Thermal break with TJI rafters
I’m a New Hampshire based builder- close to beginning construction on a small spec house. I’d like to build to a higher standard which has unique challenges: most new construction builder grade homes in this area do not employ green tactics. I need to be careful not to price myself out of the market.
I’m considering a double stud wall system with 7″ of open cell foam, 2×4 on the exterior and 2×3 on the interior. This won’t be much of a deviation from standard construction and won’t add a great deal of cost.
The gable roof has two different pitches, the front is a 12/12 and the rear is a 6/12. It’s 1 1/2 stories and the second floor will have a flat, vented insulated ceiling (likely cellulose blown to R60) under the 6/12 pitch.
The 12/12 will be mostly open to below. I’d like to fasten a layer of OSB under the the 11 7/8 ” TJI rafter flanges. This will create a substantial ventilation channel and leave 10″ for cavity insulation; I’m not sure what that will be yet, possibly more open cell foam as that contractor will already be on site and it has the added benefit of air sealing. We’d also use a smart vapor retarder.
My question involves creating a thermal break with the TJI rafters. Is this necessary? How much conductivity will happen through the 1/2″ OSB web? How does the rest of this assembly sound like a cost effective solution?
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