Adding insulation above a 2×4 framed roof with wide swept/flared overhangs and fancy exposed rafter tails
I have a 1909 house in Portland OR (zone 5) that like most houses in the area of the vintage had no insulation. The large attic is unconditioned and I’ve pumped 5.5″ of cellulose onto the ceiling joists to mitigate the heat loss upwards.
I’d like to turn part or all of this attic into conditioned space and would rather insulate above the sheathing if possible. One plan is to remove the roof sheathing but leave the horizontal batons in place and nail 2×4 or 2×6″ rafters on top of the existing 2x4s which are on 24″ centers.
I should be able to do the same over the flared eaves to maintain the look of the house albeit with a much thicker roof. I’d probably fill with dense pack cellulose or Roxul bats above and below. This would hopefully strengthen the rafters and get closer to R-39 .
I want to end up with about 3/4″ of vented space from overhang to ridge. I’ve also considered 4″ of polyiso foam above the current 2x4s with hollow 2×4″ boxes over just the eaves but worry about roof strength and weight (for seismic reasons).
Local code simply says to add as much insulation as is practical with R13 as a minimum. Complicating factors are that there are many hips and 3 gables to contend with so there’s a temptation to tear the whole thing off and replace with 12″ rafters and straight gable ends. It’s hard to know what any of this would cost. The house is 26′ x 44′ and the peak height is 11′ approximately, pitch is 10:12 everywhere.
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