XPS below rafters in cathedral ceiling zone 4
We are finishing off some attic space (most over a garage, some over main part of house) for very occasional use as a guest bedroom, kid's playroom. Total sq ft is about 350, with cathedral ceiling (8 ft at the collar ties) with ridge vent and soffits. I'm fairly certain that the roof structure is just shingles, paper and plywood. Air vent baffles were installed between all 2 x6 rafters and OSB was used as backing behind the 5 ft knee walls (and was thoroughly air sealed). JM Spider high density fiberglass was blown in to fill all cavities in the slopes and knee walls and R38 batts were placed between the collar ties (only about a 3 ft span). We ended up with R30 in the slopes (which is all that was required by code) and since the knee walls were actually almost 10" deep (had to install 2 x 6 5 ft kneewalls in front of the existing 2x4 knee walls (because the original builder only made them 4' 3" high), the R value in the knee walls is much higher than code.
I realize that this is not the most energy efficient way that could have been utilized. We didn't want to use spray foam because of cost and health concerns and did not want to furr out the rafters and lose the limited amount of head space we have. As I said, this room will be used infrequently, and there is a ductless 26 seer minisplit in place to take care of heating and cooling as needed.
Given all that, I have a question about adding a layer of XPS below the rafters before the drywall goes up. I understand the concept of thermal bridging, but other than providing greater R value, is there any other advantage to putting up the XPS? If I don't put it in, what are the chances of "ghosting" where the rafters meet the drywall? Mold on the inside of the drywall? If I do put a layer up, is 1/2" sufficient to prevent any issues, or do I have to use 3/4" or 1"?
Thanks for your help!
Posted Oct 17, 2012 10:16 AM ET
Edited Oct 17, 2012 10:17 AM ET
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