Details for Spray Foaming an Unvented Vaulted Roof
Thanks for all the insight on insulating vaulted ceilings – both vented and unvented.
16 years ago we did it wrong. Not enough vent space plus I think the space/void above our front porch did not help in generating proper airflow to the ridge vent. Inside, the top floor has a knee wall and vaulted ceilings. The rafter bays are 5.5″ deep and 24″ wide. Raftermates were placed under the roof sheathing – then 5″ of roxul (R22) – then poly vapor barrier – then the drywall. Ice damming was guaranteed each winter. We live in climate zone 6. Building code minimum is R-31 here in Sudbury, Ontario.
Now we need a new roof. I will be working from the top down. I would like to avoid needing to take down all the drywall/vaulted ceiling inside.FThe sheathing is coming off and I will be pulling the roxul and spray foaming – likely heading toward an unvented foam filled roof.
Q1. should I cut/pull the poly? will it affect the spray foam cure?
Q2. should I put up the new sheathing and then have the foam injected to fill the rafter bays / cavities?
or spay the foam into the empty rafter bays and then put on the new sheathing?
I am concerned about needing to trim off the excess foam before re-sheathing.
Q3. is it possible to flash & batt from the top down? 2″ of foam against the drywall ceiling and then put the roxul back in? being able to reuse the roxul cost-effectively somehow.
Q4. should I insulate the space above the front porch?
I will be adding horizontal strapping above the roof to correct some dips and straighten the plane of the roof. This would increase the depth of the rafter bays by 1″-1.5″ and give us a grid to work off of. At the same time I am hoping that it may also help negate some thermal bridging.
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