Asked on Sep 15, 2019

Why does mold keep growing in my under cabinet vent hood?

We're onto our third vent hood now and I'm totally flabbergasted as to how this keeps happening. We also found mold growing behind and above the one we just removed (UGH!). There's also some mold in the ducting. Our ducting goes straight up and then out to the side of the house. We have a gas cooktop. I'd really appreciate any advice before I go out and spend more money on another vent hood. Thanks in advance.

areas above and behind vent hood covered in mold

  9 answers
  • Dee Dee on Sep 15, 2019

    have you checked to make sure the duct does not have any obstructions or air leaks?

    • See 2 previous
    • Colette Johnston Colette Johnston on Sep 15, 2019

      Thank you

  • Linda Sikut Linda Sikut on Sep 15, 2019

    Hi Colette,

    Does if have doors that close? The moisture that the hood is drawing out is getting stuck there and kitchen bacteria is causing the mold. Try wiping it with 50/50 mix of vinegar and water with a few drops of Dawn on a regular basis. If that doesn't work, it's possible that there is something wrong with your hood installation. Wishing you the best.

  • Colette Johnston Colette Johnston on Sep 15, 2019

    Do you mean cabinet doors? Yes, but they were removed ages ago and haven't been put back. It's probably the hood installation. Thank you!

  • William William on Sep 15, 2019

    If the outside vent has a flapper make sure it opens and closes freely. Also the force of the hood fan running opens flapper. If there is no flapper the cool outside air can be preventing the warm/hot air from getting out. It drops back causing moisture to form and mold. A log run of pipe can cause the air to drop back also.

    • Colette Johnston Colette Johnston on Sep 16, 2019

      Thank you William. The run of ducting to is about 4-5 feet. Unfortunately we can't do anything about that, but do you think that using a larger size of ducting might help? We've been using the 3.25 x 10 rectangular type.

  • Hi Colette - There are several ways to get rid of mold, but most importantly, you need to address the root cause to keep it from returning. This article focuses on the bathroom, but is an overall great article that covers methods of how to remove mold, if bleach is safe to use, and how to prevent it from coming back. Hope this helps! Hugs, Holly

    PS - We have a FREE eBook with recipes for all of my homemade and all-natural cleaners. You can download it here -->

  • Colette Johnston Colette Johnston on Sep 16, 2019

    Thank you Holly.

  • William William on Sep 16, 2019

    In your photo it does show 3 1/4 X 10 duct up to the ceiling. Those kind of installs are standard. And also took less space in the cabinets. You need to check the new vent hood instructions for round duct size. New hoods require either 6" or 7" round duct. Some even require 8". Above the ceiling you would need a transition boot from 3 1/4 X 10 to the appropriate round pipe size. An elbow (if needed), length of round pipe to the outside wall, then the vent itself. The proper way would be to install the transition boot inside the cabinet at the hood and then go with round pipe the rest of the way. Of course, it would take up space in the cabinet.

    • Colette Johnston Colette Johnston on Sep 16, 2019

      Thank you William. I've been doing loads of research and learning a lot about this stuff and I agree that we should switch to a round duct. We can easily change those cut outs to accommodate that. Unfortunately we're restricted to a 6 inch duct though due to the soffit size that it runs through, unless we go up into the ceiling and run it between floor joists (but that would open up a whole new can of worms, so I'm trying to avoid it). Another advantage to going with round duct is that we won't need that transition piece because most range hoods provide both options of a round or rectangle connection. We would still need an elbow, that is unavoidable.

  • William William on Sep 16, 2019

    Yes. Some new range hoods do provide options for round or rectangular connections. Hope this solves your mold problem. I cannot think of anything else that could be responsible.

    • Colette Johnston Colette Johnston on Sep 16, 2019

      Thanks again. Me too. I do have one more question if you don't mind me asking because it sounds like you know what you're talking about …. do you think that insulating the ducting would help?

  • William William on Sep 16, 2019

    Insulating the duct will reduce noise and vibration when the hood is on. If the duct runs through an unheated space it can help prevent condensation and moisture problems. Before insulating make sure all connections are sealed. Use foil tape. Don't use duct tape because it will not provide a good seal and degrades quickly.