Vapor barrier?
D B| Posted inGBA Pro Helpon
Hi,
I have read your articles on vapor barriers and retarders and have a question about my cottage.
The basement is block wall construction (9′ high) and is below grade for most of it (on 3 sides – walkout on one side which is wood frame construction on that side).
The floor is poured concrete.
We are in the 5A or 6A area of Ontario Canada.
The cottage is not air conditioned in the summer.
The block walls are spray foamed with 3″ of closed cell foam.
I plan to frame out the walls in the basement – would you recommend a vapor barrier, semi-permeable barrier or nothing at all in the inside of the framed wall?
Thank you.
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Replies
DB. The closed cell foam is vapor barrier. Dana has commented on the need for rigid foam (EPS, for example) under the bottom plate to avoid moisture issues.
DB,
On the interior side of your studs (on the three sides with concrete block walls), you need to install drywall as an thermal barrier (because spray foam is flammable). You shouldn't install polyethylene or any similar membrane under the drywall.
For more information on this issue, seeHow to Insulate a Basement Wall.
Thank you for the help.
Can you point me to the link from Dana about rigid foam under the bottom plate? (ie. how to do this?).
I've read about using it under the sub flooring but if I build out the exterior walls as a first step before I install any sub flooring, how would I do that under the bottom plate?
Thanks again
DB,
If you have a continuous horizontal layer of rigid foam under your slab, you don't need to use Dana Dorsett's trick (installing a layer of rigid foam between the slab and the bottom plate of a basement partition). However, most building inspectors will require the bottom plate to be pressure-treated.
If you have a continuous horizontal layer of rigid foam above your slab, followed by plywood or OSB subflooring, you also don't have to use Dana Dorsett's trick.
Dana Dorsett's trick is used for basement partition walls installed on concrete slabs that lack a horizontal layer of rigid foam.
If you want to read more about Dana Dorsett's trick -- and reading more might just confuse you -- here is a link to a thread where it is discussed:XPS or PVC under base plate.