Replies

1. 
   Jon_R | Oct 30, 2021 09:20am | #1

   IMO, lapped WRB is more resistant to water entry than non-lapped taped joints.

   > 2” foil faced hi-R

   If you were near the code minimum for exterior R (I don't think you are, even with proper de-rating), I'd allow a little outward drying by not using foil.

   > ccspf sprayed to the roof sheathing

   Think about replacement when the roof fails and there is some rotten sheathing. R has to be according to code and I'd use something like R5.6/inch for the very long term de-rated R value of ccspf.

2. 
   walta100 | Oct 30, 2021 11:08am | #2

   I do not like the idea of foam as your primary water barrier just too easily damaged. I think a layer of house wrap before the foam sounds like cheap insurance.

   Will you have a vented rain screen? Will you have large overhangs? How much rain do you get each year? I might feel differently if you have a rain screen 4 foot overhangs and lived in a desert.

   When I see the words “spray foam” and “new construction” together I think poor planning or laziness. Seems to me if you must have the sloped ceiling it would cost less to use sheet foam over the rafters and even cheaper if you located reclaimed foam.

   Walta

3. Expert Member
   Akos | Oct 30, 2021 12:22pm | #3

   I would also not trust taped foam as a WRB.

   With outies my preferred assembly is to use fiber faced roofing polyiso for the exterior rigid with the WRB over the foam. This puts the WRB in the same plane as your windows simplifying flashing details.

   The nice benefit of fiber faced polyiso (besides the fact that it tends the be the cheapest for $/R around me) is that it is slightly permeable allowing a bit of drying towards the exterior.

4. 
   user-7031471 | Oct 30, 2021 05:23pm | #4

   Thanks for your feedback!