Best insulation method for an unvented attic with a concrete tile roof
Hi, I am building a house in Kendall County, Texas on the border of Zone 2 and Zone 3. I have read a lot on here about spray foam along with the pros and cons of open vs closed cell. It seems the consensus is that:
1) Open and closed cell foam behave about the same in terms of roof leak detectability.
2) Closed cell foam helps prevent attic moisture from building up in the roof sheathing.
These things make me lean towards closed cell foam, or at least a layer of closed cell followed by open. I was considering some rigid foam above the roof deck but opted out of it due to not finding any contractors familiar or willing in this area.
Some background: The external walls of the home are ICF. The roof deck is 5/8″ Advantech sheathing. I have a low-perm self-adhering underlayment installed (CMI SecureGrip Max HT), and I will be using concrete tile with elevated battens to form a “cool-roof” with vented eves and ridges (above the roof deck).
With the concrete tile roof, my understanding is that the underlayment is the actual weather barrier since wind-driven rain can get under the tiles and run down under the battens.
The main concern I have is that the combination of a low-perm underlayment and a roofing material that isn’t the actual weather barrier may pose a problem. If I use closed cell foam, I have essentially sandwiched the sheathing in between two water/vapor barriers.
My HVAC loads were calculated based on R30 at the roof deck. Any advice on the best way to achieve this for my build would be greatly appreciated. My goals are to have an energy efficient structure and to avoid future maintenance headaches.
Thanks in advance!
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