I'm adding a bathroom exhaust fan, which is being mounted in the ceiling of the bathroom (in the attic) and venting out through a vent in the wall.

right now, there is a vapour barrier (paper) between the insulation in the attic and the drywall. I will have to break this of course to mount the fan housing, but my question is, what do I do after? Should I put a vapour barrier around the exhaust fan and seal it to the paper?

I was also going to put insulation back over top of the exhaust fan - is that correct to do, or anything to watch out for? The vent pipe I have is a 4" insulated duct that will go to an outside vent (which I just installed this weekend).

3 Answers 3


Check the directions/specifications of your exhaust fan, it should say if it is rated to have insulation next to or on top of it.

It would probably be a good idea to seal the vapour barrier back around the fan.

  • 3
    Interestingly, says nothing about insulation in the instructions that came with it, but the Broan website has it: any of their fans can be covered up to R40 (broan.ca/support-faqs-500.asp#9)
    – gregmac
    Commented May 4, 2011 at 6:27

The important part of the vapor barrier is to keep outside moisture from reaching the cooler drywall, condensing, and creating mold on the drywall. Anywhere I've ever put a hole in the drywall, e.g. for electrical boxes, we just cut the paper around the protrusion. Any excess was either cutoff or folded back. For whatever reason, building inspectors don't seem to worry about condensation on the back of an electrical outlet, go figure.

If you're especially paranoid, you can install some fire blocking between the joists on either side of the fan housing, screw a piece of drywall on top (with the vent protruding) and install some insulation on top of that. This would minimize fire and moisture risks, though I've never personally seen anyone take it this far.


Install a vapour barrier boot where your fan will go. Seal boot to existing vapour barrier. Mount fan inside boot. Cut hole like an X where elec and vent pass through. Seal holes around elec and vent with tuc tape and with accoustic sealant. Make sure vent pipe is insul wrapped

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