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We're planning how to finish our basement, and haven't been able to figure out something related to insulation.

The home's in Minnesota, and was built in 2008. XPS insulation (installed by the builder) covers all concrete surfaces, and I haven't seen any indication of any sort of moisture intrusion. I can't see what's behind the XPS, though I took photos during construction and it doesn't look like there's anything but concrete there.

Here are my questions:

  1. When framing the exterior walls, should I add any sort of vapor barrier between studs and XPS, or will the existing XPS facing suffice?
  2. Should I add batt insulation between studs, or leave those interstices empty?
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2 Answers 2

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I finished a basement in MN and spent a whole lot of time researching it. I eventually settled upon a method using XPS, steel studs, and paperless sheetrock as recommended by Building Sciences corporation and the US Dept. of Energy. I detailed the specifics on the answer to this question here:

http://diy.stackexchange.com/a/8644/1209

As for your questions:

  1. I strongly recommend not ever using a vapor barrier in a basement. Read through my answer above for some more details. But, in summary: XPS acts a moisture retarder, but will still allow trapped moisture to dry to one side or the other. And since a basement can have moisture on either side, it's good to let it breath. When I finished my basement in MN, this was the biggest challenge I had with the St. Paul building inspectors. They insisted on vapor barriers, though invariably, in every basement I visited in MN with a VB, it smelled musty. I eventually did enough research to show them that the times have changed and VBs are not recommended that much anymore in basements by modern builders.

  2. adding the fiberglass likely wouldn't hurt anything...it's more insulation. But it may not add a whole lot, either, if the XPS is already sufficiently thick enough. So, the question is how thick is the current XPS? If it's providing a good r-value already, you might be good to go there. Save the time/effort and just stud out from there (using Metal studs, IMHO).

Good luck!

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Awesome... thanks for a detailed reply! I did read through the post you linked to earlier, and found it extremely informative. Yes, I'm gravitating towards metal studs (influenced by the post you linked to), with treated wood + foam spacers for the bottom sills. Am pretty excited to start in the next few weeks. The XPS insulation is (looks like, at least) 2" nominal, with R15.8 (for reflective sheathing). –  alt Mar 29 '13 at 15:45
    
The basement generally feels comfortable enough as is; though it's certainly colder than any of the upstairs levels. I can generally spend a couple hours only wearing a thin t-shirt and a pair of shorts, so it may not be bad, as it is now. –  alt Mar 29 '13 at 15:51
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Concurring with DA01. Usuallly, fiberglass, is not recommended in basements, because it is SO air-permeable, but paired with XPS foam sheet at wall, would be a hybrid solution.

Just be sure to use UNfaced batts (no kraft paper covering).

See How to insulate a basement wall and Tyvek or tar paper question.

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Thanks! Unfaced batts; will certainly keep that in mind when shopping. –  alt Mar 29 '13 at 15:52
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