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Old 07-03-2012, 06:59 PM   #1
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Insulating Floor of Addition

I am putting a small 2nd story addition on the back of our house. It is supported on columns, so the floor is open to the outside.

I am going to be using fiberglass batts between the floor joists, and am considering using a layer of 2" rigid insulation (XPS) below the floor joists (there is a bathroom with plumbing in the floor). My question is what do I do about a vapor barrier? Is it okay to use unfaced batts and use the 2" of XPS foam as the vapor barrier since it is vapor impermeable?



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Old 07-03-2012, 07:12 PM   #2
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Insulating Floor of Addition

If the bottom is exposed, I'd used faced batts and Exterier rated t & G plywood, or for a better look 1 X 6 T & G on the underside.


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Old 07-04-2012, 03:13 AM   #3
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Insulating Floor of Addition

Vapor barriers are, in general, a no-go any more. Vapor retarding and air sealing is what you want to do. Your rigid foam will probably stop your joists from condensing, so I would sure use it and seal all gaps w/ Great Stuff and tape. Look for Roxul instead of fg batts; fg batts are a pp excuse for insulation.
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Old 07-05-2012, 01:46 AM   #4
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Insulating Floor of Addition

No vinyl flooring;

Cover for critters and fire.

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