Ventilation baffle materials & cathedral ceiling
CZ 3A/4A
2001 gut reno
We’re trying to ensure the sloped ceilings at the knee walls in our attic maintain open ventilation channels. We had our insulation company install Provents, but they did an awful job. They’re incredibly squished, and I’m tempted to scrap the whole thing and start over.
1. Does the material selection for ventilation baffles (plastic like we have vs rigid foam vs wafer board) matter that much if we make sure they’re air tight (tape the sides to the roof sheathing, tape between each baffle seam, etc.)? Trying to prevent condensation and mold, so do we need a more vapor permeable option in our climate.
2. If we keep the plastic baffles, is there a way to create braces/blocks in the middle of them to make sure the channels don’t get squished?
3. Do we have to go from soffit up to the hip vent above second floor ceiling insulation? Or can we start at the back of the knee wall and go up above the ceiling? We used rigid foam to block at the knee wall “top plate” (on the attic side).
Thanks!
Stephen
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Replies
If we go with 1" rigid foam, that would compress our insulation about an inch. We have the 10.25" cathedral batts and a little over 11" to work with in the rafter space. With the rigid foam adding R-5, will that compression make much of a difference here?
A drawing of your roofline would be helpful here.
I think your best option is to use 1/4" waferboard here. You don't need to seal it like an air barrier, your air barrier should be on the interior under your insulation (I'm assuming you're insulating the roofline here). Waferboard has the advantages of being cheap, rigid, and reasonably vapor open. If you want some extra R value, put a layer of foil faced polyiso under the rafters with seams taped. An interior side vapor barrier with a vented roof like this can be helpful as it will limit how much moisture gets into the assembly from inside. Note that in your more Southern CZ, you won't have as much of a wintertime moisture issue as we in the more northern CZs do.
I don't like the prefab plastic baffles since they are too easy to compress. I'm OK with these for windwashing protection at the eaves (where they usually don't see much physical stress), or in catherdal ceilings where you are limited on what you can use. I think site built baffles are more durable. I like to use 1x2s to define the vent channel, then tack things together with a finish nailer.
Bill
How difficult is it to cut wafer board without a saw? We have a couple extra boards within the rafter bays (had to fur out the ceiling slopes in order to get the right R value - connected 2 2x6s to create the right depth), so we have to notch around them.
I included a couple of hand drawings and an image of the house from the exterior. Is that helpful? There aren't knee walls all the way around the house, just certain areas of the bonus room.
You need a saw. Waferboard is just thin OSB, typically 1/4" thick. I usually just use a circular saw. Even my cheap-o oddball size (5-3/17" or whatever it is) cordless cicular saw cuts it just fine. I don't think you could make a clean cut in the stuff any other way.
You don't have to use waferboard. You can use 3/8" (ish) OSB too. The 1/8" stuff you saw is probably masonite. I suppose you could use that, but I've never tried it. I'd be a little more concerned with that stuff splitting when nailing close to the edges compared with the other materials being discussed, but aside from that, it's probably fine too. There is also 1/4" masonite which will be more durable. Keep in mind though that vent baffles don't need to be super strong, so my concern with the thin materials is in regards to splitting when fastening. I like baffle materials to be able to be installed quickly, so if my finish nailer breaks things, then I tend to use a different material.
Bill
Our local hardware doesn't seem to have waferboard. Is plywood or osb an option? Is 1/8" too thin? Trying to figure out what would be best for cutting since we don't have a saw
Any thin permeable material works, OSB/CDX is fine. You can even use house wrap.
Note that those hipped roofs make ventilation much more difficult. You did mention "hip vents" above - are you installing ridge vents or another material below the hip shingles? Unless you do something there, the ventilation is pretty useless.
We were able to borrow a small saw and had success with thin plywood, so I think that's the direction we will go. Is air sealing the seams/small gaps a good idea?
Air sealing the seams and gaps in the vent channels is not necessary.