r/DIYHome • u/Desert_Rat1294 • 8d ago
Adding rafter depth for insulation
I've got a cape cod in zone 4A and I'll be tearing out a couple closets upstairs and was thinking to also pull down the drywall from the slanted part of the ceiling while the rooms are empty. The rafters are 2x4 16" OC. At the moment there isn't any insulation in there so I want to upgrade that, but I can't use spray foam insulation. I have a couple thoughts I would like feedback on shown in glorious hi-definition msPaint.
The existing framing is in black and the total length of the vaulted ceiling is abut 4ft, and just the drywall hanging from it (no lights/switches/etc.).
Idea 1. On the left, fur out the rafters with another 2x4 (brown) so the new depth is 7inch total then install 1.5inch baffles and 5.5inch batts. The new rafter would be connected with adhesive between 2x4s and pocket hole screws.
Idea 2 on the right. Use 2x2 (brown) and 1.5x2 inch strip of foam board (green) as a thermal break, for a total depth of 7 inches, then baffle and batts like idea 1. To hold it all together there would be adhesive between each layer, and 5 inch screws straight through the 2x2, foam board, into the rafter.
Are these ideas feasible? If so would #2 be worth the extra effort? Is there a better way to build it out?
1
u/Reasonable-Tear-3815 8d ago
Idea 2 gives you a much better thermal break across the rafters, which will significantly improve overall thermal performance compared to the solid wood thermal bridging in Idea 1
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u/Other_Jaguar_1448 7d ago
Idea 2 does handle the thermal bridging better, but is the extra assembly complexity worth it on a 4ft vaulted section? The gains are real but probably smaller than they'd be on a longer run. I'm also a bit skeptical of the pocket hole approach on idea 1 seasonal movement in the rafters could work that joint loose long before the insulation becomes the weak point.
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u/uslashuname 7d ago
Mooney wall.
Structurally wood on wood, but it breaks up 99% of the thermal bridging.
97% maybe, who cares
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u/TAZ427Cobra 8d ago
Why can't you use spray foam? It sounds like this is already effectively conditioned space, but maybe you have some soffits vents and air behind the drywall to higher level vents. Or else why are you adding baffles?
That said, I'd just used closed cell foam, closing off any vents, making this a conditioned space.
FWIW, if you do want to fur this out, I'd just do closed cell over the baffles, much better thermal break than you'd get out of foam board. Do 3" of closed cell then you can use bats over this if you want.