Is there a limit on how much you can add to the thickness of a cathedral ceiling?
the74impala
| Posted in General Questions on
It looks like I may have the whole winter to think about my final ceiling design.
I have planned to put a 2 inch Eps layer over my rafters, then furring strips with BIBS between the foam and the bottom side of the baffles.
How would that compare to adding ripped 2×4’s to the bottom side of the rafters to obtain R49+ vs. the R42 it can get with the existing 10 1/2 inches? What about a full 3 1/2 from the 2×4? 6 inch screw?
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I think I need a drawing (or 1000 words) to get a better idea of the stackup and construction being propose here.
Is it going to be a vented cathedralized ceiling? What climate zone?
Any wood-depth extension will have lower net thermal bridging if it's perpendicular to the rafters, letting the cavity fill of the rafter bays thermally break most of the new wood, and the cavity fill of the new wood thermally break most of the rafter area.
Yes vented, Zone 7 Northern Wi, I like the idea of perpendicular ripped 2x's. Sounds a lot easier than foam.
Thanks
Hi Tom,
So the options are R-42 with a thermal break over the rafters or R-49 without the thermal break? I'm not sure how much R-value you need on the rafter bottoms for that to be a better option than a few more inches of cavity insulation. You could have a professional model these options for you. I'd be interested to see the results. It would cost a bit, but would give you a sure direction. Whichever way you go, put a lot of attention towards air sealing the ceiling, which will add the most performance value to the assembly.