Unvented roof assembly
I am building a compact unvented roof, following Joseph Lstiburek builders guide to cold climate. What I have , is ceiling is 2×6 t&g with ice & water shield for air barrier then 9” of polyiso insulation with 1/2 plywood sheeting on top. (I’m in zone 5) What would be the recommended for roofing paper on the plywood? I was going to use a 30lbs tar paper but metal manufacture says that’s not a good choice because of temp of metal roof. Can I use a synthetic roofing paper in this application with ice and water shield on roof overhangs?
Also I am not planning to insulate the 36” overhangs on this build. my understanding that in zone.5 I would be ok doing it like this? Any insight on this would be great. Thanks in advance!
Regards Jmmr
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Replies
Hi Michael.
I believe that is a pretty common opinion and one that is accepted across the industry. Many builders use synthetic underlayments under metal roofs, but since the metal roof manufacturer suggested that you don't use felt, maybe they have a recommendation for what you should use.
You do not need to insulate roof overhangs, but the insulation should extend to the outside of the top plate and the space between the top plate and the bottom of the roof deck needs to be insulated, assuming that you are not installing insulation in the rafter cavities.
Thanks Brian, my concern is do I need to have a vapor open paper or synthetic under the metal? The metal manufacture is recommending ice& water over the whole deck.
Hi Michael.
My understanding is that unless you are installing the metal roofing on furring strips, creating an air space between it and the underlayment, it doesn't matter if your membrane is vapor open because the metal is an impermeable material. If you did have an air space, a vapor open roofing membrane would let the plywood dry into the air space should it get wet.
Thanks Brian for your help!
Note that an impermeable material with even tiny air leaks creates an assembly that is permeable. This is a large and critical difference in some cases - but data is limited for metal roofs.
Just a couple of comments:
9" is a lot of foam, and not super green (unless using reclaimed foam), and considerably higher than code-minimum performance.
Hitting code min on a U-factor basis only takes 6" of continuous polyiso, which is substantially cheaper on both the foam and fastener fronts.
Hitting the same performance point as 9" of polyiso could be done with 6" of polyiso and installing R20-R23 fiber in the 2x6 rafter bays, which would have huge dew point margin at the roof deck for a zone 5 climate.
Hitting code min on a U-factor basis could also be done with just 4" of polyiso up top + R20-R23 in the rafter bays, and would still have more than adequate dew point control.
If the overhangs are deep (18"+ ) extending the foam 6"-8" beyond the wall insulation on south facing pitches can sometimes be worthwhile in snowier parts of zone 5. With deep overhangs ice-dams can get started by localized melt-out from the rising column of air on that large unglazed solar collector called "siding". It's common to find siding surface temperatures 30F+ higher than the ambient temps when it's in full sun. With deep overhangs the melt outed area from that solar gain doesn't always extend to the roof edge and it re-freezes. With shallower overhangs (<12" ) it's never likely to become an issue, but with 36" overhangs it's worth thinking about if you're in a location that can get 18" of snow in one go, or 5'+ in a single season. Of course metal roofing helps mitigate that issue too, so maybe I'm overthinking it.
Thanks Dana, I am using reclaimed polyiso, I found 3” thick sheets so I’m going to stack them on top of t&g roof deck to get my r value. I like your thought on extending insulation on south facing wall!
Michael,
If the t&g roof structure extends out past the exterior walls to form the overhang, you can get a lot of air leakage there. Even if you don't insulate the overhang, make sure to extend the air-barrier out to the fascias and do as good a job air-sealing the t&g at the walls as you can. Otherwise you can end up with a lot of moisture from the interior of the house making its way into that roof space at the perimeter.
Thanks Taylor, That is the plan to install air barrier all the way to the eave and seal and insulate really well at the top-plate. I was going to air seal with exterior sheathing were it meets top-plate & the rafter-tails with zip caulking & same on the interior then add polyiso in cavity above top plate and then spray foam the perimeter of board.
The way I've tackled this issue (provided the T&G is not needed to support the overhang) is to rip a 1/2" slot in the T&G in along the middle of the top plate and spray foam this gap. This does a good job sealing up the air leaks along the grooves. Backer rod with caulk would also work.