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Community and Q&A

Can I put foam board insulation on top of one floor and lay another floor on top of it?

Greg Logan | Posted in GBA Pro Help on

I have a enclosed porch that I am trying not to crawl under to put insulation. I also want to make the floor level. So I want to know if I put foam board insulation on top of the old floor and level the new floor, will the new floor keep the cold out?

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Greg,
    Q. "If I put foam board insulation on top of the old floor and level the new floor will the new floor keep the cold out?"

    A. Yes. Of course, the thicker the foam, the higher the R-value of the floor assembly. Pay attention to air sealing when you install the rigid foam and the new subfloor.

  2. William Goodwin | | #2

    I have seen several posts like this where the original poster wants to know how much load foam insulation can carry, where the answers posts say "it can do that". Can ANY foam board be used under ANY floor or ANY roof deck? We used to always use sleepers every 16" or 24", was that just wrong? Will building inspectors generally allow this?

  3. Expert Member
    Dana Dorsett | | #3

    With a fully supported slab or floor below and standard thickness subfloor resting on top the dynamic & static weight loads are distributed over quite a bit of foam surface, and any rigid foam would be able to handle residential floor loading without permanent deformation. Sleepers add very little, and only serve to thermally-bridge the foam, reducing the average R value and making it more difficult to air-seal.

    Stagger the seams of the subfloor and foam by a foot or so for better air tigthness and better weigh distribution at the subfloor seams.

    SFAIK there are no code prohibitions from that approach as long as there is at least 1/2" of subfloor to serve as an ignition barrier, but that doesn't always keep inspectors from trying to take exception to non-standard assemblies.

    On slabs you'd be advised to stick with EPS or XPS though, not polyiso, since there's potential for the iso to become saturated with moisture. At 1.5lbs or heavier density EPS and XPS have better closed-cell structure (particularly EPS), and won't take on water or lose much performance even if fully submerged. Some unrated very low density EPS can become waterlogged, but even 1lb "Type-I" goods will not have that issue.

  4. William Goodwin | | #4

    Any restrictions on thickness of the foam board?, in a house with 6" attic floor joists could you put 3" or more of foam over the old floor with plywood on top of the foam?

  5. GBA Editor
    Martin Holladay | | #5

    William,
    Q. "In a house with 6" attic floor joists, could you put 3" or more of foam over the old floor with plywood on top of the foam?"

    A. Yes. However, if you do it, you should avoid point loads (for example, a column that bears weight).

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