rafter insulation under skip sheathing in unvented assembly
I’ve seen the recommendation that there be no air gap between the sheathing and insulation when filling the rafters for an unvented assembly, but I missed the explanation on why it’s important.
I’m going to be filling rafter cavities where the sheathing consists of a layer of 1×4 skip lap (originally a cedar roof), which was covered over with plywood sheathing for asphalt shingles. If I put rockwool batts in there, there will technically be a gap above the rockwool in between the 1x4s.
I’m in Climate Zone 4C, the rafters are 2×6 and I’ll also be adding 4 inches of GPS Foam on top of the roof deck. Cathedral rafters will be filled with dense pack cellulose (or JM Spider?).
I have two traditional attic areas with some of the space inaccessible, or nearly so, from inside. I’m not sure I can do dense pack in those areas.
Is it problematic to have a rockwool bat up against the 1x4s with gaps in between to the plywood?
Edit: attached a picture
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Replies
Peter, I believe the reason it is strongly recommended not to have a gap between interior insulation and the underside of the roof sheathing is moisture, primarily condensation. Under the right conditions (in the winter scenario) warm wet air would condense against the cold surface at the inside face of the roof sheathing. However, in your planned case with 4" of external rigid foam you are keeping your roof sheathing warm, so likely not a concern. There is a table that shows the ratio of inside to outside insulation for different climate zones in order to avoid condensation, with 4" in zone 4 sounds like you'd be fine but find the table and confirm it.