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I've got my exhaust fan installed in the ceiling of the bathroom, now I'm wonder how best to seal the gap around it. The fan housing is obviously sheet metal, and the ceiling has the plastic vapor barrier on top of the drywall. Would metal tape work for this or is there something better to use?

The fan can be covered up to R40, so once the physical gap is covered I can back-fill the insulation. I'm just wondering what will stick reliably to both sheet metal and the plastic sheet.

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2 Answers

up vote 3 down vote accepted

Sealing the vapor barrier

You'll want to seal the vapor barrier to the fan housing using an acoustic sealant like OSI® SC-170™, or similar product (you might have to do this from above). Just make sure the sealant is approved for use with vapor barrier, some sealants won't adhere well to the vapor barrier.

If there is enough plastic to lap up the side of the housing about an inch, this will be a simple task. Fold the plastic back away from the housing, apply a bead of sealant around the housing, then push the plastic into the sealant.

If there isn't enough plastic to attach to the fan, you'll have to make a gasket. Simply cut a square of plastic larger than the fan housing, then make a cutout that will fit the fan. Leave the excess from your cutout attached, so you can attach the plastic to the housing. Use housewrap tape to attach the gasket to the existing vapor barrier, then follow the steps above to seal the plastic to the fan housing.

Patching the drywall

Depending on the size of the gap surrounding the fan, you'll have to use different approaches to fill it.

Very small gap

If the gap is really small; as in the drywall basically touches the sides of the fan housing, you're done. Just put the fans face trim on, and enjoy. If you really want to, you could run a bead of silicone caulk around the housing.

Small gap

If the gap is ~1" or less, you should be able to easily fill it with joint compound. It might take a few coats to fill the gap, since you don't want to put it on really thick or it will take a long time to dry. Once the gap is filled, feather the compound out to blend with the surrounding wall. You can use a 10"-12" taping knife for this.

Medium gap

Once you have a gap ~1"-2", it's time to start thinking about making the hole larger and patching it with another piece of drywall. You might be able to fill the gap with joint compound, but it will be much more difficult than with smaller gaps.

Large gap

If the gap is larger than 2", you're going to have to make the hole bigger before you can make it smaller. Cut out a square of drywall around the fan, until you get to the center of a joist on each side of the fan. Cut a new piece of drywall to fit the hole you just made, then cut out a hole in this piece to fit your fan housing. Make sure the hole for the fan is very close to the outside dimensions of the fan housing, so you'll end up with a very small gap. Tape and mud the joints, prime, and paint. Finally, follow the steps above from the "Very Small Gap" section.

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I tried the metal tape cause I had it on hand and it seemed to work fairly well, but gasket+sealant sounds even better. –  Brad Mace Oct 20 '12 at 3:24
@BradMace The metal tape will probably hold, but possibly not long term. The acoustic sealant will create an air tight seal, that should last a long time. It stays flexible, and will handle the expansion, contraction, and vibration of the fan housing better. –  Tester101 Oct 20 '12 at 15:42
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You could use a spray foam insulation like Great Stuff. It sticks to just about everything and expands to fill voids.

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I'm pretty sure using spray foam sealant around that area is not up to fire code. I'm not sure the acoustic sealant is fire rated either. –  user12205 Mar 23 '13 at 21:51
@user12205, if you need fire rated (not sure that would be required in a single family unit, but check your local codes) the expanding foam in orange (vs the typical yellow foam) is fire rated. –  BMitch Mar 23 '13 at 23:13
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