Vented vs unvented roof in 1960 Cape Cod
We have a 1960 Cape in Zone 4/5 (Philadelphia) and have gutted the “2nd” floor to the rafters in preparation for insulation. The rafters are 2×6, 16″ on center. Insulating above the roof is not an option unfortunately. We are undecided on 3 options (all of which will include “conditioned” space behind the kneewalls. None of these options get us to R38, but we had almost no insulation before to this will be a major improvement to the existing building:
1) Vented roof with air barrier, but insulation between 1-10 perms (semi-permeable). This plan would included site built baffle made from 1″ Amvic perforated EPS SilverBoard and a 2″ air gap under the decking from soffit to ridge. The remainder of the cavity would be Roxul 3.5″ Mineral Wool. Furring the rafters would be needed. Below the rafters would be a 2″ continuous layer of Rigid Roxul Comfortboard, smart Membrain and drywall.
2) Vented roof with site built 1/4″ plywood baffle and 2″ air gap below decking from soffit to ridge. 2″ of closed cell spray (non-permeable) foam under the baffle, and the remainder filled with Roxul Mineral wool (flash and batt). Furring the rafters would be needed. Below the rafters would be a 1/2″ continuous layer of Amvic SilverBoard (about 3 perms), smart Membrain and drywall.
3) Unvented roof with 2″ closed cell spray foam directly to decking, and the remainder filled with 3.5 ” Roxul Mineral wool (flash and batt). Ridge and soffits would be sealed with foam. Furring the rafters would be needed. Below the rafters would be a 1/2″ continuous layer of Amvic SilverBoard (about 3 perms), smart Membrain and drywall.
All 3 scenarios would dry to the interior, but also have a complete interior air barrier.
I’ve been reading theory on GBA and have read the most popular articles on “how to build a cathedral ceiling”, “vented attic” etc. I’m trying to pick the best option. I’m a bit worried about spraying closed cell to decking in consideration of future repairs, leaks, etc. I believe “venting” would also assist is drying any moisture between the outside brick and the wall cavity (2″). This open channel could vent from soffit to ridge. Calculating “Dew point” condensation on the closed cell is very ambiguous depending on many factors, so option 1 seems to make the most sense with that consideration.
I am looking forward to any expert opinions (and constructive disagreements!)
GBA Detail Library
A collection of one thousand construction details organized by climate and house part