Mineral wool roof
We are in the beginning design phase of our new house. The current concept is a 1,600 sqft ranch with a 4/12 pitch shed roof to allow clerestory windows along the south side. We are in the middle of Iowa, on the edge between climate zone 5 and 6.
The single roof plane would be a cathedral roof with both interior and exterior insulation. No valleys, no skylights, no dormers, no can lights, just a single roof plane with fascia mounted vents at the top and bottom.
I am curious if we can make an option work using mineral wool batts on the inside and mineral wool boards for the exterior insulation. We are considering the Roxul Comfortboard 110 under slab, exterior foundation wall, and exterior wall so by having it on the exterior of the roof as well we may be ordering enough that a truckload would be economical.
After reading several articles, including how to insulate a cathedral ceiling, the below concept appears to be a slight modification to a vented approach since the ventilation channel is above the exterior insulation. With that in mind, I think I have it correct that the following should work.
From the inside out:
– 5/8″ GWB
– 3/4″ Furring Strip Electrical Chase
– Intello Plus Smart Air Barrier and Vapor Retarder
– Rafter cavity filled with 11″ Mineral Wool Batts tight to sheathing. R46.
– Delta Vent SA Vapor Permeable WRB which should also act as air barrier.
– (2) Layers of 3″ Mineral Wool Rigid Boards with seams offset. R24.
– 2×4 on flat Furring Strips running diagonally to allow air flow and to support metal roof. Otherwise 2 layers of 3/4″ furring stiprs first vertical for ventilation and 2nd horizontal to support roof. Likely attached with Heco Topix Therm Fasteners.
– Standing Seam Metal Roof
Please let me know if you think this detail would pose any concerns.