Returning member here lastly posting first question sorry if I picked the inaccurate Category.
We are changing original roofing on 1991 single story home in SE PA Zone 4a and doing a chainsaw retrofit. Home is 36 ′ long with 30 deg pitch roof. 20 ′ of that length is open room cathedral ceiling, the other 16 ′ is traditionally vented. Cathedral area has real 2 × 9 rafters on ridge beam. walls are 2 × 6 fg, plywood, 1 ″ pi, stucco (good condition however there is a strategy cover w/ rainscreen and siding)
Cathedral side layers are:
-drywall
-.5 ″ pi foil dealt with both sides
-R30 yellow manville fb craft dealt with toward interior
-Styrofoam baffles 1 ″ from soffit to ridge
-sheathing 1/2 ″ plywood
-linking vertical wall to other half is drywall
-penetrations in ceiling are 8 can lights and a skylight
Conventional side is
-drywall
-2 × 6 ceiling
-r19 fb batts attic
-Styrofoam baffles 1 ″ at vented soffit a
1.5 ″ pi on vertical wall that links to cathedral side.
I’ve read “Musings of an Energy Geek -How to Set Up Stiff Foam on Top of Roofing System Sheathing” where in Climate Zones 4A and 4B, you’ll need a minimum of R-15 of rigid foam (about 4 or 4.5 inches of EPS, 3 inches of XPS, or 3 inches of polyiso);
On managing the shingles/tar paper/sheathing you could see the traditional vented side sheathing was still great ok however cathedral side its badly distorted and breaks on removal amazing it never ever dripped, 1 area with moisture on drywall around can light closest to heating system.
My rebuild from leading plan is this
Cathedral side:
-cut off rafter tails scabbed in zipR6 sheathing flush to existing wall below
-drywall (existing)
-constant pi (existing)
-keep r39 batts (existing)
-deleted skylight include insulation there (done)
-seal 1 ″ foil dealt with poliso boxes foil taped to existing pi and lined with 1.5 ″ rigid rockwool around can light holes for usage lower profile LED component. (1 of 8 done)
-5/ 8 ″ sheathing
-henry blue skin vp 100 covered to wall below sealed with acrylic wrb
— 2– 4 inches of xps taped
-flat 2 × 4 vent channel
-5/ 8 ″ sheathing
-grace ice and water over whole surface
Conventional side:
on outside like above
on inside i can move very same insulation into 2 × 9 rafters if required.
Questions:1) is the foil faced polyiso directly above the drywall make a problem in that any air that gets caught in my 2 × 9 cavity between pi and the xps can’t go out?
2) According to post ref ‘d above in my 4a zone I need 3 ″ (r15) of external foam. Or 30% of overall on outside.Going by 30 if i have R30 bat + r3 pi on within (33 x. 3 = 9.9). Can I squeak by just using 2 ″ of XPS (r10)? The XPS was type of sustainable sourced, I ‘d rather not perpetuate its need and avoid that extra roofing depth. In principals you layed out in short article 3 Code-Approved Tricks for Decreasing Insulation Density I’ll attempt to get the U element values to operate in my favor or go purchase 1 ′ xps. If you combine 2 ″ and 1 ″ any advantage to one or the other being the top/taped layer?
ty