Interior rigid foam to reduce thermal bridging in sloped ceiling?
I’ve been reading and learning a lot of the articles on this site, thanks very much for all the experience and knowledge and guidance! I’ve recently started a project to finish my upper level, a previously unfinished attic of a colonial. It’s a pretty straightforward colonial with no hips, valleys, or dormers but 2 skylights in the rear.
I’m in Northeast MA (Zone 5). I previously had a vented attic but have since closed off the gable and ridge vents and filled the 2×8 rafter bays with ~7″ of closed cell spray foam directly to the underside of the roof sheathing from soffit to ridge. Expensive, yes. But I’ve already noticed my gas bills are less than previous years (used ~35% less therms Dec 2018 vs. Dec 2017)
However, I have noticed thermal bridging. On the frosty mornings I can see lines with less or no frost at every rafter, with wider lines on either side of the rear skylights where there are double rafters.
From what I understand, I could help reduce the thermal bridging in the sloped part of the ceiling by adding a continuous layer of internal rigid foam before I cover it with drywall. I should note that the contractor had added the strapping for drywall prior to the spray foam (something the spray foam contractors weren’t too happy with but were able to work around.)
So my questions are:
1) If I just add the drywall and do not address the thermal bridging, will it create any problems for me? Not so much heat loss, i’m more concerned with moisture/mold/rot.
2) If I add the continuous interior rigid foam, is 1″ enough? XPS or Polyiso? I was thinking Polyiso for the higher R-value but then am I creating a double vapor barrier?
3) For the areas above the flat ceiling and behind the kneewalls, would an additional 2″ layer of open-cell on top of the existing closed-cell help or what would you do there?
4) Does the air-gap between the rigid sheets and the rafters (caused by the strapping) take away from the thermal break?
5) Are the air gaps between closed-cell spray foam and rigid boards going to trap moisture, doing more harm than good?
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