Hi there.

I have a 1980 house with cathedral ceiling in main area. We added an addition with matching roof line to this area. Problem is, 2x6s was used as rafters, then 2x4s attached as furring. These 2×4 are not structural of course.

That’s pretty structurally sufficient.

Now get me R-38 in 8” with 1 inch air gap vent. I can’t figure it.

Cathedral fluff foam is R-38C is 10.5″ thick. Too much.
Rockwool or Thermafiber R15+ R23 is 3.5″+5.5″…too much.
R30 rockwool is 7.5″…too much.
R21 rockwool + R15 either fluff or rockwool is 2″ too much.

I challenge anyone to find Rockwool R23 or Thermafiber R-24 in single bags instead of pallets. A home owner just can’t do it without a 12 bag minimum.

Now try find R23 + R15 at above 16 OC. Can’t, and astronomical if going pallet.

I approached my city building inspector and ask if he’d fail my idea of iso R-6 Rmax rigid foam on the rafter face. He said no he wouldn’t fail the interior foam, though he’s never seen it this way, but couldn’t see how that wouldn’t be a vapor issue for me. I’d still have to cheat compression to have 1 inch for vent.

Short of expensive spray foam that is kinda questionable when you start digging into claims…my only option is to fur down the fur down either again or perpendicular.

I want R38. I do. From the bottom of my concerned planet soul. But lest we forget, my house was built with R13 and R21 throughout. It’s unlikely any house like mine will ever get a total insulation replacement.

So…now I am stuck in a situation where the solution is foam that is a significant percent of the total construction or two fir downs.

This isn’t adding up as good outcomes for people with homes. What am I missing?