Why not just use more closed cell foam and be done with it -- keep the 2x4 walls ? Why go to all the expense and trouble of furring it out ?
KISS .....
Yes -- 2x6 walls would have allowed an easy hybrid application -- but the cost and time of adding the furring and doing the two part insulation should be folded back into the cost of more foam. It's a better way to go. Thermal bridging is a minor issue .. very minor through 3.5 inches of wood and 5/8 of drywall.
GBA is great for an interesting read -- but, they have been incorrect many times over the years. They love all the outside rigid foam and sealed sheathing products .. I think it's going to be a big disaster.
I guess if they fully embraced spray foam (closed) -- they would be out of business.
My time is free and the project is enjoyable. The extra 2x4s needed, cost less than $250. The cost difference between 3 1/2 inches of spray foam and the hybrid 2x6 wall I surmise will be much greater, but that is one of the things I need to find out.
The R value difference between the two choices is greater than one might think. The spray foamed 3 1/2 inch wall is R25 according to the manufacturer. The hybrid with 1 1/2 inch spray foam (R10.5 roughly) and the remaining 5 1/2 inch spider (R20-R23 depending on density sprayed) could theoretically have an R value in the mid 30s going hybrid.
People may argue that time is money which is true, but I have more time than money currently. Every decision is going to be a trade off...
