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Soffits and other large ceiling penetrations are essentially unintentional air ducts. Learn how to fit and seal foam to close up even the biggest holes in your attic.

Video Transcript:

We’re up in an attic; downstairs, there’s a kitchen with a soffit above all of the upper cabinets. When we came up here, we were on the lookout for one of the biggest holes you find in an attic, which is the area above the soffit. Drywall usually doesn’t get applied to the ceiling before the soffits are framed in, and it ends up being a big leaky hole. This one was easy to find because I could already see the insulation drooping between the ceiling joists. When I pull the insulation out, I uncover the cavity of the soffit below.

In the cavity there is a recess light can (which isn’t air sealed), a ceiling or junction box, and there’s also a gap around the bottom of this plumbing pipe. In the back there are holes that are going down and into the wall cavity between the studs and the adjacent wall. There are a lot of places where air is leaking up into the soffit. That is certainly evident by the insulation. If you look closely at the insulation, you can see a lot of fine dust that has collected along the sides as the air has leaked through over time.

This is one of the biggest air leaks, therefore it’s one we want to ignore. The easiest way to deal with this is to use some sort of rigid panel. There are a couple of choices. One would be a piece of rigid insulation, which is what I prefer because it gives you insulation value as well as it's lightweight and easy to handle. Another alternative is scrap drywall, or even scrap plywood or OSB. Fit whatever rigid panel you choose into the cavity, caulk around the perimeter, and you’re done.

To start the process of installation, I’m going to run a bead of acoustical sealant. It’ll stick between the ceiling drywall and the underside of the blocking panel. Then press the panel and finish it off with a bead and finish down the ceiling joist. If I want to make sure I have a really great seal, I can continue running all the way around the rest of the board; there’s no harm in a double seal. Then I use my glove to protect my finger and smear that sealant in. It’s important to have a good bond between the seal and the surfaces.

I’m always impressed when people air-seal their attics and spend all their time focusing on the penetrations for pipe wires, the little gaps between the top plateIn wood-frame construction, the framing member that forms the top of a wall. In advanced framing, a single top plate is often used in place of the more typical double top plate. of walls and the drywall. People often overlook big holes like this, where the soffit has a big, gaping hole into the attic. They’ll just step over it to go after the small things. You get more bang for the buck for both time and materials sealing up a soffit penetration like you see here.

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