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Old 09-28-16, 12:01 AM   #4
nibs
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Thank you for the input. It makes sense.
I would hope that the trusses are better than R3, because they make up about 1/4 of the volume of the roof structure. If I calculate the truss as being 6" of wood and 6 inches of mineral wool insulation (Roxul) I come up with about R37 and with the rest of the roof being R60
To arrive at the truss value, I took the bottom and top chords thickness of 4.5 inches, then took the 12" on center jack studs and laid them flat for another 1.5 inches, I ignored the 3/8 plywood on each side of the box truss and added in 6 inches of Roxul for R 28 + R9 for the wood structure for R37, & the 12 inches of styrofoam between the box trusses at R 60, should give me an R value of 54 - 4 for conservatism so I think we have an R50 roof.
May have been better to go with 2X12 rafters, but we are pretty happy so far.
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