R-value at roof/wall intersection
I am located in the Albany, NY area zone 5A.
I purchased a dilapidated house and will be doing a deep energy retrofit. The siding will be torn off, most of it is rotten or soft, and I will install huber zip which should provide the air and vapor barrier. Larsen trusses will be installed and the cavity depth will be 12″, ~R-40.
The roof is 12:12 and peak is roughly 10′ high. New asphalt shingles were supposedly installed approximately three years ago- they appear to be in decent shape. Roof framing is dimensional 2×4’s 24-30″ O.C. There is a shoddily built dormer which will be removed. From exploring around the dormer it looks like there was 1x decking, wooden shakes, 1/2″ plywood, tar paper, then shingles. The exterior walls are balloon framed with a double 2×4(dimensional) top plate. The second floor ceiling joists are approximately 7″ deep.
I have attached a an image with a basic drawing. Brownish color represents framing, greenish is cellulose (didn’t extend it into the ceiling joist like I did for the roof rafter for clarity), orange line air barrier.
My concern is the R-value achievable around the top plate and roof/wall intersection. Once the 2″ ventilation channel is considered, the top outside corner of the ceiling joist has about a 2.75″ cavity that can be used for insulation which ultimately presents moisture (cold spot on the ceiling) and energy issues. However the minimum depth from the top of the double plate when including the joist cavity is 7″ which is acceptable if spray foam is used.
Now since this is a complete rehab project, there is no issue with running a continuous interior soffit to allow for the proper insulation depth. Anyone have any input on whether that would satisfactorily address the issues? It seems like it would reduce, but not necessarily eliminate, the issues.
I have contemplated tearing the whole roof structure off and going with a flat roof (labor to be done by me and not to fond of working on steep roofs) because the dormer has to come off which represents about 1/4 of the roof area. I was originally concerned with 2×4’s 24-30″ O.C, but it seems to have worked for the past 100 years. It does have a pretty new 30 year roof on it. I do like the 12:12 appearance. It would increase the expenses. Ultimately, I am struggling to justify whether the energy problem at the roof/wall intersections merits redoing the whole roof structure.
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