I'm interested in installing a bath fan and am currently planning ou the project.

The fan specs (https://www.homedepot.com/p/Delta-Breez-GreenBuilder-Series-100-CFM-Wall-or-Ceiling-Bathroom-Exhaust-Fan-ENERGY-STAR-GBR100/204347760) say that the port should be 4".

This leads me to believe that the tube entrance should be larger than 4", but I'm not entirely certain. I was thinking of insulated ducting similar to (https://www.homedepot.com/p/Master-Flow-4-in-x-12-ft-Insulated-Flexible-Duct-R6-Silver-Jacket-F6IFD4X144/203420974) for the attic.

Does 4" ducting connect to the 4" fan or does the ducting need to be larger (Such as 6" ducting)?

  • I don't see any reason to use insulated ducting for an exhaust fan. That is used for HVAC to keep the heat/cold from dissipating before it gets to the destination. For an exhaust fan that really isn't much of a concern. Dec 21, 2018 at 5:00
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    @manassehkatz A lot of people disagree everywhere I've looked. In NY the winters get cold and the attic in not environmentally controlled. People say that uninsulated ducting will cause water to condense and re-run back into the fan/house and cause general havoc. Dec 21, 2018 at 5:03
  • Interesting. I don't see that insulated ducts would hurt anything, Just don't see it worth the expense, though with a quick search it looks like less of an upcharge than I expected. Dec 21, 2018 at 5:19
  • I live in NY as well. and yes, its true, you literally MUST and truly do mean MUST insulate ducting. For the exact reason noted.
    – noybman
    Dec 21, 2018 at 5:33
  • Also, (while everyone uses it), I would try to avoid the ribbed ductwork. If you are going to do it, do it right. Go rigid, and if in the cold climates insulate it like you said. I say this from experience.
    – noybman
    Dec 21, 2018 at 5:36

2 Answers 2


If your ductwork is in an uncontrolled environment, you truly should insulate it. This for the reasons you list - to help prevent vapor from becoming water prematurely.

While it is not necessarily a lot, any is bad since gravity pulls it right back down the pipe into the ceiling, or the fan, etc.

If your air column is smooth, the work necessary to move it is less. The goal is to get the water out, not let it stay in. If the fan is properly sized for the room and there arent many obstructions (rigid tubing, 90 degree elbows, or even wind blowing a flapper shut...) you have a recipe for success.

The insulation is just 1 more defense in avoidancehelping to promote the vapor escaping.

This site has quite a bit on it: info on others experiences with venting

  • PS... they list guidance (at least in NYC) where insulated is required.
    – noybman
    Dec 21, 2018 at 5:45
  • I'm on Long Island, but thanks for the wonderful answer! Dec 21, 2018 at 5:53

Fans that are listed for 4" ducting in specifications, like the one you linked to, are designed to connect to any 4" ducting such as the one you linked to.

They will work together.

I would not use insulated for a bathroom exhaust fan but if it makes you feel better, why not.

I have replaced many bathroom fans here in Alaska and never have found one that had insulated ducting, of course they are all older homes and maybe newer ones do?

I try to use rigid ducting when possible instead of the flexible kind.

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    It is supposed to be insulated if the airspace the tube vents from is not climate controlled.
    – noybman
    Dec 21, 2018 at 5:34
  • I have also replaced my fair share of fans and never seen an insulated one. This may be why local code requires the fan to run after the lights are off once the humid air is out any moisture would be pushed out, I have not seen moisture problems with flex or rigid uninsulated.+
    – Ed Beal
    Dec 21, 2018 at 14:34
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    As I mentioned, I speak from experience. My attic/crawl space is not temperature controlled and I live in Western NY. I actually installed 6" (because I could) rigid, "straight" shot besides the initial bend where I used dual adjustable elbows as straight as possible. I bought insulation but didn't insulate. Low and behold, winter came, and in one bathroom I now have warped drywall near the outlet where it dripped. It took almost all winter to notice it was so slow. After use, you could see the "sweat" on the duct.
    – noybman
    Dec 22, 2018 at 2:47

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