Florida Roof Insulation
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Hi all, building a new home in Florida with a metal roof and trying to decide how to insulate it.
Most of the metal roofs that pass the Florida code are screwed directly to the plywood sheathing. This makes sense to me as it limits the uplift potential and should be much stronger through a hurricane than mounted above the deck on purlins. Those hurricanes also lead me to want an unvented assembly. I’m leaning towards a synthetic underlayment (possibly pell and stick) which has me wondering which insulation method to go with.
Closed cell under the roof deck and creating the double vapor barrier seems to be a relatively safe solution with my design and wind concerns.
Does spraying 10″ of open cell buy me anything for vapor permeance/drying? It seems a much riskier solution to the bottom side of the roof deck without active dehumidification in the attics.
I am leaning towards 2″ closed cell and them open cell over that to get to R-38. Possibly something other than open cell, but it seems the most time effective solution with trusses.
What would you guys do?
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Replies
Does your code allow for vapor diffusion ports or are you willing to condition/dehumidify the air in the attic?
https://www.buildingscience.com/documents/guides-and-manuals/gm-2101-guide-building-conditioned-unvented-attics-and-unconditioned
https://www.buildingscience.com/documents/building-science-insights-newsletters/bsi-119-conditioned-unconditioned
If so of the two choices I would go open-cell.
In FL, you will likely need to dehumidify the attic, if not thoroughly condition it. Open cell with attic dehumidification would be fine.
Thanks John. I was debating on the vapor diffusion port and love Joe's writeups. His statement about hurricanes is what really had me going down the closed cell approach. Reading it again, that's not exactly what he said, so I'll have to study some more and decide if the extra effort is worth it.
If Peter is correct and I'll need dehumidification anyway, why struggle with the vapor diffusion? I guess it adds a layer of protection...
I need to keep studying, but appreciate the brainstorming.