Addressing Top Plates for Deep Energy Retrofit
Is there a way to address the thermal bridge around the top plates – roof truss connection on a deep energy retrofit without having to insulate over the top of the roof with layers of rigid foam (trying to avoid the cost and complexity)?
I know this is a topic that’s been addressed a few times here on GBA. I’m assuming it’s still pretty common for those trying to retrofit existing homes.
In a PH Accelerator episode (17 minutes):
Just wondering if anyone’s come up with a solid alternative solution?
Since the existing roof needs to be replaced, I was wondering about the following as an option (see attached diagram):
Repair existing roof sheathing as necessary.
Add roof underlayment.
Add 2×4’s through sheathing and into existing roof trusses. This extends out the existing overhangs, which are shallow (8-9”), and serves as a nailing base for a second ½” layer of roof sheathing.
Add second layer of roof sheathing.
Apply roof underlayment.
Put down shingles.
The point of all this would be to take advantage of the full height between the top plates and the existing roof sheathing. We think there’s about 4”, give or take, in this space. Not having to maintain an air gap in this space to ventilate the roof will give us an additional 1.5” of insulation.
We will maintain an air gap for ventilation by adding the 2×4’s and then a second layer of sheathing. With new soffits (continuous vents), ample air flow from new soffits up the roof to the ridge vent between the 2×4’s.
With spray foam over the top plates, this should be about R-20 to R-25 (?).
The rest of the attic would be blown-in cellulose (R-60 to R-80).
Joe suggests matching the R-value over the top plates with the R-value of the existing wall. Is this still the correct goal to shoot for?
Paul and Jesper seem happy with their solutions. Any chance the potential risk for moisture/mold/rot in this area around the top plates is overstated as long as the air sealing in this area is perfect?
Welcome any feedback.
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