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Building envelope for reconstruction of Roof

Robert Binder | Posted in Energy Efficiency and Durability on
My 90 year old brick house in Zone 5 (Pittsburgh) has a simple gable roof that covers a finished third floor. This space has knee-walls under a flat ceiling, above which is an unfinished attic space about 4 feet high at the peak. Below all this is a typical stick frame. The exterior walls are comprised of plaster lath, studs, ~1.5″ Celotex-type sheathing nailed to studs, tar paper, ~1″ airspace, and then a brick veneer. There is no exterior wall insulation. The house has central air, and the air handler/cooler is tucked into a third floor knee wall of the stairwell along with a single return. Ducts run through the attic and behind the knees to the rest of the house. Heat is provided from hot water circulating through cast iron radiators.
 
Re-roofing will be needed in a few years. I plan replacing the existing slate shingles with steel shingles. To improve thermal efficiency at the same time, I began evaluating an “over-roof”, as described in 
 
But, I am now leaning towards a different approach, which is to remove the entire existing roof and 3rd floor partition walls (leaving the floor joists), and then reconstruct the third floor so as to achieve more usable space (adding two dormers on the perpendicular axis) and raising the roof planes about a foot. The new roof would be insulated to at least the minimum R under a new roof deck. This seems to offer the most overall value.
 
My question is how to integrate the new framing with the existing exterior walls to achieve an improved building envelope. I plan to relocate the air handler and ducting within conditioned space. For the second floor, I am considering adding cellulose blown-in through holes punched through the lath to fill the stud cavities, but I’m unsure if that is wise, owing to the risk of condensation and resulting rot, mold, etc. and how that would work with the new structure.
 
Thanks in advance for suggestions, etc.
Bob B.
 

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