I am renovating a home we bought recently which has a cathedral ceiling. The roof structure is comp. shingles over 1.5" rigid insulation over 1.5" T&G. This assembly sits on the 4x10 rafters with the T&G exposed to the interior.
We want to raise the R Value to 38 as economically as possible.
My question is how to build a working insulation system over the existing rigid insulation to reach that R38? It looks like it's RMax which should be 9.6 R Value. What would be the least expensive options for either a vented or non-vented assembly to get that to R38?