What’s the best way to retrofit interior with rigid foam?
I’m located in Portland, Maine. These questions concern a Cape Cod with a shed dormer running across the back side of the home — 2×10 rafters, 4/12 roof pitch, with 3 feet of original 10/12 pitch left on either side of the dormer roof. The opposing side of the roof has 2×6 rafters with 10/12 pitch in full.
My plan is to apply Dow-R 2″ rigid foam on the interior over the studs and rafters – 2×4 walls. I will be meticulous about air sealing seams etc.
I will run the rigid foam down the ceilings first, then up the walls. Dense-packed cellulose will be professionally blown in following this.
I have been doing a lot of reading! My question is the air/moisture permeability against the roof sheathing/rafters. I have read that this is a must re: rot and mold. If this is a must could I use an Ice and Water self-adherring membrane (like Grace) against the sheathing and rafters to create that barrier?
I also am planning to apply 5/8″ fire-code gypsum following this and right up against the foam board. Would I really gain a large advantage if strapping is used for an air space before the gypsum is attached? Would the answers apply to the exterior wall sheathing also?
Hope there is enough detail in my question. – Dean
Oh, nonvented, conditioned roof, full Ice & Water Shield on the exterior of the dormer roof, then architectural shingles. First 6 feet of opposing roof have Ice and Water, the ‘turkey runs’ have full Ice and Water Shield. Dormer roof sheathing 5/8″ Advantech tongue-and-groove, the remainder is 1′ pine space 1/4 inch apart.