Placement of rafter baffles with non-soffit intake vents (smartvent/eyebrow vent/shingle-over vent)
I’m in Seattle, climate zone 4C. I’m insulating my attic with blown cellulose and trying to make sure I install the insulation baffles correctly. My house doesn’t have eaves, so there are no soffit vents, but there are “smartvent” vents that are basically corrugated plastic under the shingles, so the air comes in at the drip edge. It enters the roof about 6-12″ up along the slope of the roof. When installing insulation baffles, where do I need have the bottom edge of the baffle? Everything I find talks about positioning the baffles with respect to the soffit. My intuition is that I just need the baffle to extend down below the intake vent and “cap” the bottom end. This would allow the thickest insulation possible out to the top plate, with the insulation in contact with the roof deck. Maybe this would create a problem though because the space down the slope of the roof from the baffle could then potentially trap moisture. So the alternative would be extending the baffle down further, but then I get almost no insulation at the top plate, since the rafters sit right on top of it. I guess with a vapor-permeable baffle, the moisture issue wouldn’t be a concern. I currently have some durovent that I picked up at the big blue box, but having them in hand they seem pretty flimsy, so I was considering upgrading or building my own (having read the non-paywalled part of the site-built baffles article).
Additional details: this is in the lower attic space before a kneewall and the exhaust vent is a not-particularly-well-placed RVO/box vent a little ways up the ridge. I don’t seem to be having moisture problems though, so I’ve decided to just leave the venting as-is until I have to replace the roof and then switch to exterior rigid foam insulation and a conditioned attic.
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Replies
It depends a little on how the soffit is constructed. If all your vents open into a soffit space, then you really just need the baffle to tie into that space, so you could come up from the outside of the top plate on the exterior walls, then go over along under the roof sheathing with your baffle. If you don't have an open soffit, then you need to get the baffle all the way out to the vent, wherever it may be.
What you're trying to do with those vent baffles is to maintain an air space under the roof sheathing so that air can circulate and remove any excess moisture. Code requires a minimum 1" air space here, although 1.5" is better, and it's easy to create a 1.5" air space using 1x2 furring strips. You want your loose fill insulation to be able to cover the top plate of the exterior walls, but you don't want to fill up any open soffit space you may have past that. That's why you would generally want to stop the baffle just past the top plate, and put a baffle up from the outside of the top plate to tie into the air baffle. If you have no open soffit space, then there is no issue with loose fill insulation filling that space, so you would just need the vent baffle to tie into the vents you have up near the sheathing.
Bill
I haven't got any soffits at all, so it sounds like I just need to get the baffles underneath the vents. That makes, sense, thank you. I just wanted to make sure there wasn't something I was overlooking.