Venting at Intersection With Uninsulated Cathedral Roof
* How can I prevent soffit-less rafter bays from drawing conditioned air out from the house? (current thought: EPS/XPS blocking between the rafters, sealed in with canned spray foam, and then sealed against the drywall using acoustic sealant. Making that up myself, so it’s likely wrong).
* Is a vapor-impermeable ventilation channel (such as foam baffle or EPS foam + foil) more risk than benefit? That is, as a condensation surface next to sheathing in a leaky house/roof assembly, will that make moisture problems worse?
* Where there is no soffit for a soffit vent, should I install an off-ridge vent down low instead? (from other answers, it sounds like the small cross-ventilation is probably not enough)
* If you were me, would you just close it all back up “like it was”, without ventilation channels or ridge vent, and hope that if it (mostly) worked for the past 40 years it’ll keep on working?
Renovating a 1960’s cabin which was extended a couple of times. Zone 4, PNW
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Replies
Note: I re-discovered this, which describes why the condensation on foam baffle is not a big problem: https://www.greenbuildingadvisor.com/article/site-built-ventilation-baffles-for-roofs
However, I think that the fact that the most problematic rafter bays terminate within the conditioned space makes this more of a challenge than usual. And it reinforces that I need to find a way to seal out the air from those bays!
Am I asking the wrong questions or in the wrong place?
I think your approach with a ridge vent and very good air sealing from interior space will be enough to get adequate air flow. If you are worried about that last rafter bay you noted had damage you could possible add a couple of vents on the gable that fed into the new vent channel you are creating.
If you are really worried about it then add intello right above your Sheetrock to limit even further your interior air exchange with that space.
If you are worried about the cross gable that I’m assuming is cal framed over main gable, drill some holes in the main gable shear to couple that space with the main roof.
Haus,
If it has only had minimal damage up to now, it should be fine with the suggestions feyr_design made.
If you want to be sure, pick the appropriate assembly from one of these five:
https://www.greenbuildingadvisor.com/article/five-cathedral-ceilings-that-work