Venting vents in a vaulted ceiling
Hello everyone,
I’ve had some very informative feedback to some of my earlier queries here, so I thought I’d ask another related question. I recently learned that there is the possibility of condensation on the underside of plastic baffles, when installed in vented cathedral ceilings (for example: https://www.greenbuildingadvisor.com/article/condensation-with-plastic-baffles-in-a-vented-attic and https://www.greenbuildingadvisor.com/question/moisture-between-insulation-and-baffle).
I’m planning to use 14″ x 48″ plastic baffles as part of adding roof vents to a section of my house where I have a cathedral ceiling (ADO Provent: https://www.adoproducts.com/vents/provent/). I believe these are polypropylene, and therefore practically impermeable as is. I’m trying to identify ways in which I can still use these baffles, but allow for “breathing” above and below the baffle. Here are some ideas that I’ve come up with:
1. Cut the 48″ length into two 24″ sections, or three 16″ sections, and install these with 1-1.5″ spacing between adjacent baffles (this would be my preference)
2. Have the original 48″ baffles butted against each other, but drill a grid of 0.5-1″ dia holes in, say, a 2×8 type grid to make this more permeable (this is doable, but will be more labor-intensive for a 10-square roof rework)
I realize that using something more permeable (e.g., cardboard, etc.) from the get-go will be more effective, but I’m wanting to stick with the Provents (mostly for logistical reasons that I can explain if anyone wants to know more).
I’m curious if anyone has thoughts on whether either of these strategies are viable, and if one will work better than the other (and if #1 is a good plan to proceed with).
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Replies
User...619,
Those vents are specifically made for use at the eaves on trussed roofs to get the air past the insulation and into the larger space. For cathedral ceilings you want the baffles to span the whole width of the rafter bay, which is why site built alternatives are usually recommended. Those baffles will work with either of your alternatives as long as you are insulating with batts - but don't give you the same drying potential as baffles spanning the whole bay.
That makes sense, thanks. The idea definitely is to put batts right under the vents, and have 2-4 in. of face polyiso right behind the drywall as the insulated vapor barrier (taped and all).
Deleted
Those poly vents are not meant for cathedral ceiling install, the are for attic install with loose fill insulation.
You want either the thin foam ones, cardboard or site built with something permeable.
Most cases it is simpler to skip the vent baffles (only install them by the soffit) and use high density batts.
I used 1 1/2" foam ripped about 1 1/2" wide, and used that to make the vent channel with 1/2" plywood ripped to appropriate width. Then used narrow crown staples to push ripped pieces, about 6 inches long, about 3/4" wide of the same plywood, flat against the rafters to hold the plywood. Then air sealed. Batts underneath that. Air sealed and permeable.
Hi again, everyone,
Based on everyone's feedback (here and elsewhere), I decided to take the plunge. I opted for the ADO Provent baffles, spaced 1-2in apart. Also opted for 2+2 in of polyiso to try and claw back some of the R-value from my ceiling. I realize this isn't the "best option" with the Provents, but I thought I'd post this as a sanity check for myself, in case I'm missing something obviously catastrophic.
Progress photos in a single bay and schema here: https://imgur.com/a/nkf2YLt