Basement Insulation: What to do wen exterior is insulated?

chwtomMarch 15, 2012

I am getting ready to finish the basement on my 5 year old home in Wisconsin. I am wondering what to do about insulation.

The basement foundation walls are 4" thick concrete. There is a 1" layer of rigid foam insulation on the exterior, and the foundation was damp-proofed with foundation sealant. The floors are 4" thick, with another " of clean stone base and a layer of visqueen. There is spray foam insulation in the joist bands.

Looking at the basement science page, they recommend using xps on the basement walls followed by a framed wall with batting, but say the best strategy is to insulated from the outside. Well, I did insulate from the outside, so now what do I need to do?

Can I just frame and bat since we have the foundation sprayed and a layer of rigid foam on the exterior? Or should I still put the rigid foam on the inside? I'm only finishing half my basement (it's a ranch), so it'll probably only be about 60 feet of wall, but I'd still rather not waste money on something that isn't needed.

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Are your basement walls really only 4" thick? I find that hard to believe--my impression was that 8" is typically the minimum.

Chances are there's still a good portion of the concrete wall that's above ground and not insulated. Were I in your position, I'd go with a thinner layer of XPS (probably 1") on the inside of the concrete, then fiberglass between the studs.

    Bookmark   March 15, 2012 at 9:53PM
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The recommended level of insulation depends on your climate. In the Lower 48 above the Mason Dixon Line, R-20 is the recommendation of Building Science Corp.'s Dr. Lstiburek. One inch of XPS = R5. So you've got a ways to go. In a cold climate, one-inch of XPS on the interior is sufficient; I'm not sure if one inch to the exterior will keep the interior walls above the dew point, thus protecting the fg from dampness.

    Bookmark   March 16, 2012 at 10:17AM
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In a cold climate, one-inch of XPS on the interior is sufficient

Sufficient, that is, to keep the wall warm enough to prevent condensation. However, it must be supplemented by further insulation to achieve energy savings.

    Bookmark   March 16, 2012 at 8:09PM
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Thanks for the followup. Is there a way to test the wall to see if it is warm enough already--something like the floor test where you tape a square of plastic down?

    Bookmark   March 16, 2012 at 8:42PM
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Not that I know of.

But my thought is that the external insulation is so minimal--BSC and Building America recommend a minimum of 2"XPS for high performance in a cold climate--that the wall might still be cool. Plus there's likely extensive thermal bridging at the top and wicking up of moisture through the walls (unless there is insulation under the wall or a thermal break). If it were my home, I'd add at least an inch of high-R foam on the interior as well.

And don't forget the rim joist area, either with foam board well sealed, or easier and surer, spray foam.

Spray-foaming the rim joist area is an essential step in
creating a healthy energy-efficient basement.

Here is a link that might be useful: High R Value Foundation Assemblies

    Bookmark   March 17, 2012 at 5:03PM
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