking on it. It would be much better to locate the insulation between the joists as is conventionally done.

  2. Expert Member
    BILL WICHERS | | #2

    You can put subfloor down, a layer of foam, then another subfloor. That works, since the two layers of subfloor material (OSB/plywood) distribute loads over large enough areas of the foam that you don't usually have issues with excessive distortion of the foam. You MIGHT be able to put the foam UNDER the joists though (such as would be the case if the joists are right over a slab), then possibly filling the cavities between the joists with batts.

    It's difficult to really give you good advice here without knowing more about the way everything is going together on your project. I agree with everything Michael said about the potential problems if you put the foam between the top of the joists and the subfloor too.


  3. Expert Member
    DCcontrarian | | #3

    Keep in mind that if you change the height of your finished floors it screws up the stair spacing for the first step.

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