Standing-seam roof, closed-cell spray foam, self-adhered membrane: Double vapor barrier?
I am designing/building (my profession) a 1378 sf 2-story house (no garage) in St. Helen CA.; which is California Climate Zone 2 or IECC Zone 3-c.
The average January temperature is 48 degrees F and average July temp of 68 degrees F. The extreme conditions from Manual J are 31 degrees F and 97 degrees F respectively. Average Rainfall of ca. 40″ from Oct to Feb. Average Relative Humidity 80% January and 55% July.
I plan to build a simple 4:12 Hip roof with cantilever trusses at 24″ o.c. over a conditioned attic space that does not contain any mechanical equipment. I will be using 6 of closed cell spray between the top cords of the trusses and will provide 12″ of clearance from the top chord to the top plate which will be solid blocked and allow the foam to tie-in the top plate. On the roof deck I will be using 3/4″ cdx, some sort of synthetic underlayment, and a standing seam metal (cool) roof. The roof will be unvented. The framing will be done in the hot summer, the moisture content of the plywood and trusses will be well below 15% at the time they are “dried” in.
In looking for help with the underlayment I turned to Hot Dry & Mixed Dry Climates by Joseph Lstiburek pg. 163 which is somewhat similar to my plan except for no knee walls and the space will be unoccupied. Why is he specifically calling-out fully self-adhered underlayment for the entire surface? This clearly produces an assembly that will have extremely low permeance on both sides. I like the minimal risk the roof could get wet from wind-driven rain but I don’t understand why I couldn’t use the self-adhered on the hips, ridge, etc then use a high-temp mechanically fastened synthetic on the rest of the roof. The ATAS spec sheets for both products state.1 perm for the self-adhered and .05 for the mechanically fastened.
I will be using an ERV coupled to my Carrier Greenspeed Furnace & Air Handler. My total heating load will be about 15k BTU and cooling load of 1 ton.
Any consideration to my confusion would be greatly appreciated.
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