Insulating roof with 2×6 rafters in Eastern Massachusetts
We are renovating our 110 year old house located just north outside Boston. Planning on upgrading the insulation of the top floor (under sloped roof). Currently a living space with what looks like 50 year old mineral wool insulation. Most of the lath and plaster covering the underside of the roof will go.
The gable roof’s rafters are actual 2×6″ 24″ o.c. Not vented. No exterior insulation, nor there will be any anytime soon (roof is 3 years old; asphalt shingles over 6′ of Grace Ice and Water Shield at the eaves, with double lapped 15# felt above). I am planning on 2″ of closed cell spray foam directly to the roofing boards, so ~R12-R14, followed by 3.5″ of Roxul batts, so R14.
I fully realize it does not meet the current code, but due to headroom issues I am not able/willing to fur out the rafters. It will be miles better than what it is now: moist interior air condensing on the inside of the deck. No rot, the deck manages to dry out.
While substandard, will the stack up be reasonably safe mositure-wise? Should I add interior vapor retarder (Membrain)?
I’ve been considering retrofitting venting, but then I’ll lose at least an 1 inch of space for vent baffles, and retrofitting the offsite vents will be difficult on this house. Adding ridge vent would not be a big deal. So I have pretty much decided to keep it unvented.
I’ll appreciate any feedback very much!!
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