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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https://www.buildingscience.com/documents/reports/rr-0108-unvented-roof-systems/view
On a curved roof like that it may make more sense to build out trusses on the inside (losing some headroom) to a total depth of 19", installing a couple inches of HFO blown closed cell foam (R13-R14) and 9" of 1.5-1.8lbs JM Spider or Optima (~R36-R37) which would meet the current code minimumum.
https://www.jm.com/content/dam/jm/global/en/building-insulation/Files/BI%20Data%20Sheets/Resi%20and%20Commercial/BID-209-JM-Spider-Plus-Data-Sheet.pdf
https://www.jm.com/content/dam/jm/global/en/building-insulation/Files/BI%20Toolbox/T07-016JMSpiderOverheadApplicationV2.pdf
For the retrofit truss structure, rip half-inch OSB into 11" wide strips and cut them in to ~14" long gussets and nail them to the side of a milled 2x4, every 4 feet, then nail that gussetted rail to the sides of the existing rafter. The resulting truss is considerably stronger than the original 2x4, (stronger than adding on another 2x4) and has the requisite depth to hit R49 with a couple inches of foam and 9" of fiber, at a fraction of the thermal bridging of a 2 x 12, adding less insulation weight than 6" cellulose.
The interior side would need to have an air barrier and would need a Class III or tighter vapor retarder. Using Intello Plus as the interior air barrier & vapor retarder could also serve in lieu of blowing mesh, a method recommended by the 475 High Performance Building Supply folks (who sell Intello).
https://foursevenfive.com/blog/how-to-install-dense-pack-cellulose-with-intello-plus-airtight-membrane/
Thanks Dana and Bryce,
My roof has two distinct angles so not really a sweep and the angle changes at the edge of the wall. I could see adding a 4" rigid foam layer on top of the envelope and tapering the roof out to the edge with lumber triangles between the old T&G decking below and new decking above around the 3 foot wide eves. I'm not sure how I'd vent that area but drilling holes in the T&G from below might be an option.
I'd thought about interior only trusses but I'd loose too much headroom in many areas and would have endless numbers of creepers to deal with. Here's a google sat view of the roof with the ridges and valleys traced in.