Feedback on Roof Retrofit Plans
So after pouring through the wealth of information on this site I’ve made some large changes to plan to redo my second floor. If you could point out any glaring errors it would be appreciated, it’s hard to keep it all in line sometimes.
Current situation: gutted second floor, Cape Cod, 5″ true rafters, Zone 6 Montreal. Soffits are closed but there are 2 roof vents (not ridge).
1. Drill 2 x 2 1/8″ hole per rafter bay in soffits, staple insect screening overtop, vinyl soffit covering
– attic is around 300 sq ft, with 40 rafter bays I get about 144 sq inch of soffit ventilation
I’ll redo the soffits in the upcoming years and will install a continuous soffit vent, but for now I need a quick and easy way to open them up.
2. Insulate rafter bays, soffit to attic. Outside to inside design (roof is staying as is unfortunately, so no wiggle room there)
– 1 inch air gap
– 1 inch polyiso rigid foam for baffle (sealed in the soffit above top plate with rigid foam to stop wind washing)
– 3 inch dense blown cellulose
– 2 inch polyiso foam directly over rafters
– 1×4 furring strip
I attain a fairly miserable R29 on this section, but I’m not sure how else to improve upon this without redoing my roof. Would the ideal for the dense blown cellulose be to install the rigid foam first, and the installer shoot behind it, or would it be easier if they install netting and then I install the foam?
My ducting runs fairly close to the roof behind the knee wall. Should I consider wrapping in fiberglass even though it’s in the conditioned space, as the insulation isn’t great?
3. Insulate the attic
– 18 inches of loose blown cellulose
I attain around R60 here.
When I did the demo on the second floor there was 6 mil poly all along the ceiling. From what I’ve been reading here I shouldn’t worry about re-installing it, but simply ensuring I have solid air sealing. Does this go for the bathroom as well, which has a fan, but will still see a lot more moisture?
4. Insulate the exterior walls, outside to inside
– 4 inches of dense blown cellulose
– 1 inch of polyiso rigid foam
I attain an R20 on these walls. Do I need to add furring strips overtop of the foam, or can I attach the drywall directly into the studs? Worried about space on an already small floor.
From what I’ve been reading this seems like the usual recommended plan for insulating a vented gable roof. Ensure there’s ample soffit ventilation (I know 1″ is the minimum, but again there’s not a lot of wiggle room here), rigid foam over rafters to stop thermal bridging, focus on air sealing above all else … I’m wondering if there’s anything I’m missing, or anything I can do to improve the design. I’m staying away from spray foams for a number of reasons which obviously lessens my attainable R value, but it seems to out weigh the risks for me.
Does this system allow me, down the road, to redo my roof to reach a higher R value, or if I do that will I have to remove everything I’m currently doing and start from scratch?
Thanks a lot as always for all of the valuable information provided on the site.
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