Hi,
We are building a home in coastal Maine, zone 6, with a low-slope roof (2/12). We'd prefer exposed beams so have been exploring rigid insulation. Currently we plan the following layup (top down):
Solar Panels--
EDPM rubber roofing--
3/4 ply--
rigid foam insulation--
1/4 birch ply--
exposed beams.
This will primarily be a 2-season house (summer & fall) but heated during the winter to 50. A decent percentage of the walls will contain a lot glass (triple pane as long as fits within budget).
Our town has no codes. Question is whether insulating the roof above R-30 rigid foam board is worth the extra expense? My thought is that above 6" of foam board, additional framing would be necessary because otherwise it would be difficult to attach two layers of ply and 10-12" of foam with fasteners alone. Additional framing increases cost quite a bit and would probably also require deeper beams to carry the additional weight of the add'l framing on top. To go from r-30 to r-60 will probably double the cost of the roof structure. The house's heat load will probably be handled by the solar power generation.
My research has led me to believe that in a properly sealed house, the cold-weather difference between r-30 and r-60 is not dramatic. Am I missing anything? |