Opinions on this Insulation Retrofit Plan
1960s split level with 2×4 construction in Zone 6. Along with the construction of a 500 sqft grade level addition (with 500 sqft basement), I’m planning to retrofit exterior insulation to the existing walls (currently thinking 4” semirigid rockwool, given the existing polyethylene vapour barrier) and significantly improving the roof/attic insulation. The roof is a hip roof with a 4:12 slope. The truss heels are only about 4” high and there is only about 4’ to 5’ of vertical clearance in the attic at the peak. It’s a tight space. The existing roof is vented and has insulation consisting of two layers of fiberglass batt (~R24) without ventilation baffles. Some of the batts are quite dirty as the air must be flowing through them. Given the minimal amount of insulation I can provide on the inside at the eaves, I’m considering converting the vented roof to unvented and installing roof top insulation.
The Plan (also see sketch, Option 2):
– Install 6” of GPS/XPS/Polyiso (~R30) on top of the roof sheathing,
– Seal between the top of the walls and roof sheathing with spray foam,
– Carefully rip out the poly vapour barrier above drywall from above,
– Install 2 layers of overlapping ~R12 batts directly above the ceiling or blowing in about 7” of cellulose (~R24) for about R50 total effective R-value.
1. Does this plan make sense? Am I missing anything?
2. This is going to be expensive, isn’t it?
3. Is it actually okay to have the large air space between the batt insulation and the underside of the roof deck in an unvented attic? Typically I’ve see details for cathedral ceilings where the batt is installed right against the roof sheathing. I could do this here, but it would be more difficult given the limited access, also it may be hard to get more than R12 between the 2”x4” top truss cords.
4. How easy is it to join roof top insulation panels at hips and valleys? Would you fill gaps with spray foam and perhaps reinforce the joint with tape/mesh at each layer?
5. Given that I want the addition’s roof to match the existing (shape style and elevation), could these insulation details work just as well for new construction or is there a better way?
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