Does the AccuVent baffle leave a gap above the top plate?
I installed 41″ tall x 22.5″ wide AccuVent baffles in the truss bays of my 4:12-pitched roof, and no matter how I tried, I could not get the part that covers the top plate to sit flat upon the top plate reliably. The way the plastic curves back toward the outdoors creates a slight curve above the top plate that would allow cold (or hot) outdoor air to come between the baffle and the top plate, thereby preventing the insulation above the baffle from doing much good to insulate the top plate. Any wind pressure from the outside would enlarge the gap. The weight of blown-in cellulose will not push the plastic down against the wood; it takes more pressure with my hand than the cellulose would exert.
The solution I’ve devised is extremely tedious. I’m pressing the plastic down with 1-1/4″ drywall screws through pieces of 1×4 and similarly shaped pieces of 5/8″ drywall that I cut up from a scrap. This basically works, but inserting the screws with just 3″ or so vertical clearance above them has proven very difficult, even with a right-angle drill adapter, although not the smallest one available.
Has anyone else encountered this problem, and if so, solved it in an easier way?
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Replies
I used a pneumatic stapler to hold ours down. It seemed to work well enough and then we sprayed foamed them before installing cellulose. So maybe I wasn't trying to get them as air tight as you figuring the spray foam would take care of any gaps.
Thank you. Spray foam wouldn't help the problem I'm encountering unless I applied it from the outside, which isn't feasible, as I installed soffit first. Maybe I should have used a lot of caulk or glue to get the plastic to stick to the wood and thereby prevent cold outdoor air from reaching the top of the top plate. Unless I'm missing something, the AccuVent baffle seems to have a major design flaw.
Jeff,
I see what you mean. All I can suggest is to attach a piece of foam board to the top plate in each bay before installing it. That's annoying.
Thank you, Malcolm. That would have been a good idea if I had thought of it in time and could have found a way to staple the bottom to something that would hold the staples. I just ordered a smaller right-angle drill adapter that I hope will make it easier to screw wood or drywall onto the plastic to press it against the top plate. None of the reviews of the AccuVent that I read address this problem. I wonder how many people who installed it checked whether it would allow outside air to reach the top of the top plate directly. It looks fine, and one would only detect the flaw by pressing the plastic to see whether there's a gap between it and the top plate.
If you don't mind - do you think construction adhesive on the outside edge of the top plate would have solved the issue? Looking to retrofit my attic with these as they seem to be the best value way to treat this edge condition after the fact.
The main difficulty I would expect with using adhesive would be keeping the PVC of the baffles pressed against the wood while the adhesive sets. There may be a fairly large gap otherwise, and any breeze might move the baffles.
I ended up cutting pieces of cheap 1x4 board and screwing them down to press the baffles onto the top plate. There were also too-large gaps between the edges of the baffles and the trusses going up the underside of the roof sheathing in many of the truss bays, so I nailed 1-1/2"-wide strips of leftover foam board to cover those gaps. The insulating properties of the foam didn't matter; it was just what I had on hand, and it was very easy to handle.
I installed those same baffles in the same attix (4/12, 24 OC) I used 3 staples as far back as possible, and one near the interior part of each bay.
I put 4 staples in the top plate using hand, 120v and battery staplers. It wasn't perfect, but it seemed to be pretty tight. Reccomendation is 2 in the top plate, but staples are cheap. Drywall wasn't up so that made the process much easier. When I put in fiberglass batts, I made sure to stuff that cavity.
I'd reccomended using the most shallow stapler you can find.
For consideration
I am watching this install video
https://youtu.be/FI0cXfR7gog
At 1:19 in the video it is clear that the guy is stapling the plastic to the inside face of the top plate.
Even if the Accuvent is pushed down over the top plate and screwed down, I fear that it will not protect the top plate.
With no exterior insulation
If there is no exterior insulation over the walls, then the top plate is getting cold/hot as the air comes up through the soffit and washes over the exterior vertical face of the top plate.
Exterior insulation on wall
If there is exterior insulation on the walls, trying to close the gap with wood and screws may not help that much. If there is for example 2” EPS on the wall, the only ways to keep the top plate warm is to install rigid insulation (say smaller pieces of polyiso) over the top plate.
But you would need to air seal the gap where the EPS and POLYISO meet - with tape or other sealant. But you would need to also seal between the POLYISO and the rafters and/or ceiling joists. Unless there is a continuous air sealed insulation from the face of the exterior wall to the inside face of the top plate and around the sides of the POLYIISO, that top plate will always be cold.
Going back to the scenario of no exterior wall insulation.
If you are pressing down the thick black plastic of the Accuvent onto the top plate, you are not air sealing the gap between the top plate and the Accuvent on the back side. You cannot possibly reach that gap if I understand correctly. Unless you remove the soffit (not suggested) and air seal that gap from behind, cold air will always and forever reach the top plate. But, sealing the backside would not do much anyway because the cold air, I would predict, is still going to easily squeeze between the ACCUVENT and the rafters and/or the ceiling joist. That cold air will reach the insulation at the edge and keep the top plate cold. Also consider that the Accuvent is between the insulation and the top plate in the image you attached and what I saw in the video. Even if everything is sealed and all that I said was false, the black plastic Accuvent will get cold/hot and will transfer the cold/heat to the top plate that is permanently touching.
If anyone thinks I am wrong, I would appreciate knowing where my understanding is flawed.
This is one recent GBA article that talks about one possible wall to roof detail to keep the top plate warm.
https://www.greenbuildingadvisor.com/article/detailing-a-wall-to-roof-truss-transition
This is a 2nd recent article addressing the same issue with a different solution.
https://www.greenbuildingadvisor.com/article/zip-system-panels-as-primary-air-barrier
Here is a 3rd one
https://www.greenbuildingadvisor.com/article/air-barrier-for-a-vented-roof