Flash & batt and rim joist insulation
I read an article – I believe on GBA.com that talked about the risks of flash and batt due to thermal break issues caused by framing. Basically if code calls for 2″ of closed cell foam before I lay batt insulation, there still won’t be enough insulation where the closed cell insulation, 2×6, and batt insulation all meet – creating risk for condensation in the wall structure.
I can’t find that article any longer and see plenty of articles on GBA.com suggesting flash and batt is ok. So is it ok – or should I have concerns with doug this? I am doing with blown in cellulose or fiberglass (still researching which) in the walls, but my insulator reca flash and bad batt for rim joists – if the point raised above is valid – should I just use blown cellulose or fiberglass instead of flash and batt?
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The risk is primarily to the sheathing at the mid-point between the studs. This is not at all about risk to the studs. The lower-R of the studs keeps them warmer (= drier) than any sheathing that has 5.5" of insulation between it an the interior. The low vapor permeance of the foam limits the amount of wintertime moisture from the interior reaches that cold sheathing.
Flash'n'batt is pretty safe, but also pretty low performance:
The thermal bridging undercuts the performance of the closed cell foam pretty drastically, since there is about half an order of magnitude difference in the conductivity of the studs compared to closed cell foam. Even though the framing fraction represents only 20-25% of the total face area of the wall, the studs end up conducting far more heat than the 2" of closed cell foam that is covering 75-80% of the wall.
If the equivalent R is installed on the exterior of the sheathing it covers 100% of the wall, including the framing fraction area, yielding quite a bit higher overall performance. It also keeps both studs & sheathing warmer/drier. A 2x6/R20 wall with 2" of exterior rigid polyiso suffers only a bit more half the heat loss of a 2x6 flash'n'batt with 2" of closed cell foam. In other words, it's nearly twice the total performance of the flash'n'batt solution.
Matthew,
Dana Dorsett is correct about the fact that a continuous layer of exterior rigid foam is preferable to the flash-and-batt approach. But the exterior rigid foam option may not be possible for you.
Thermal bridging through the studs is not a risk; rather, it is a heat transfer mechanism that lowers the thermal performance of the wall assembly. In other words, if you use the flash-and-batt approach, your wall sheathing won't rot -- you'll just have somewhat higher energy bills than some other options.
For rim joists as well as stud bays, flash-and-batt can work -- as long as (a) you use closed-cell spray foam, not open-cell spray foam -- and (b) the spray foam layer is thick enough for your climate zone.
Here are links to two relevant articles:
Flash-and-Batt Insulation
Insulating rim joists