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Old 04-09-2014, 06:40 PM   #1
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Another basement insulation question..

Hello All, I am new here, been lurking for a while though. I am about to start finishing my walk out basement in Chattanooga, Tn. Two walls are block, one of which is 80% below grade, the other about 50%(on a hill). I checked the IECC website and it specifies R10/13 for my area. My question is do I use 2"xps to achieve R10 or should I use a combination of thinner xps and f.g. batt? The basement is dry, I am in the coatings business and can effectively measure humidity. That being said, I don't plan to skimp and hope for the best because it has been dry so far. I know you guys have answered this for people a hundred times(I searched), but I'm still not sure. If you have time to answer, thanks!


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Old 04-10-2014, 07:20 AM   #2
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If fiberglass batts can be avoided, then avoid them. Either do it with all rigid foam or rigid foam and Roxul or another rock wool product.


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Old 04-10-2014, 08:36 AM   #3
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1" XPS would be minimum thickness I would run.

Get the bands and ribbon boards sealed up prior to basement finishing.
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Old 04-10-2014, 10:00 AM   #4
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I would go with XPS combined with closed cell spray foam. You can either use spray foam to seal any seams around the foam board, or you can bulk up on R-Value and coat the XPS with 1" of foam.

Closed cell foam has an R-Value of 7 per inch, will seal air leaks and resist moisture penetration. To get to your IECC R-Value recommendation you can either do 1" XPS and 1" spray foam, or 2" XPS and seal the seams with closed cell foam.
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Old 04-11-2014, 01:22 PM   #5
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How high a relative humidity in the basement are you expecting... using your 3 coldest months average high/low temps equals the basement wall (above grade/1' below grade) temps at 49.6*F and safe to 55%RH using R-5 (1/2")XPS;

If paying attention to details; you can DIY with only foamboard/wall cavity (for wiring) and no SPF, for a lot less money. BTW- you can mix/match using continuous/cavity insulation to meet ICEE, check with local AHJ.

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