insulating where stud wall is built away from poured wall

codnuggetsJanuary 21, 2009

Greetings from a long-time kitchen/bath lurker/poster. It's time to tackle the basement. I've been pouring through all the insulation threads I can find (XPS ones especially) and I can't find one piece of information. I've seen how to handle channelled XPS with furring strips and a stud wall directly adjacent to XPS, but how does one insulate properly where the stud wall will be built 9 or so inches from the poured concrete wall? I think maybe I read that the air space will invite moisture, and I don't see where a foot of insulation would be a reasonable answer. If it matters I'm in Portland, OR, and my basement is 75% below grade. The basement stays 62-65 degrees year-round and despite the fact that it rains a lot it has only seen water infiltration once in a single crack when the timer on my drip irrigation broke and the spigot basically ran non-stop for a week. However, I can't say that's true for the 83 years the house stood before I moved into it.

Thanks for the guidance,


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the experts may chime in But I think the 1.5 - 2" of XPS is your starting point. Whether you biuld up gainst it or 9" away should not matter UNLESS that 9" of concrete floor comes into play...which I would bet they say to XPS that portion of the floor.....????

    Bookmark   January 22, 2009 at 10:44AM
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To clairfy, I am planning to use Delta-fl on the floor with an OSB layer on top and butt it up tightly to the XPS on the wall.

    Bookmark   January 22, 2009 at 12:25PM
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I agree with andrelpaume2. But why are you building the stud wall 9" away from the foundation??

    Bookmark   January 22, 2009 at 6:36PM
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The 9" gap is to build in front of existing plumbing and HVAC obstructions. After some more planning, I don't have to go to 9", but I'll probably still have about 4" airspace. I'm guessing this is not a problem then?

    Bookmark   January 23, 2009 at 1:51AM
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I presume that the plumbing and HVAC are going parallel to the wall. Are they at the ceiling or partway down? You may be able to build a soffit if the obstructions are near the ceiling. If going vertical then enclose within the wall.

I believe I would attach XPS to the concrete foundation which would fill 2" of the 4" gap you're now anticipating.

Your greatest need, being that your are in the Pacific NW, is to control the humidity year 'round. A good dehumidifier in your case would be a priority no matter how you insulate.

    Bookmark   January 23, 2009 at 12:03PM
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We have a large pvc pipe along one wall of the basement foundation wall that slopes at an angle to meet the septic tank. There is no way a soffit would work. The finished basement wall sits about 10" from the concrete foundation and was constructed as though it were up right against it (vapor barrior, insultation, etc etc). There has never been a problem. I can actually see into the cavity from my workroom and it is as dry as dry can be. We do not have a dehumidifier either.

    Bookmark   January 29, 2009 at 1:04PM
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Portland is in the Marine Climate, which necessitates a building structure that dries to the interior and the exterior depending on the season. (See link for Building Science Corp. recommendations; they're still working on specific Marine recommendations.)

The US Department of Energy has recommendations for insulation levels by zip code.

Here is a link that might be useful: Guidelines for Mixed-Humid Climate

    Bookmark   January 29, 2009 at 5:01PM
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