Sealing gaps around a fan/heater combo - what would you actually use?

I've had a similar unit installed in my downstairs powder room for years, and my expirience says you absolutely want to seal that gap, even around a heat source. The feeling of drafts coming off the ceiling is heat escaping directly into your attic, and that energy loss is significant. I wouldn't gamble with the foam on the whole housing, though, because that would definitely compromise ventilation. I found that using a silicone caulk first, around the perimeter of the unit where it meets the drywall, was essential to handle the moisture and temperature fluctuations. Then, for the gaps where the housing meets the ceiling cavity, I used low expansion backer rod, about a half inch wide and two inches long, to fill the major void before applying a bead of fire-rated sealant over the rod and the housing. This allowed the material to move slightly without cracking while still blocking air movement. The key is to seal the static gaps, not the whole thing. The backer rod is great because it doesn't completely fill the space, allowing some of that necessary thermal expansion. My unit is a standard 4x4 inch housing, so the backer rod fit perfectly into the wider gaps on the left and right sides of the fixture. I did this three years ago and I haven't noticed any performance changes, and the heat output seems just as consistent as it did initially. It’s much quieter too because the air flow around the motor is less turbulent now. It's a small project, but it really pays for itself over time.

I spent a few winters doing this exact project, and I think the advice about total isolation is a bit aggressive, especally with the heaters. My concern shifts from the heat loss to the thermal expansion of the metal housing itself. When that unit cycles on and off, that metal heats up and contracts pretty dramatically. If you seal it completely with anything rigid, like standard foam or grout, you create stress points that will eventually crack the seal or compromise the unit's operation. What worked for me was focusing on a thermal break approach. Instead of stuffing the entire perimeter with expanding foam, I focused only on the largest, most noticible gaps. I cut strips of thin, flexible fire-rated insulation and compressed them into the void behind the unit, then used only a thin bead of high-temp silicone caulk along the exterior edges where the ceiling drywall meets the flange. This way, I maintained a seal against the attic air, but the flexible material allowed the unit to expand and breathe a little without putting pressure on the housing.

It felt like overkill initially, but after the first winter, the temperture inside felt much more stable, and I didn't have any worry about the unit becoming detached from the ceiling due to stress.

Complete beginner on this stuff so apologies if this is a dumb angle, but I had the same draft issue with a plain bath fan, no heater on mine. Instead of working through the grille I went up in the attic, pulled the insulation back, and sealed the gap from above where the drywall cutout meets the housing. Flipped the breaker first since I didn't know what wiring was up there. Worked great and the draft is gone. Is your combo unit reachable from the attic side too? Seems like it might be easier than caulking blind over your head.

Check the duct connection while you're up there. On my own place the biggest leak wasn't around the housing at all, it was where the flex duct met the fan box. The old tape had let go and it was dumping warm air straight into the insulation. Re-taped the joint with foil tape and a skim of mastic over it, and the draft through the grille dropped way down before I even touched the drywall gap. For the heater side, keep any foam back a couple inches from the can and let the fire caulk do the work right at the metal. Might save you sealing the wrong hole.

One thing I'd check before stuffing anything fiberglass near that heater side is whether the housing is actually rated for insulation contact. A lot of the mid 90s combo units have a marking on the box or in the manual saying whether they can be buried or need clearance, and mine definitely needed a couple inches of space around the top. Sealing the drywall gap with caulk is a different animal from covering the housing, so you can do one and not the other. Worth a look before you buy materials.

Reading this made me poke at my own fan tonight and I noticed the cold air isn't coming from around the housing at all, it's coming right through the fan blades themselves. Like the little flap door that's supposed to close when the fan is off just doesn't close all the way anymore. So now I'm wondering if I seal the drywall gap like you guys are saying, will the air just find its way out through the flapper instead? Is that flap thing somethig you can buy on its own or do you have to swap the whole unit?