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Sealing around air baffles

4901 Views 11 Replies 4 Participants Last post by  Windows on Wash
I added air baffles to each rafter bay and all new insulation last year. I didn't have time to properly air seal around the baffles. They are straight baffles. They do not have the wrap around on the soffit side.

I am planning on installing a continuous vent soffit. Is it ok to use foam board (with spray foam around the edges) to seal around the baffles from below when I am replacing the soffit? Would the foam board be considered exposed in this location? Should I use a different material?
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Just to clarify, I want to seal the large gap between the top plate of the wall and the bottom of the baffle.

The way it is now, some of the air is getting pulled under the baffle and through the insulation, making it dirty.

2 sides of the house have slanted ceilings, 2 have flat. They both have the same size gap to fill.

Thanks for the reply.
Spray foam will work as will a dense batting material. Roxul will do better than fiberglass as it is far less subject to wind washing.
Is this what you mean?

I don't have soffit vents, so in your case you wouldn't want to block the vent airflow, but those are the outside top-plates sealed with spray foam. A 30" foam gun will do wonders for those tight spaces.

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Yes like that, but from below when I am changing out my soffits. Would it be possible to do something like that from below? Any ideas?
Lol. Yes, air seal around it for better air-flow up and toward the roof deck. Some of that incoming air will fall anyway (the colder portion at least) and mix with the main area attic air creating convective loops, three of them, on top of and in the top few inches of your insulation when the temps get cold enough. Fiberglass, rock wool and cellulose are all fibrous air permeable insulation. You need a rigid plywood or foil-faced foam board with canned foam (fire code) around the baffles.

How tall is the gap?

I have seen folks over apply foam in some of these cases and create issues by either letting the foam expand into the soffit (creates and unsightly mess) or over expand and crush the baffle (effectively limiting the ventilation).

Depending on the gap size, you would be well served to seal the top plate first, add in layers of rigid foam to close the gap, and then insulate with a low expansion (closed cell) foam to close the final gap and add R-Value.

It doesn't need to be airtight there if we are talking about a vented attic design, but the more R-Value the better.

This all works in a perfect world with proper access and ease of installation.

Roxul insulation is available in different densities. If you opted for the higher density batts, they are more resistant to air flow as compared to FG.
I sure wouldn't use rock wool unless in a panel form (R-8) and even then you are installing from the outside, if I understand correctly- so it would insulate between outside air/attic air... waste of money. With your limited HAP, a rigid foam is best (from the inside over plate line), with the rigid (or plywood) air seal against the outside from the roof sheathing to the wall broken by the added chute vent. With a fibrous insulation touching the roof sheathing, it will rob you of heat from the room below by conduction. At least wrap any fibrous insulation in a plastic bag which also stops air and the moisture from depositing on the fibers degrading its R-value.
1. sealing around the baffles from the outside (which your post is about),
2 insulating/sealing on the inside- over the wall plate. Without the air-sealing on the outside, air will be drawn in through your fiberglass (as a furnace filter) leaving the dirt behind. Unless you block it off by layers of rigid on the plate and canned foam. Sealing around it also makes the attic draw from the baffles more, without a competing, easier air flow path.
Hopefully, the soffit supply vents are next to the fascia board, not in the mid span or near the house, for optimum air flow without moisture/snow due to air currents there. Also, air seal the attic, all top plate intersections of ceiling/wall drywall joints, any pipe/wiring holes, chases, drop ceilings, etc. with appropriate rated canned foam.

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How many baffles will you be retrofitting?

From the exterior makes it more manageable and expands your options to be certain.
The baffles are up and everything has been air sealed except the exterior top plates. There are probably 50 or so if I had to guess. Not sure what the gap will even look like from below until I open it up. I'll take both of you guys' advice and put it toward my project. Hopefully I remember to take some pics so I can share what I learn. It could be a while though before I get around to it. Thanks for the replies.
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