Iam presently in the planning stages for a 24′ x 24′ great room addition to a home in WI. The roof will be a 12/12 pitch with asphalt shingles and will have a 3x 16″ LVL ridgebeam. The present thought is to use 14″ I-joists for rafters- plenty of room for insulation- with 5/8 osb roof deck. We are toying with the idea of using strips of R- Board cut to fit snugly between the webbings of the I-joists and fastened to the underside of the upper flange with poly caulk and min. fasteners. This would be to create a venting channel from top to bottom. Below that would be r-38 FG batts,1/2″ foil-faced with taped joints, drywall and t+g carsiding. The goal is to “seal” the fiberglass away from the cold air channell above with a more breathable material on the upper side and a tighter vapor barrier below to avoid moisture problems and ice dams. I’ve heard that r-38 next to an open airgap loses some-maybe a lot of it’s R-value-true/false? R-BOard has a value of about r-5, but will it breath enough to let out any vapor,-any better products for “breathability”? Energy efficiency is a big concern of the homeowner. I get widely varying answers around here with little consensus.My experience being mostly with trusses I am looking for other opinions from builders with more experience in this.

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