Hey There, Fellow Tradespeople! 

I’m planning a wall system for a renovation project that will likely be (from inside out), 1/2” drywall, Intello Plus Smart Vapor Retarder, 2×6 studs with dense packed cellulose (between the studs), 1/2” cdx pine sheathing, WRB over cdx, a 3.5” layer of Timber HP Timberboard, ( R-13), over the WRB, 3/4” strapping (rainscreen) over the Timberboard, and wood clapboards—(R-33 total with R-13 continuous).

Does anyone know if the old rule of thumb for minimum required R-value for 2×6 walls, applies to Timberboard HP (wood fiber board)? I’m re-reading Martin’s article (and table) that specifies for Climate Zone 6, a minimum R-7.5 for 2×4 walls and R-11.25 for 2×6 walls, but believe this article was for foam sheathed walls–not wood fiber board exterior sheathing. 

What I’m hoping to find out, is if I can safely get away with a thinner layer of the Timber HP, to save money– like maybe the 2.5” thick (R-9) Timberboard, due to the robust inner vapor control layer, carefully detailed air tightness on the warm side, and “different than foam board” approach/ characteristics. Any takers on this one?