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Thread: basement floor XPS foam

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    DIY Senior Member diyfun's Avatar
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    Default basement floor XPS foam

    My unfinished basement has a 87" ceiling height right now. I don't know I should use 1", 1.5" or 2" XPS foam on the floor? The whole basement floor plan will be 6mil poly + XPS foam + 23/32" T&G plywood + engineered wood floor.

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    DIY Senior Member dlarrivee's Avatar
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    Personal preference, how tall are the people who live there?

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    Test, Don't Guess! cacher_chick's Avatar
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    One thing to consider is that a standard door rough-in is 82-1/2 tall. Additional height will be required to install a standard casing around the opening.

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    DIY Senior Member diyfun's Avatar
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    Quote Originally Posted by dlarrivee View Post
    Personal preference, how tall are the people who live there?
    5'10" for now.

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    DIY Member grahamW's Avatar
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    Can I ask the purpose of the poly? If your basement floor is allowing water up from beneath then you're likely going to have issues no matter what you do (IMHO).

    I use ship-lapped, rigid foam insulation and use 2" on the walls and 1" on the floor. Glue to the walls & floor with the correcct adhesive and each seam should be tuck-taped, and spray foam used to fill any gaps around the edges. All rigid foam insulation is mould and mildew resistant and won't hold moisture, even if you have a flood in your basement. You can use mould-resistant drywall as well.

    The goal of insulating in this manner is creating a thermal break between the air inside your basement and the air outside. It will eliminate any air movement behind the walls that could lead to condensation.

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    DIY Senior Member dlarrivee's Avatar
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    Quote Originally Posted by grahamW View Post
    Can I ask the purpose of the poly? If your basement floor is allowing water up from beneath then you're likely going to have issues no matter what you do (IMHO).

    I use ship-lapped, rigid foam insulation and use 2" on the walls and 1" on the floor. Glue to the walls & floor with the correcct adhesive and each seam should be tuck-taped, and spray foam used to fill any gaps around the edges. All rigid foam insulation is mould and mildew resistant and won't hold moisture, even if you have a flood in your basement. You can use mould-resistant drywall as well.

    The goal of insulating in this manner is creating a thermal break between the air inside your basement and the air outside. It will eliminate any air movement behind the walls that could lead to condensation.
    Mold only grows on organic materials, concrete, poly, foam, etc. etc. none of them support the growth of mold.

    If your basement floods, having gimmicky drywall wont make a difference.

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    DIY Senior Member diyfun's Avatar
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    Quote Originally Posted by dlarrivee View Post
    Mold only grows on organic materials, concrete, poly, foam, etc. etc. none of them support the growth of mold.

    If your basement floods, having gimmicky drywall wont make a difference.
    I plan to attach the drywall at least one inch above from the plywood. That means as long as there is no more than 1" water in my basement, the drywall will be safe. I have floor drain and french drain. I think my basement probably should be good enough.

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    DIY Senior Member diyfun's Avatar
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    Quote Originally Posted by grahamW View Post
    Can I ask the purpose of the poly? If your basement floor is allowing water up from beneath then you're likely going to have issues no matter what you do (IMHO).
    The poly is to reduce moisture vapor from the ground and to prevent small amount of water from ground if any. But it will be bad if water is from inside (water pipe), I think.

    In my case, that poly layer is for moisture only because I have added french drain inside my basement and outside grading has been addressed as well.

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    In the trades Dana's Avatar
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    Quote Originally Posted by diyfun View Post
    My unfinished basement has a 87" ceiling height right now. I don't know I should use 1", 1.5" or 2" XPS foam on the floor? The whole basement floor plan will be 6mil poly + XPS foam + 23/32" T&G plywood + engineered wood floor.
    Since you're not spanning 16" o.c. joists, there's no rationale for going with 23/32" plywood subfloor- XPS hard-sandwiched between even half-inch t&g sheathing for the sub-floor and a concrete slab has PLENTY of structural strength, and flexes far less than 23/32" plywood spanning joists. You don't want to go thinner than 7/16" though (for fastener retention reasons.)

    There's an economic rationale for R5 (1") XPS on space heating savings (as well as a comfort rationale) in a MA climate zone & subsoil temp. There's a long term economic rationale for R7.5 (1.5"), but R10 only makes economic sense if

    A: It's a DIY and you discount you fully discount the value of your labor

    B: You use reclaimed XPS at 25-30% of virgin-stock costs

    C: You are stuck only with very high space heating fuel options (propane, or oil, or >12 cent electricity & no heat pumps)

    D: You are installing radiant floor to heat the place (in which case R12-R15 might even make sense.)

    If you stagger the seams of the subflooring with that of the foam by at least a foot or so for least mechanical creepage you can just float the floor, using foam board construction adhesive between the foam & subfloor. If there's any detectable flex to it you can throw in a few tapcons through-screwed to the floor per panel of subfloor.

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