Spray Foam Roof Line or Not?
I am doing a new build in Tampa, FL (hot climate), and have been given a choice to either spray foam the roof line or go with a traditional blown/bat solution in the attic. An important distinction is that if I choose spray foam, it will be the roof line only and not the walls. I am told that traditional insulation will do as good a job as foam on the walls. My concern with that statement is the concept of foam providing a sealed envelope. And if the walls are not foam then is there any point in foaming the roof line. I know there are structural pros to foam, but from an insulation perspective am I voided out the benefits by not doing the walls as well? I tend to like the increased structural integrity being in hurricane country, so I feel almost inclined to do it even if the insulation benefits will be diminished. Thoughts? Also the choice is to go R19 or R30 on the spray foam. The installer is telling me the increased R rating is not worth the money (something like 95% to 98%).
That says it all, but if you need further reading: Table #3: http://www.buildingscience.com/documents/reports/rr-1001-moisture-safe-unvented-wood-roof-systems
For your new house: http://www.buildingscience.com/documents/reports/rr-9302-humidity-control-in-the-humid-south
R-19 isn't good enough at 5-1/4", need R-30 for code or at least R-25+ (7") for value, check with local AHJ; http://www.enersavesystems.com/pdf/Economic-Thickness-of-Thermal-Insulation.pdf
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