Exterior rigid foam plus celluose rafter insulation questions
I have a difficult-to-insulate second floor with as-yet uninsulated side attic kneewalls and an insulated attic above most of the living space. The rest of the 1917 house has gotten lots of energy upgrades: minisplits instead of 80 y.o. furnace, 3 inches of foam under siding, etc. It’s just the attic left.
Reading GBA and other sites for years, I’ve decided to bring the side attics into the building envelope, rather than insulate the kneewalls. Since we need a new roof soon, I’ve decided to do what Martin Halladay details here (and elsewhere): http://www.finehomebuilding.com/design/departments/energy-smart-details/an-unvented-superinsulated-roof.aspx
We can’t afford a superinsulated roof (and I can’t do this work myself), so I’m thinking 2 x 1.5 in. polyiso, staggered, taped, with the roof slopes in the side attics blown with cellulose (4 inches). We’re in Zone 4 marine, so R18 exterior foam will keep the sheathing warm. So far, so good.
One question: where should the air barrier go? Martin suggests taping the foam as the air barrier (at least one layer). But the very similar roof structure shown on the govt’s energy site shows the air barrier *under* the foam: https://basc.pnnl.gov/resource-guides/above-deck-rigid-foam-insulation-existing-roofs
Does it matter?
Second question: to cut costs, I’d like to only put rigid on the roof slopes over the side attics, then just fur out the sections of roof which don’t require insulation (and install the roof, probably standing seam, over the furred out areas): 1. the 8 foot roof overhang on one side of the house, which extends past where the side attic ends. 2. the roof over the already-insulated (loose cellulose, insulated years ago) attics which is over the second-floor living space. (so this won’t be a ‘chainsaw retrofit’) Should the foam overlap a foot or so into the parts of the roof which don’t require insulation? (in other words, run the foam a foot down into the area above the overhang, for example)
Here’s what the house looks like from outside: https://www.flickr.com/photos/arthurcts/sets/72157648832196257/
Third question: I’ll seal base of roof slope/attic floor with canned foam, along with top of the kneewall/roof slope junction. Anything I’m missing?
This represents my best effort to balance cost and energy efficiency, based on years of insulation bids I wasn’t happy with, and efforts to process what I read on GBA. Thanks for any final suggestions, if anyone has any!
Art
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Replies
Arthur,
Q. "Where should the air barrier go?"
A. The air barrier can go anywhere you want, as long as your are consistent and take steps to assure that the air barrier is continuous. Options include (a) taping the roof sheathing seams with a high-quality tape like Zip System tape or Siga Wigluv; (b) installing an air barrier membrane like Solitex Mento (with taped seams) above the roof sheathing; (c) taping the seams of your foil-faced polyiso. If you like redundant systems, you can even choose to implement two of these options.
Q. "Should the foam overlap a foot or so into the parts of the roof which don't require insulation?"
A. I don't know if you need a full foot, but you have the right idea. Make sure that you seal the perimeter of the insulation with caulk or high quality tape. And make sure that your insulation is continuous at the wall/roof junction.
Q. "Anything I'm missing?"
A. That's a hard question to answer, but it sounds like you have done your homework and you are on the right track. Just make your air and insulation barriers continuous, without gaps, and pay attention to airtightness -- and I think everything will be OK.
Thanks, Martin. Good to hear that this more or less makes sense. I hadn't thought about sealing the perimeter of the insulation -- that makes sense with this kind of arrangement. I'll see which of those air-sealing system the roofer prefers. I like the idea of the zip system tape, because the sheathing seems less likely to move/shift than the foam. I'll also try to at least run the tape over the edge of the roof sheathing a little way down the wall, on the sides where there aren't any overhangs.
thanks again for your response, and for all the great information over the years from various discussions I've read.
We're proceeding with the re-roofing and exterior foam insulation described above, but we're now planning on putting exterior foam on the whole roof, including over the peak attic above second-floor living space, since with the various roofers we've talked to, there seems to be little cost savings to just putting foam on part of the roof. My question: the roofer we're working with insists that we ventilate the roof peak above the peak attic, despite the fact that the rest of the assembly is unvented. Do we need to do this? The assembly would follow Martin's and Building America's guidelines: cellulose between rafters in side attics, taped, staggered polyiso above sheathing, layer of OSB to attach roofing, standing seam on top of that. The one exception: there is existing blown-in insulation in the peak attic floor. I would leave that in place and not blow cellulose between the rafters up there. Ok to avoid cutting ridge/soffit vents up there?
Arthur,
This type of roof shouldn't have a ridge vent. You are aiming for airtightness. Your tiny attic near the peak will be fine without any venting. The rigid insulation will prevent the roof sheathing from getting damp.