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Community and Q&A

Insulating existing slab on grade – need for sleepers?

deanm | Posted in Energy Efficiency and Durability on

After completing a strategy for insulating my slab-on-grade floor (which sits on sand ONLY) as part of a major building renovation in Zone 5a, I discovered the GBA construction detail for the same situation here:×4-sleepers-two-layers-1-12-rigid-insulation#ixzz46QEEMp00

The GBA solution calls for treated lumber sleepers in each layer of insulation.
a) Given the high compressive strength of XPS, what purpose is served by the sleepers?
b) In my project, we need to do some leveling of sloped floors (as much as 1″ over 9 feet). We’ve thus far called for pouring a yet-undetermined cementitious material for leveling, then installing the insulation as described above. Do you have an alternative solution to suggest?

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  1. GBA Editor
    Martin Holladay | | #1

    Some carpenters don't like installing a plywood subfloor with TapCons. They prefer to fasten the subfloor into lumber, and therefore use the method with sleepers.

    Either method can work. I'll post several images below. The top image is the detail you're talking about. The other details and photos show the method using TapCons and no sleepers. (If you click on each of the images, one at a time, you can enlarge them.)

    For more information on this topic, see these resources:

    Finishing a Basement Floor;
    Green Basement Renovation;
    The Stay-Dry, No-Mold Finished Basement;
    Fixing a Wet Basement.


  2. deanm | | #2

    Thank you, Martin.

    Any response to part "b" regarding leveling?

    P.S. after reading further about the XPS blowing agent, EPS is my new intention, I'll attempt that edit in the original post, should that change any responses.

  3. GBA Editor
    Martin Holladay | | #3

    Google "floor leveling compound" and you'll get lots of results. Choose a product approved for slabs.

  4. DIYJester | | #4

    Does this same method apply to a slab on grade vice the basement?

    Also what about a floating type floor? Is there any need to add the plywood?

  5. GBA Editor
    Martin Holladay | | #5

    Yes, the same advice applies to a slab on grade.

    Concerning a floating floor: you have to follow the installation instructions provided by the flooring manufacturer. If the flooring manufacturer allows installation over rigid foam, go ahead. If the flooring manufacturer requires plywood or OSB subflooring, that's what you should use.

  6. StollerB | | #6

    Dean- depending on the thickness of your insulation, one good reason to use wood sleepers is that you can use fewer expensive concrete screws. You can bolt down your sleepers to the concrete every 24" o/c, and then use construction adhesive and shorter screws to attach your sheathing to the sleepers. Otherwise, you will be using much longer, and more frequent concrete screws to run through your foam into the concrete. To bolt down your sleeper, I suggest getting something better than Tapcons. They are a pretty poor quality fastener. They tend to snap off, or else the threads strip before they really suck down. For a fantastic fastener into concrete, use GRK Caliburn concrete screws ( Or, Simpson Titen concrete screws are not bad, as long as you get the ΒΌ" diameter ones ( Those GRK's are the Cadillac option though; you can even back them out and re-insert them a few times and the threads still stay sharp!

  7. johnarch1 | | #7

    Here is our situation - similar but not the same.

    We have and existing slab on grade with 2x4 sleepers on edge with 3/4" ply-wood on top.
    We do have a plastic vapor barrier under the slab. We have opened up several areas of the existing construction to move plumbing around - the vapor barrier and concrete was patched to match the existing +/-.

    We now have several area of the new / existing concrete that are exposed - the plan is to install new P.T. sleepers and ply-wood to match the existing. We then plan to install a new engineered wood floor system on the entire house. An existing hard wood floor on felt paper (?) has already been removed.

    Here are my question:

    1 - should we install new 1 1/2" EPS rigid foam between the existing / new sleepers in the areas that are open. We will only be able to do this is several smaller area.

    2- Is a gap between the EPS and the existing / new sleepers OK or should this be sealed?

    3- Vapor from the new / existing slab:

    The other concern is protecting the new engineered floor from vapor from below.
    We are considering two options. One is to install a liquid moisture vapor barrier coating on the of the areas of the new / existing concrete that is exposed (and then the EPS rigid insulation or not).

    The other is to leave the new / existing concrete as is and patch the sleepers and ply-wd with or with out the rigid foam insulation between the existing / new sleepers.

    We would then install liquid vapor retarder on the new and existing ply-wd (entire house) and then install the engineered floor. The liquid vapor retarder would in essence take the place of the previous felt paper ?

    Any advice or insight as to the best way to proceed would be greatly appreciated.

    Thanks, John

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