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cutnail 08-25-2011 09:09 AM

rigid foam board on basement floor is it good or bad
Hello All,
My question is; I want to install 1" rigid foam on my basement floor to complete the "envelope" but when my basement was built I know the mason placed thick plastic over the stone before he poured the floor. The mason also placed drain tile on both sides of the footer that exits via a natural drain to daylight down near my pond.
I have poured walls and I have 2" foam on the walls and sills sealed with spray foam and I plan on framing a 2x4 wall 1" away from the foam wall. I live in Upstate New York. I don't want to cause a problem by installing the foam on the floor Any info is welcome. Thanks:confused1:

Gary in WA 08-27-2011 02:31 PM

Welcome to the forum!

I wouldn't leave an air space;
I would use p.t. bottom plate on a poly sill sealer with s.s. nails. The floor foam wouldn't cause a problem, rather it will solve any moisture there; end of article-


cutnail 08-27-2011 04:14 PM

Thanks for the info. The 2" rigid foam is adhered directly to the basement walls. The space I'm going to leave is on the warm side of the foam. I wanted to leave the space so there will be no contact between the 2x4 wall and the foam. I was also planning on leaving the space between the studs empty(no insulation) and then I will drywall the wall. Does that sound like a good idea, please let me know.
Once again thanks.

Gary in WA 08-27-2011 10:50 PM

Sounds like a plan. The foam will keep the concrete at a good temperature not to condense any water vapor passing through. You shouldn't get any convective loops due the cavity being at similar temperature because of the foam. Be sure to tape and/or mastic the foam board joints against air reaching the concrete, figs. 14, 15; Floor foam is also mentioned there.


cutnail 08-28-2011 08:39 PM

Thanks Gary.
I was planning on use PT for the sole plate and was going to install the sole plate on top of the 1" XPS. What your thought on that?

Gary in WA 08-28-2011 10:22 PM

Should be fine with that non-bearing wall. The foam has a strong enough compressive strength not to crush and will act as a thermal and capillary break. Add a bead of caulking under the foam board below the wall to completely seal out any air to the concrete, or stud bays. Use ADA on the drywall;
Check with local B.D about using p.t. wood plate, may not be required with the foam. Fire-blocking every 10' lineally is required per code, where the f.g. batts may possibly work there;

Be sure to close any openings to the floor joist bays above, behind the wood wall. One poster a while back, inspector required drywall directly on the foamboard…….


cutnail 08-29-2011 07:48 AM

I appreciate all the info and links.

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