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Venting at Intersection With Uninsulated Cathedral Roof

Haus | Posted in General Questions on
(re-post since I put the first under the wrong topic)
 
Hello community – questions up-front:

* 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

 
Roof form is a T, and the trouble is at the intersection. Not sure how to vent it correctly. Moisture damage discovered from previous assembly suggest I should fix something! It’s not bad enough to need repair, and I’d like to keep it that way.
 
Previous installation was 2×6 fiberglass filling the rafter cavity without room for ventilation, even though there were soffit vents cut for most rafter bays. Tore it all out because of rodent issues. No ridge vent, although I intend to install one when the weather improves, if needed.
 
Please see the attached diagram (top-down view).
 
Note that there is 1×6 cross strapping under the sheathing, spaced to allow for limited cross ventilation.
 
Insulation is currently out, and it’s getting cold for that. Currently planning to reinsulate with 2×6 rockwool behind EPS-cut foam ventilation channels. 2×3 furring strips on all rafters to make enough space for that. Planning to air-seal the ventilation channel edges with foam as per https://www.greenbuildingadvisor.com/article/creating-a-conditioned-attic 
 
Thanks, all!

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Replies

  1. Haus | | #1

    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!

  2. Haus | | #2

    Am I asking the wrong questions or in the wrong place?

  3. freyr_design | | #3

    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.

  4. Expert Member
    MALCOLM TAYLOR | | #4

    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

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