Distance of Wood Framing from XPS

the-tech-guyJanuary 25, 2012

Here is another typical basement remodel question I could not seem to find from searching this forum. What is the recommended distance if any should I place the wood framing from my basement walls that been covered with XPS? Opinions seem to vary on the web and some say to have no space because it induces convective currents which would rob the heat. What have you guys done?

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According to Building Science Corp., framing is tight to the XPS. That is the way I do it.

    Bookmark   January 25, 2012 at 11:03AM
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even if you frame tight, won't you still get an air gap in a traditional framing scenario (not tacking 2 x 2's to the XPS and drilling thro the XPS to the foundation wall). If I were to insulate over the XPS, do I frame first and insulate between the studs or over the insulation?
I need one of my perimeter walls to be framed with 2x 6's. Any special instructions or handle it like every other wall? The -tech Guy, worthy knows his stuff. he answered several of my posts.

    Bookmark   January 26, 2012 at 7:57PM
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I asked similar question yesterday. I'm in NJ so climate may have something to do with different insulating approaches. I'm seeing contractors putting up 2x4 walls about 1 or 2 inches from foundation wall and using regular fiberglass insulation. When they come to house waste lines they just keep wall away about 4 or 5 inches. Same with electrical panel, they come away about 3 feet put up a nice straight wall and have a regular size interior door that you go into to get to electrical panel. This wall may be entire width of room and people use it for storage. Actually the job I was on the electrical panel was in corner as is typically the case, they came away 3 feet with wall and continued it the entire width of room. There is usually other stuff entering the basement where panel is anyway. Keep in mind these are pretty good size basements so losing some space isn't that critical.

    Bookmark   January 27, 2012 at 9:11AM
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even if you frame tight, won't you still get an air gap in a traditional framing scenario

Extruded polystyrene attached to foundation wall followed by wood framing.
Fibrous insulation can be added between the studs. It does not have to be covered
with a vapour barrier unless Code-required.

Yes, here and there behind the studs on a poured wall. But with a layer of foam board on the wall already, you've eliminated the bridging and will not have condensation on the foundation wall. I insulate afterwards with fibrous insulation between the studs.

Warm side vapour barrier inhibits the moisture flow in the winter,
but the reverse flow in the summer in the above grade portion of the
basement leaves moisture trapped in the insulation, feeding mould.

(There are a couple of alternatives I should mention: 1. directly spraying the wall with high or medium density polyurethane foam that can be supplemented with fibrous insulation between the studs; 2. fibrous insulation with a "smart" vapour retarder. Membrain is the only one I know of at the moment and it has been independently tested below grade in a cold climate.)

Remember the Rim.

climate may have something to do with different insulating approaches.

Yes. But not in this case. It's simply ignorance of modern building science--which is already at least 20-years old. The huge gap between insulation and foundation wall supports convective currents, thus reducing the effectiveness of the insulation. However, the spacing allows condensation on the walls--and the moisture naturally coming through the walls--to dissipate in the space rather than directly moistening the fibre glass. At least, that's the hope.

All photos from Building Science Corp.

Here is a link that might be useful: Building Science Corp on Basements

    Bookmark   January 27, 2012 at 10:50AM
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The idea is to get any frost line inside the insulation thickness.

    Bookmark   January 29, 2012 at 11:30AM
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Or dew line (condensation point).

    Bookmark   January 29, 2012 at 11:31AM
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