To vent or not vent cathedral ceiling
Hello. I am in climate zone 4a and I need to fix a poorly insulated/vented cathedral ceiling in my bedroom and bathroom.
I am removing the entire ceiling and debating if should 1) spray foam with closed cell and open cell mix or 2) make a vented assembly however I’d like to use an air barrier like membrain so have an air tight ceiling since the Sheetrock will have penetrations. Also the bathroom cathedral is a hip roof and not sure how i would do soffit vents.
My concern with spray foam besides the price is being able to properly reach all areas like top plates and not have air leakage.
What do you think is the better option? And any specifics i need to know? Thank you .
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Replies
GROUNDUP,
The two things that make vented cathedral roofs hard are shapes (like hips) that make continuous venting difficult or impossible, and shallow rafters which do the same for getting adequate insulation into the cavities.
If you have those conditions, and you don't want to work above the sheathing to either add exterior insulation or strapping to vent, you are basically down to assemblies #4 or #5 in this link, both of which unfortunately involve spray foam.
Thanks so much
Groudup,
Sorry - I didn't include the link: https://www.greenbuildingadvisor.com/article/five-cathedral-ceilings-that-work
Thanks Malcolm. What is strapping to vent? Also the hip roof is only the master bathroom not bedroom. Would it be wrong to vent the bedroom and spray foam the bathroom? They are connected though. Thank you
Groundup,
You can vent roofs by applying a WRB to the rafters, strapping on top of each one to form a vent space, then sheathing above that. It allows the ventilation not to be interrupted by the framing on hips and valleys.
That approach, like roof top rigid insulation, is fairly straightforward on new construction, but pretty disruptive on renovations.
Yes a mix of vented and un-vented roofs works fine. Spray foam the problematic ones, and vent the rest.