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Interior rigid foam to reduce thermal bridging in sloped ceiling?

Ben_Jamin | Posted in General Questions on

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?

Thank you!
Ben

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Replies

  1. Expert Member
    Dana Dorsett | | #1

    Leave no air gaps between insulating layers. An air gap is a potential thermal bypass & fire spread path.

    XPS is the least environmentally friendly insulation in common use. Polyiso is much friendlier, using hydrocarbon rather than HFC blowing agents.

    Don't install a continuous layer of foam. Instead, cut 1.5" wide strips of 1.5" or 2" thick polyiso attached the rafter edges 1x furring for a 2.75" thick "Bonfiglioli strips", and install compressed R11s or R13 batts between the strips, hanging the gypsum on the furring. That will allow at least some drying path for the rafters, whereas a continuous layer risks a moisture trap, and it's a lot of foam for minimal performance gain.

    https://www.finehomebuilding.com/membership/pdf/9750/021250059.pdf

    Cutting foil-faced polyiso is easier and cleaner than XPS or EPS. See:

    https://www.finehomebuilding.com/2009/04/15/theres-a-better-way-cutting-rigid-insulation-2

    Even 1.5" of polyiso would more double the R-value of the 2x8s which is robbing the R42-R49 foam of it's potential performance. An R13 compressed to 2.25" would perform at about R9, about the same R-value as the 1.5" polyiso. An R11 compressed to 2.25" would perform at about R8.

    If 3/4" or 1" foam + 1x furring is all you can tolerate for loss of headroom, do it. The thermal break isn't as great, but would still leave the framing fraction at a higher R than 2x12 rafters.

  2. Ben_Jamin | | #2

    Hi Dana,

    Thanks so much for the quick response! I like the idea of strips instead of continuous, allowing for some drying path for the rafters to the inside in case it's ever needed. As I mentioned in my post there is strapping installed perpendicular to the rafters, so the polyiso strips won't work on the rafters, I would have to add them to the strapping (I don't want to pull the strapping because it's nailed in and kind of stuck in place with the foam in some areas, and i really don't want to mess with the foam job). So could I add the polyiso strips to the strapping, and then use longer screws for the drywall through that to the strapping? This gives me about 1.5"-2" space for fiberglass or rockwool, but it would be going across the rafters. Would this help create the thermal break I need?

    Also, still have the question - what if I just install the drywall directly to the strapping, and leave the ~1/2" air space between the drywall and the spray-foam. I understand i don't get the full R49 R-value that they promised but it's still a huge improvement from what I had before. Am I just losing a little heat? or is there a bigger potential problem?

    From soffit to kneewall and flat ceiling to ridge... add some open cell over the closed cell? or something else?

    Thanks,
    Ben

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