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Basement slab retrofit questions

mfredericks | Posted in Green Building Techniques on

I have an older home built in the 1950’s. I’ve insulated the basement walls with rigid foam and a stud wall with batts. This has made the basement more comfortable, but I still have a lot of moisture in the air based on the dehumidifier that I run frequently. I’m fairly certain that the existing slab must be un-insulated and is missing any poly or other moisture barrier under it.

I’m using part of the basement for long term storage and would like to use it without worrying about moisture wicking up through the slab and into the stored items. I’d also like to run the dehumidifier less! I’m considering options to insulate the slab.

2’x2′ Barricade subfloor tiles (½” XPS laminated to tongue and groove OSB panels)
4’x8′ Sheets of 1” EPS + 5/8″ plywood (left floating, using tapcon screws only where needed like high traffic spots)

I’m limited by an already low ceiling so there isn’t much room to add more thickness to the floor.

I contacted Barricade customer support to ask about their recommended gap around the perimeter of the room. They suggest that ¼” be left open where the tiles meet the walls for expansion. I asked them if I should fill this gap with spray foam or tape, but they didn’t like either idea. Air sealing here seems important to me, but it goes against the manufacturer’s instructions. What’s a good solution here?

If I go with sheet goods instead of insulated subfloor tiles, I’m wondering if a layer of poly is worth adding first or if its better to allow for some drying by skipping the poly. Are there any disadvantages to laying a sheet of poly on the concrete and then topping it with EPS and plywood?

Thanks for any help
Located in Nova Scotia, Canada. Climate Zone 6.

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Mark,
    You don't want to allow any drying toward the interior. That's what's happening now: moisture in the damp concrete is evaporating, raising humidity levels in your basement. You want to minimize the flow of water vapor from the damp concrete to your basement, so a layer of polyethylene is appropriate. You can install a layer of rigid foam on top of the poly.

    The 1-inch later of rigid foam is better than the Barricade option, since the Barricade option has only 1/2-inch of rigid foam. If you do end up installing the Barricade, don't worry about the 1/4-inch expansion gap at the perimeter. Only a tiny amount of moisture will evaporate from this tiny gap -- it isn't worth worrying about.

  2. mfredericks | | #2

    Great, thanks Martin!

  3. mfredericks | | #3

    Martin, to follow up on your comment above - do you think leaving the 1/4" gap at the perimeter of Barricade subfloor tiles would create much risk for summertime moist air condensing on the cool concrete slab?
    I'm thinking of just taping this gap to allow expansion of the Barricade tiles, but maintain an air tight connection between the tiles and the bottom plate of the wall.
    Thanks again!

  4. GBA Editor
    Martin Holladay | | #4

    Mark,
    Q. "Do you think leaving the 1/4 inch gap at the perimeter of Barricade subfloor tiles would create much risk for summertime moist air condensing on the cool concrete slab?"

    A. No.

    Q. "I'm thinking of just taping this gap to allow expansion of the Barricade tiles, but maintain an airtight connection between the tiles and the bottom plate of the wall."

    A. Can't hurt.

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