Vapor Barrier / Insulating Below Grade

moteef17May 11, 2010

I'm a little confused about vapor barrier. I'm getting mixed information from contractors and other web-info and the thought of putting up a plastic sheeting directly against the masonry and then framing walls is.....well....scary. Any moisture collected has to go somewhere, so even if i don't see the problems within the finished space, where's it going/what's it doing behind that plastic sheeting? Something about that is disconcerting.

Framing walls with 2x4 and the interior walls would be insullated with R-13 batts (unless i hear differently). My question is how best to insulate the exterior walls. I like the idea of simply framing my walls with a 2" space between the studs and masonry and i'll finish the interior with drywall. But, is leaving that airspace enough to help avoid moisture build up? Here's what i think i need to consider


1) XPS against masonry, framed wall against that with NO batt insulation in studs

2) XPS against masonry, framed wall against that WITH batt insulation

3) XPS against masonry, framed wall with ROXUL (product my lumber store recommends for basements)

4) No XPS on masonry, 2" airspace with 2x4 walls and ROXUL or batt insulation in framed wall (not touching concrete)

Feedback is welcome,


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Authoritative information on building science is available free from Building Science Corp. (see link) whose assorted engineers are also the source of much of the US Dept. of Energy Building America programme.

As long as you have sufficient XPS against the wall to keep the temp below the dewpoint (min. 1" in cold climates) options 1-3 will work well. However, you would get best results by insulating to the min. R factor recommended (scroll to map) by the US Dept, of Energy for new wood-framed homes in your climate zone.

Here is a link that might be useful: Building Science Corp. on Basement Insulation

    Bookmark   May 12, 2010 at 9:03PM
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