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Community and Q&A

Rigid Foam Above an Uneven Slab

ScottC7 | Posted in General Questions on

I am working out the details of installing rigid foam above an existing attached garage slab that is becoming part of the conditioned house.  The slab is currently uninsulated and has not vapor barrier.  I am in zone 5, Central Iowa.  The exiting concrete has a 2-3 inch drop from on end to the other.  The contractor is recommending to rip 2x sleepers to match the floor slope as a solution for leveling the surface.  Rigid foam would then be installed in between the sleepers and the top would be covered with 3/4 in Advantech sheets.  I am concerned about thermal bridging through all the sleepers on 16 in centers.  I am also concerned about the rigid foam not filling the full depth of the cavity in between the sleepers (with will vary due to sloping concrete) and leaving an air space under the plywood.  The foam depth will probably range from 1-2 inches thick.  the high end has a doorway the limits how high the new floor level can be.

The final assembly as proposed from bottom up is:
existing concrete slab
thick poly layer
ripped 2x sleepers 16 in o.c. on end to level the floor
rigid foam in between the sleepers.  1-2 in thick.  Maybe more on low end.
3/4 in Advantech sheets, single layer
finish floor to be determined

Does anyone have comments about the proposed plan?  Will it work as proposed or should some changes be made?  I appreciate your responses.

Thank you,

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  1. Expert Member
    BILL WICHERS | | #1

    I would put the foam on the concrete and the sleepers over the foam. Most XPS and some EPS is rated for compressive strength for applications like this. This also means there is nowhere for moisture to accumulate between the insulation and the slab, and no thermal bridging issues with the sleepers. You could even use some loose fill insulation over the rigid foam in the lower spots to fill the void with some extra R value. Shake and Rake style loose fill fiberglass insulation would be good for this since it can be installed in small areas without the need for a blowing machine.


  2. ScottC7 | | #2

    Hi Bill,
    Thank you for the recommendation. After some back and forth, it was decided that having the sleepers directly on the concrete will provide a level surface that is more solid than other options. Rigid foam will still be cut and placed in-between the sleepers. gaps between sheets and between the foam and wood will be sealed with spray foam. The lower section of the concrete floor will have more foam. There will probably be a small air space above the foam and below the plywood subfloor. I understand that having a continuous layer of rigid foam would be more ideal, but will the proposed assembly still work without issues?

    Thanks, Scott

  3. Expert Member
    Michael Maines | | #3

    I agree with Bill; it would be better to run continuous foam under the sleepers. But I have done exactly as you propose, in climate zones 5 and 6, with no issues that I know of other than slightly increased energy bills and slightly reduced comfort.

  4. user-6623302 | | #4

    The contractor is just trying to keep all the work in house. Get a concrete guy in to level the floor, then foam and floor, Maybe a footing and foundation across the large door opening so the wood is not on the ground.

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