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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Replies
Opt for at least R5 of closed cell foam, then the remainder can be open cell or even netted and blown cellulose. Note that HFO or water blown foam has much lower GWP. You can do a hose test on the roofing prior to insulating if you want to check for leaks.
With the addition if a continuous "vapor diffusion vent" at the ridge, you could skip the foam altogether and use all netted cellulose but it sounds like you're past that point in the build.
https://www.buildingscience.com/documents/building-science-insights/bsi-100-hybrid-assemblies
https://www.buildingscience.com/documents/insights/bsi-088-venting-vapor
Best,
j
Hi Jason, thank you for your comment. I read about the vapor diffusion vent to prevent damp sheathing. My idea for that was to monitor attic humidity and install a dehumidifier if necessary so I didn't install such a vent.
It sounds like you don't see any issue with the sheathing sandwiched between a low-perm underlayment and closed cell foam. Perhaps I am making something out of nothing. I do like the air sealing qualities of the closed cell foam to protect the sheathing from inside humidity.
Does anyone else have any opinions on this issue? Thanks again!