Sealing living space ceiling J-box from a vented attic
Using J-box mounted LED “can” lights is encouraged to limit the holes in the gypsum that is often the air barrier between conditioned space and a vented, insulated attic. I know about sealing/caulking around the perimeter of the J-box where it butts the gyp.
These j-boxes have wires running to them, and there is an air “leak” at each of these j-boxes where the tabs are bent back and the wires enter and leave. Although significant insulation will be over the ceiling J-boxes, it does seem like a significant amount of air can pass through these gaps, as well as through the same gaps in wall outlets if the gyp is the primary air barrier. What is the typical method of sealing these gaps? Putty pads before gyp on walls, and before insulation on ceiling? Spray foam from above around the tab and then remove any of the foam that might get in the box? Other?
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Replies
CL,
I'd have to see the fixtures that you have to make a recommendation, but here's an article on how to air seal old-school can lights safely. Perhaps it is the same for today's LED fixtures.
https://www.finehomebuilding.com/2015/01/07/air-sealing-can-lights-safely
CL, the easiest way to handle this situation is to not use the drywall as an air barrier, but to put a separate air barrier against the bottom of your trusses or attic floor joists, furr the ceiling down with 2x material, and use low-profile LED fixtures such as these: https://www.ndrelectric.com/_prod/rzr-led-400-en.php. Then you don't have to worry about air leaks.
But it you are using drywall as your air barrier, and you have junction boxes, you can fill the gaps between the cables and the cable retainer with expanding foam, a sealant such as Pro Clima Contega HF, or a flexible tape such as 3M 8067. There may be electrical codes to check as well.
Alternatively, I have used the approach that Brian linked to many times, and it works well. There are also now products that do the same thing, available at big box stores: https://www.tenmatusa.com/insulation-protection-covers/ff130e-recessed-light-draft-stop-cover/.
@Brian - per articles on this site, j-box mounted LED's are better than having a can light penetrate the gyp. so the only thing to seal is the j-box, not the fixture - so does not matter what type of fixture you install. Issue is sealing the traditional j-box.
@ Michael
- the separate air barrier is expensive.
- Agree, sealing with expanding foam would certainly work and be the easiest; I was trying to figure out if that method is code compliant. Putty pads also work, and those are code compliant, but far more labor intensive to install after gy is installed (and more costly than foam)
- The approach of building a "hat" over the j boxes does not seem to solve things. The hat displaces much needed insulation, and more importantly it just moves the problem of the wiring causing an air leak from the j-box to the bottom edge of the hat...
CL,
Use caulk or canned foam. Don't forget to do all the tabs, not just the ones with wires through them, unless they have gaskets. Metal boxes are the devil. You are better off covering them with pads of self-adhesive membrane, then sealing the wires.
C.L.,
You've been given good advice, with several possible approaches suggested.
Here's one more (if it's not too late): use airtight electrical boxes. There are several manufacturers; Google "airtight electrical box" to find them.
I’ve found that the white fiberglass “hard boxes” are better than the more common blue plastic boxes. The “hard boxes” are sealed — no gaps or holes — so there are less places to leak making them easier to air seal. The only place to seal on the hard box is right around where the wires enter the knockouts.
For metal ceiling boxes, I’ve found the “fan boxes” have fewer extra gaps and holes than the regular octagon boxes. They’re a lot more expensive though.
I use canned foam to seal the boxes from the back. If I had to seal the boxes from the front, I’d use red silicone fire caulk. All you need is a little bit around the wire entry, and a head around the edge. You might have to plug extra gaps and holes depending on the type of box you used. I haven’t had much trouble with the foam expanding into the box enough to be a problem, just be careful and don’t overdo it.
Bill