Opinions on this Wall Assembly with Spray Foam and Mineral Wool Rigid Board
I have a 1960’s cottage, climate zone 6. There are 2′ overhangs all around the cottage. Original assembly was T&G, 2 x 4 x 24 cavity with R8 fiberglass, 1″ fiberboard continuous, 1/4 painted plywood as siding with strapping covering seams. My plan is to remove the exterior plywood & fiberboard. The new assembly will have the cavity spray foamed (closed cell) from the exterior against the back of the tongue and groove, 1.25″ roxul Comfortboard 80 R-5 continuous, typar housewrap, then reinstall the 1/4 plywood against the comfortboard. I know this doesn’t meet the continuous insulation R-value to cavity R-value recommendations for condensation. I chose the comfortboard for its perm rating of 30 and think the spray foam will stop most moisture anyway. I also realize that there is no true structural sheathing and the comfortboard is rated as non structural. The old assembly had this existing issue and building hasn’t shifted at all in 60 years. The closed cell will add some rigidity to the exterior. What am I overlooking? Will moisture still be a concern in the wall assembly. Any recommendations on taping seams and securing the comfortboard? Manufacturer also recommends securing the comfortboard by sliding boards in between strapping which I don’t have the space form in my assembly. Will the .5″ air gap between the closed cell and the back of the 2×4 where the comfortboard is attached create an issue? The goal is to leave windows and trim as is. Thanks in advance.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part