Need to reinsulate attic in older house

razlFebruary 18, 2009

I recently purchased a 38 yr old colonial in RI. Inspecting the attic I noticed very thin white batts (wool, maybe fiberglass) with deteriating kraft paper. I removed all the old insulation, sealed all gaps/penetratins with spray in foam and now want to add blown in cellulose to an R50-R60 level.

I'm a little confused about the best approach for a vapor barrier. Should I lay down 6-mil plastic or just put down a cheap krafted fiberglass (say R11-R13) and blow cellulose on top? Batts seem straigh forward but if plastic, how do you properly install it?

I was initially thinking of just spraying in a 1" foam but this stuff would cost about $800 alone.

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Ron Natalie

Use the plastic continuous air barrier. 6mil would be OK but 4mil will work just fine. You'll need to wrap it around the joists but that's not too hard if you roll out perpendicular to them. The kraft paper stuff is pretty ineffective.

    Bookmark   February 19, 2009 at 7:42AM
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razl

To cover or not to cover the joist was one of my questions. So I roll it perpendicular like you said. Can I assume I need to try to matched the up/down contours so there's no air pockets or streching when I blow in the cellulose on top? I guess I need to overlap the plastic a couple of inches and tape it off, right? If so, any special kind of tape? Thanks.

    Bookmark   February 19, 2009 at 12:54PM
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katdip

I wouldnt worry too much about taping the plastic sheeting. Drape it loosely perpendicular over the joists, tucking it loosely into the corners but leaving slack for the insulation to push it in tighter. Overlap the sheets by 6-12" to avoid big air gaps, but you've fixed a lot of the air leaks through the caulking.

    Bookmark   February 19, 2009 at 2:45PM
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