Climate Zone 6: Low-slope warm roof materials / assembly
I am designing a large mono-pitch shed roof in Climate Z6. The slope is 1.5 in 12 or about 7 degrees. A ventilation space seems impossible due to the scale of this roof plane. 60 feet sloping up and about 25 feet wide. I would like the finished roofing to be standing seam metal as the roof is visible from various vantage points on the property.
It my understanding that we will need to install a high-temp waterproof membrane like Grace Ultra over the entire roof sheathing to prevents leaks. This means the roof assembly is only able to dry to the inside.
As I see it I have three options.
1. Specify a warm roof with closed cell spray foam to limit the vapor drive to the backside of the sheathing. For instance 10 inches of SPF @ R6/in is an good roof. But this technique is risky if there is a leak as condensation that occurs on the back side of the plywood would not be able to dry out. I’m also not trilled with the idea of using so much SPF. The idea of SIPS have been discussed by the structural engineer.
2. Another option is to use a smart vapor retarder with dens packed cellulose between the rafters. The dens packed cellulose would limit vapor drive but if and when the condensation does occur it would seem that the cellulose acts as a storage medium for the liquid and that eventually it would dry back to the inside, especially when the summer sun heats up the dark colored metal roof. Possibly add rigid insulation above the deck as well?
3. Expose all the rafters as part of the interior aesthetic and then place rigid board above the sheathing. I’m least familiar with this concept and it will result in a really thick assembly.
Does anyone know of a waterproof membrane that is vapor open and approved for use on low slope roofs with metal roofing?
Any advice would be greatly appreciated.
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