Bathroom Fan Ventilation Duct Repair

By |Last updated on |Bathroom, Heating & Air|10 Comments

During my annual fall inspection of the house I noticed the soffit vent for the bathroom ventilation fan had been painted over, almost clogging the grate. I replaced the soffit vent and turned on the bathroom fan to check the air flow. The fan was blowing but no air was coming out of the soffit vent.

I went into the attic to check the duct work between the bath fan and the soffit vent to see if there was an obstruction in the flexible duct pipe. The 3 inch bathroom fan duct is the long skinny pipe on the right in the photo below.

3 inch Bathroom Fan Ventillation Duct

3 inch Bathroom Fan Ventilation Duct

“Everything is built by the lowest bidder”…

I removed the attic insulation from the bathroom fan and discovered the 3 inch diameter uninsulated flex duct was laying loose inside the 4 inch uninsulated diameter fan connector allowing the air to blowing around the duct and into the attic insulation. There are several problems here:

  • The fan air flow is constricted because it requires 4 inch duct. The 3 inch duct is too small.
  • In the winter, warm humid air from the bathroom will condense and make a moldy wet spot on the drywall ceiling because it’s not being conveyed to the outdoors.
  • Humid will condense inside the uninsulated flex duct, pool and eventually leak on the insulation and ceiling.

The worker who installed this was too lazy to do the job right and just left it laying there hidden by the insulation where no one would see it. Fortunately this was a seldom used bathroom so a mold problem never developed.

Loose 3 inch Duct Pipe

Loose 3 inch Duct Pipe

The correct way to fix this is:

For now, I’ll temporarily connect the duct to the fan so it doesn’t leak air. Afterwards I’ll replace the entire section of duct when I add extra insulation to the attic.

In the next photo I’ve removed the 3 inch duct pipe for comparison with the 4 inch fan connector.

3 inch Duct Pipe, 4 inch Fan Connector

3 inch Duct Pipe, 4 inch Fan Connector

To fix the problem, I used 4-inch vent duct, wire snips and metal tape. HVAC metal tape is specifically designed for sealing air ducts; do not use the common “duct tape” because it will not last.

4 inch Conduit, Metal Tape and Wire Snips

4 inch Conduit, Metal Tape and Wire Snips

A short length of 4 inch uninsulated duct pipe will be installed to bridge the bath fan and the 3 inch duct pipe.

4 inch Adapter

4 inch Adapter

The 4 inch duct pipe is snugged over the fan connector and taped securely to the fan with metal tape for an air tight seal. The 3 inch duct is slid inside the 4 inch duct and securely taped. This completes an air tight seal between the bathroom fan and the outside soffit vent.

4 inch Adapter

4 inch Adapter

The fiberglass insulation is replaced around the fan and flexible duct, taking care to use a gentle bend in the duct to prevent kinks which would block the air flow.

Repack the Insulation

Repack the Insulation

I turned on the bathroom fan and went outside to check the air flow from the soffit vent. I felt a good breeze on my hand confirming everything was working correctly.

Take care,

Bob Jackson

Copyright © 2018   Reproduction strictly prohibited.


  1. Michael J. Klein November 11, 2012 at 8:23 pm - Reply

    Thank you! My kids bathroom always had an odor despite the fact the fan was on when they were in the washroom and we clean in their regularly. Then we noticed a water spot in the ceiling next to the vent. We had a friend who does house repairs inspect the spot and he said the roof was fine but he noticed the duct work was torn in two. Hence, the humid air, as you said, led to the water spot.
    I bought all the necessary supplies as it seemed like a relatively easy fix but noticed the air duct that connected to the fan was smaller than the outside duct. I had 4″ flex pipe but couldnt figure out how to connect it properly to the 3″ duct. I never ever thought about sliding 3″ tubing inside the 4″ tubing!! Shame on me…
    Im tentatively holding the tubing in with metal tape until I buy the 3″ tubing tomorrow. Thanks so much for posting this article. Lord knows what would’ve happened if I hadn’t installed the tubing properly.

    Thanks again1

    Michael J. Klein

    • Bob Jackson November 13, 2012 at 5:50 pm - Reply

      Home Depot sells a plastic 3″ to 4″ duct adapter that would be a better way to correct the problem instead of the quick-fix shown here. I’ve since installed the duct adapter, but the quick fix is better than dumping the humid air in the attic!

      Thanks for writing.

    • Steve December 13, 2015 at 2:32 pm - Reply

      I am now struggling with replacing my bathroom vent as well. I removed the old one and was cruising along with replacing it until I realized the existing venting was 3″ and the new fan has a 4″ exhaust. I am having difficulties trying to get too the old pipe, I need to cut it back and it is in a tight hard to reach space. Do you have any ideas on how I might go about cutting it?

      Appreciate any help you may be able to provide.

      • Bob Jackson December 13, 2015 at 9:01 pm - Reply

        The best solution is replace the old 3 inch duct with 4 inch.

        If that’s not possible a 4 in to 3 in round duct reducer will do. However it will significantly decrease the fan air flow volume.

        A plastic reducer is available from

  2. Craig Salhoff October 2, 2013 at 7:30 am - Reply


    If the exhaust duct runs between joists with heated floors above and below, in a 1.5 story house does the duct need to be insulated? I can transition to insulated ducting when I get to the attic part of the duct run beyond the metal joist bridging but I do not think I can run the insulated duct through the joist bridging. What do you think?

    • BobJackson October 2, 2013 at 9:48 am - Reply

      Bathroom exhaust duct within the building’s thermal envelope (heated space) does not have to be insulated, but best to call your Building Department to check local code requirements, if any. It is a good practice to install insulated duct to minimize the possibility of condensation within the exhaust duct, especially for metal duct.

      Your plan to transition to insulated exhaust duct inside the attic is essential to reduce condensation and the possibility of water pooling in the duct which can cause mold and/or a leak.

      City of Spokane Building Services – page 1, bullet #2 “If the duct leaves the heated space or goes above the insulation, it must be insulated to a minimum R-4 rating.”

  3. craig November 9, 2013 at 2:54 pm - Reply

    Thanks for publishing this website. I have used “Bathroom ventilation duct repair”, “How to replace a dryer vent” and “How to replace a bathroom exhaust fan” to get to my current impasse.

    I have completed he installation of the Panasonic Bath WhisperCeiling fan FV-08VQ5and most of the duct work in the first-floor bath ceiling space. I installed R8, 4-inch flexible duct between the ceiling joist through the unheated part of the duct run. The duct terminates at the exterior wall but the duct needs to continue through the wall to the ventilated eave. The end of the intra-joist-space needs to be closed with OSB [oriented strand board], covered with Tuff-R insulated sheathing and the duct work caulked so no air enters the area between the ceiling joist, above the drywall, and make its way back to the bathroom.

    How do you suggest I attach the R8, 4-inch flexible duct to the OSB, connect another piece of flexible duct on the outside of the insulated sheathing and continue the duct through the ventillated eave to the dampered outlet in the horizontal eave?

    I have a 4-inch start collar with crimp that will not work [see It seems to me that I need a 4-inch metal duct with male fittings on both ends attaching the flexible duct to the interior side that can pass through the wall system. At least one of the two sides needs to have a metal flange that can be nailed to the OSB. Then another piece of flexible duct can be attached to the outside of the wall system and the system can be caulked on the outside to prevent air infiltration through the wall system.

    What would you call this connector? What do you think of my plan for continuing this work? How should I proceed?

    • BobJackson November 9, 2013 at 4:27 pm - Reply

      > How do you suggest I attach the R8, 4-inch flexible duct to the OSB, connect
      > another piece of flexible duct on the outside of the insulated sheathing and
      > continue the duct through the ventilated eave to the dampered outlet in
      > the horizontal eave?
      You need a 4 inch sheet metal duct spice connector for the flex duct to flex duct connection through the OSB sheet. That item at Menard’s is a start collar designed for connecting flex duct to rigid duct board and won’t work.

      Saw the round hole in the OSB draft block sheet large enough so the raised bead on the duct splice connector fits through the hole. Cut four or five 1 inch wide by 4 inch long (use your judgement) mounting brackets out of 26 or 28 gauge scrap sheet metal. Bend the tabs in an L shape. Center the duct splice connector in the OSB sheet, then fasten the L brackets with 1/2 inch long self-drill sheet screws to the duct splice and OSB sheet. Space the tabs equally around the splice connector. The home made L brackets will hold the duct splice connector in the OSB sheet to prevent movement and tilting. The mounting tabs are installed only on one side of the duct connector and OSB.

      The round hole sawed in the OSB sheet will need to be slightly larger than the raised bead on the duct splice connector, thereby creating a ~3/8 inch gap between the spice connector body and hole in the OSB. Fill the gap with outdoor rated silicone caulk to prevent drafts. Wrap a layer of HVAC metal foil tape over the screw heads to seal.

      If I were mounting the OSB sheet between the ceiling joists, I’d use four Simpson Strong-Tie Z-Max 1 -1/2 in. x 2 in. x 1-3/8 in. Angles and skip the difficult toe-nailing or support blocks.

      If you’d like to send photos, I’ll post them here so everyone can benefit. Email to bob (at)


  4. Joanne Kumar May 3, 2016 at 5:37 pm - Reply

    I would like to have a dryer vent put in place in my attic. Also need to have a bathroom fan vent that was recently installed 4 years ago, checked for reverse air flow. It seems that the outside air is flowing back into the unit and making the bathroom air warmer than the house temperature. My house is in the 32828 zip code, can you say who can do this improvement in attic for dryer and bathroom vent checked for what the problem can be?

    • Bob Jackson May 4, 2016 at 9:56 am - Reply

      Hi Joanne,
      The clothes dryer vent must not exhaust into the attic space because the humid air can cause condensation and mold. The proper way to do it run the duct through a roof vent to the outdoors. See Clothes Dryer Vents by Ask The Builder for details. If at all possible it’s best to run the dryer duct through a side wall of the house which avoids long runs of vertical duct which promotes lint buildup.

      > Also need to have a bathroom fan vent checked for reverse air flow.
      The bathroom vent fan should have a back-draft damper (see the damper at the center top of this photo) and the outdoor vent may have a damper, too. On windy days you may hear the damper closing with a “thup, thup” stopping the back flow. Sometimes the damper can get stuck due to dust buildup. Birds may build a nest in the outdoor vent if it’s not protected by a bird-proof cage. It’s a simple job to remove the vent fan cover to check if the damper is closing and vacuum out the dust if needed.

      You can find appliance installation/repair and handyman services on Also check the Better Business Bureau (BBB) reviews for prospective companies.

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