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Community and Q&A

Cold climate basement subfloor questions

Kent Wothers | Posted in General Questions on

I’m finishing a 400 sq ft section of my basement to be used as a general purpose rec room and TV area in our home in Winnipeg, Canada. Our winters are extremely cold and dry but our summers are quite hot and can at times be humid. In 15 years we’ve never had water issues in our unfinished basement other than very short-lived signs of moisture “darkness” on the concrete floor in times of summer high humidity (especially if the central air conditioning had not been left running).

I want to insulate the floor but I’m concerned about moisture and headroom. Headroom between concrete floor to the bottom of the floor joists above is about 7′ 6″ except where there are beams and ducting where the headroom is only 6′ 9″. I plan a drop ceiling everywhere except where the beam/ducting is located and will probably drywall that.

There is so much contradictory information but I believe the best option is to stack up poly, then XPS, then 5/8″ T&G plywood (tapcon’ing the perimeter sheets as a minimum) followed by the finished flooring (planning for carpet to improve acoustics).

Because of the headroom (and concerns about altering the stairway and adjacent doorways, etc), I’m considering using 1/2″ XPS under the plywood. Is that acceptable or do I need to go 1″? 1/2″ doesn’t seem like much but every bit counts.

Do I have to be concerned about the moisture I sometimes see on the concrete floor on hot, humid days in summer? This might only occur a handful of days in a typical year. If so, should I lay down a dimple product (e.g. Delta FL) under the XPS? That of course would further hurt my headroom and I’m not sure if it helps or hurts my problem since it will actually allow air to get to the concrete?

I’ve also read that I should avoid gluing the XPS as mastic can promote mold. True?

Any other advice is appreciated.

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  1. GBA Editor
    Martin Holladay | | #1

    I suggest that you refer to this article: Fixing a Wet Basement. You'll probably want to read the section called "Insulating an existing basement slab."

    In addition to reading that section of my article, you should check out the links there.

    I think that you will want at least 1 inch of rigid foam, and 2 inches would be better.

  2. Kent Wothers | | #2

    Thanks for the response. Your article contains a lot of good information.

    Assuming I'm wiling to go a little thicker, it's not clear to me whether I'm better off going with 1" of XPS and doing my best to seal it all around the perimeter or instead putting down something like Delta-FL and then top that with 1/2" XPS. I read somewhere that you shouldn't combine these two concepts but I see your article suggests it. Given my moisture concerns really only relate to occasional summer day dampness, am I better off dealing with it by limiting air exposure and going with maximum R value (i.e. the 1" XPS) or accepting that there will be moist air contact and going with an air gap (via Delta-FL) and adding some R-value on top of that with the 1/2" XPS?

    I very much appreciate the feedback.

  3. GBA Editor
    Martin Holladay | | #3

    Assuming you have never seen signs of basement moisture, I think that you would be better off going with 1 inch of XPS than 1/2 inch of XPS plus Delta-FL.

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