Unvented Cathedral Ceiling Insulation Strategy
Architip
| Posted in General Questions on
I am insulating a 2×8 framed gable roof with board sheathing and asphalt shingle in zone 4A (Maryland) no attic – ceiling will be open to a bedroom and a bathroom.
My plan is to insulate underside of sheathing with 2 1/2″ (min R15) close cell Spray foam and 5 1/2″ of rockwool batt. Additionally, to up overall R values and to try and lower the thermal bridging of the rafters I was thinking of adding 1 1/2 -2″ of insulation board to the underside of the rafters.
Am I making a mistake with the rigid board? by sandwiching the mineral wool? or if I work with a simi vapor permeable board will I be ok?
Appreciate any Guidance!
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Replies
The place to start is this article:
https://www.greenbuildingadvisor.com/article/five-cathedral-ceilings-that-work
Thanks DC,
I have read through that one. And what seems like a million others. So may just be confusing my self.
But my inclination is that rigid foam (EPS or Polyiso) continuous below the rafters in my case would be akin to a sheet of poly - which according the the referenced article would be bad unless the assembly is vented. I have also read (or at least understood) from Building Science Corp. that as long as everything is dry to begin with then you are probably be ok?
Rockwool Comfort board 80 might be the solution as its vapor open? but will need to fasten GWB through to the rafters whatever I install.
Appreciate any practical guidance - what would y'all do to mitigate thermal bridging that is not adding insulation above the roof deck (cost prohibitive and roofers that I know are not familiar with the details which makes me leery)
Thanks
Architip,
I think Building Science Corp is referring to sandwiching materials like sheathing between two impermeable layers, not a cavity with batts or cellulose. Those definitely need a drying path.
I would not place rigid board under rafters, the code specifically prohibits an interior vapor barrier with flash and batt. It is not worth it just to reduce thermal bridging. You should just cut strips at rafter width and use thicker batt like a bonfig wall if you are worried about it.