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1” sleepers with 3/4” EPS on a concrete basement floor?

alliecat8979 | Posted in Green Building Techniques on

I have a home in Zone 5 with a walk out basement. The basement floor is concrete and the current concrete to bottom of ceiling joist measures at 7’-4”. I would like to float an engineered floor, but ceiling height is an issue. We plan on finishing the walls and ceilings so I’d like to keep the floor as thin as possible. Would 3/4” EPS be too thin? I have read that if you go too thin it will never dry out and that 1” is preferred, but we would like to do 1” sleeper with 3/4” EPS then 1/2” Plywood then 1/2” engineered floor. So 2” thick, but even that is pushing it. Or is it recommended to use SIPs in a low ceiling situation like this? I would prefer to deal with low ceilings and a warm room temp then higher ceilings and a frigid basement. Also, the insulation contractor I’m using spec’d Polyiso for the concrete walls, am I right to prefer that he install EPS instead?

Thank you!

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  1. GBA Editor
    Martin Holladay | | #1

    Either polyiso or EPS will work for your wall. Here is a link to an article on that topic: How to Insulate a Basement Wall.

    Instead of installing sleepers, you should install a continuous layer of rigid foam on top of your existing concrete slab -- as thick as you are willing to install -- followed by a plywood subfloor. The plywood can be fastened through the rigid foam to the concrete slab below with TapCon fasteners.

    The information below comes from my article, Fixing a Wet Basement:

    If you intend to finish your basement, and your basement slab lacks sub-slab insulation, you’ll need to install some rigid foam above your slab. Otherwise, the slab will be damp during the summer.

    The usual technique is to install 1 or 2 inches of XPS or EPS foam insulation on top of the existing concrete, followed by a layer of plywood that is fastened through the foam to the concrete with TapCon fasteners. (If you are still worried that your slab may sometimes be damp, you might want to install a layer of dimple mat under the foam.) When installing this layer of foam, it's important to make the installation as airtight as possible, to make it impossible for any humid interior air to contact the concrete. Seal the edges of each piece of foam insulation with a high-quality European tape, with caulk, or with canned spray foam.

    If you don’t want to lose the height required for rigid foam, you could try installing a dimpled subfloor product like Delta-FL. (Note that some similar products, notably DRIcore, have mixed reviews from some builders....) While Delta-FL is worth considering, especially when the basement ceiling is already low, it's not the preferred solution. If you plan to install finish flooring in your basement, it's always better to install rigid foam insulation on the slab than to proceed without any floor insulation.

    For more information on insulating existing basement slabs, see:
    Finishing a Basement Floor;
    Green Basement Renovation; or
    The Stay-Dry, No-Mold Finished Basement.

  2. Expert Member
    Dana Dorsett | | #2

    With engineered floors you'll be fine with 3/4" EPS in a zone 5 climate, but lose the sleepers- they're nothing but a thermal bridge and wicking mechanism.

    Put down a 6 mil poly vapor barrier as a slip-surface under the foam, and glue the subfloor to the foam with foam board construction adhesive, anchoring it with TapCons to the slab if necessary.

    Check to make sure that a 1/2" subfloor has sufficient fastener retention for the flooring. If it needs more, double-layer half-inch goods with the seams staggered and you can probably just float the subfloor, no TapCons required. (You'll notice if it's' flexing anywhere.) With 3/4" EPS + 1" subfloor + 1/2" wood you're at 2.25".

    If that's too much, back off and use a single layer of 3/4" subfloor, which would come in at 2" exactly. Huber's AdvanTech OSB subflooring would be a good choice if that's what you end up doing, since it's highly moisture resistant compared to plywood or most OSB.

    On the basement walls polyiso is fine, and will deliver higher performance per inch than EPS. Make sure that the floor EPS extends to the concrete walls so that the bottom edge of the polyiso is resting on EPS, not the slab, and not the sub-floor. An all-polyiso code-min wall solution would be 2.5", but with 1" you would have enough dew-point control on the above grade portion to be able to insulate a studwall with unfaced or kraft faced R13 -R15 batts and still hit code-min performance.

  3. Carfar96 | | #3

    I know this is an older post, but we are doing something similar. We are finishing a basement and are installing laminate flooring on the slab. The plan is to do the following:

    Install 1" XPS foam board on the concrete slab. Then put a layer of 1/2" advantech on top of the rigid foam and then installing our flooring. Should we also put down another layer of poly (we do have a layer under the slab) before the XPS?

    The other question is about how to frame and insulate the walls. We plan to put 2" XPS of polyiso on the walls and then frame out the walls on top of that. I am confused about whether to install the wall foam insulation before the plywood (but after floor foam) or after the plywood. I saw an article on here that showed it being installed after the plywood. Also, do we frame out the studs after the installation of the subfloor or before?

    I also have a question about separating the unfinished vs finished space. We will not be putting any flooring on the unfinished space. Thus, the framing will be on bare concrete. How do we best connect the two spaces to prevent any moisture from getting under the flooring?

  4. GBA Editor
    Martin Holladay | | #4

    1. If you have polyethylene under the slab, you don't have to install polyethylene on top of the slab.

    2. There's more than one way to do this work, but I prefer to see the rigid foam extend all the way to the wall before you frame your stud walls (see illustration below). Remember that the stud wall isn't structural -- it's just there to hold wiring and (in some cases) insulation. The bottom plate of the wall can rest on foam or on the plywood above the foam.

    3. If you end up leaving some of your slab uninsulated, you won't have any moisture problems. But if you ever get a chance to insulate the uninsulated portion of your slab in the future, you should do so.


  5. Carfar96 | | #5

    Thank you, Martin! In the unfinished area, should we put the wood framing on a 3.5" piece of rigid foam instead of directly on the concrete? Or maybe on a piece of sill pad? Or is directly on the concrete fine? For the finished part, we will install both pieces of rigid foam (wall and floor), seal and tape that joint, then install plywood layer, then frame stud walls. We only have the head room for one 1/2" piece of Advantech flooring above the rigid foam. Do you think that is okay? THANK YOU!!!!

  6. GBA Editor
    Martin Holladay | | #6

    Either way will work, although I think putting a strip of rigid foam under the bottom plate of your wall is a good idea. If you install the bottom plate of your wall directly on the concrete, building codes require that the bottom plate be pressure-treated.

  7. Carfar96 | | #7

    We are using pressure-treated lumber, but I am still concerned about it being in contact with the concrete. I think we will just put the rigid foam under the bottom plate to be safe. Thanks!

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