Cathedral Ceiling Insulation Sandwich Plan
Hello: been lurking on this site for years an it has been INCREDIBLY helpful– THANK YOU.
Based on all my reading and gathered knowledge, I believe the below plan will work for rehabbing the uninsulated, very leaky attic of my 125 yr old house. I am shooting to get R-40 but would be happy-ish with about R-36 (I’m in Dayton, OH).
I plan to insulate the entire roof using a cathedral style approach. All the roof slopes are 12:12. There are 3 gable ends and one hip roof end — two ridges meeting at a right angle at the top.
I plan to insulated the whole thing cathedral style because even though the attic is finished in the style similar to a cape cod, and I could theoretically go on top of the ceiling with batts or cellulose, I really want to have a nice attic to work in because have a lot of electrical and HVAC work ahead of me, and long term the HVAC will be in the “half attic” above the finished ceiling. Of course I will be removing the currently uninsulated drywall in the cathedral portion of the ceilings to do this, and will be insulating behind the kneewalls as well. But I’m willing to do that.
Here’s my “sandwich” design:
Air Gap (0.5″) – open to vents top and bottom of rafter bays
Tyvek or similar vapor permeable air barrier and radiant barrier
5.5″ high density fiberglass batts (but compressed to 5.0″)
— BOTTOM OF 2X6 rafters —
3.5″ foil-faced polyiso (Firestone fire-rated, taped at seams)
Questions about this plan:
1. Is it a good plan? My main concern here is I am a little scared of getting condensation against the air/radiant barrier under the sheathing. Should I be concerned about this if I have good air continuity above the barrier, and if the polyiso foam is taped and all penetrations sealed well, and the air gap goes all the way down to the soffits?
2. Since I am compressing my bats against the top foil face, I am not benefitting from the radiant barrier. Would I be better off using 3.5″ batts with the reduced R value, but now with an air gap between the batts and foam so that the top polyiso radiant barrier can work properly?
3. If this is a good design, are there any suggestions on a good product for the vapor-permeable air/radiant barrier? Does such a product exist? Should I be considering using baffles instead? Are they vapor permeable?
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