Installing my own subfloor question..

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Joined: Thu Jan 24, 2019 8:44 pm

Installing my own subfloor question..

Post by WillLee » Fri Sep 06, 2019 8:38 pm


I've decided to install my own subfloor using just 1/2 inch XPS rigid foam on the concrete and 1/2 inch OSB. What I am debating on is whether I tucktape the joints of the XPS rigid foam so there are no air gaps between them.

Some points to consider:
1. I plan on securing the OSB and rigid foam to the concrete floor using tapcons
2. My vapor barrier around the basement walls is Certainteed Membrain and the insulation is 1 inch rigid foam and roxul.
3. My furnace room has bare concrete on the floor and walls.
4. I am based in Ottawa. Dry cold winters and humid summers.
5. Taped vapor barrier to my concrete floor for a few days and observed no moisture.

I've been trying to wrap my head around the whole moisture management of my basement. Which option do you think I should go with?

Option 1: I am realizing that IF I was trying to mimic Dricore R+, I would tucktape all the joints.

Option 2: If I wanted to go further, I could spray foam the 1/4" space that I leave around the subfloor and bottom plate studs. This would essentially prevent any moisture from getting into my basement from finished rooms of my basement. During the winter, I could be certain that no humid air is hitting cold concrete in the finished rooms. The furnace room with bare concrete may have condensation on that concrete but it would be able to breath and evaporate. In the summer, humidity in the concrete walls could come in through Membrain, and slowly through the floor as 1/2 rigid XPS does allow some moisture through (I think??). THis is fine as it is preferred to allow any moisture in the walls to dry inwards as opposed to sit inside the wall cavity.

Option 3: If I don't tucktape any rigid foam joints and don't spray foam the perimeter. During the winter, humid air will likely contact colder concrete through the cracks between rigid foam and create some condensation. Would this cause any issues seeing that the condensation is in contact with the rigid foam? In the summer, it would allow a little more moisture in the floor to come inwards.

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Re: Installing my own subfloor question..

Post by Shannon » Fri Sep 06, 2019 11:45 pm

Most basement floors are not actually as cold as you may think ,compared to slabs on grade which are more exposed to exterior air and weather . They are cool though.
I don't think the warm air inside is as much of an issue as you think. In the end I think it is a toss up and not sure there is a complete perfect answer.
I do have a concern with the 1/2" foam and 1/2 plywood. If you are placing anything other then carpet over this you may find that the plywood sheet may not be true enough to each other. I would consider T&G wood subfloor so that the joints don't give you issues.
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