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Community and Q&A

What’s a good damper for a bath fan/range hood vent?

Nate G | Posted in Mechanicals on

My bath fan vent is basically a big hole in the house to the exterior and sucks air out of the house in the high New Mexico winds, which today reached 30 mph. There’s no damper or air check valve or anything like that installed up there, and I’d like to rectify it. What do people use for this situation? A standard butterfly damper? Something louvered? This thing?

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  1. GBA Editor
    Martin Holladay | | #1

    A high quality bath exhaust fan (like those manufactured by Panasonic) has an integral backdraft damper. So you might want to try installing a new fan, especially if your existing fan is old, cheap, or noisy.

    However, no damper is perfect. Because of the low air flows involved, a perfect seal is impossible.

    The damper shown in your link is the Cape backdraft damper (manufactured by Tamarack Technologies), and it will work, although it will introduce considerable static pressure to your duct system and will therefore reduce the air flow rate provided by your fan. Here is a link to the product information page on the Tamarack site:
    Cape backdraft dampers.

    By the way, I would ignore the advice on the Battic Door site about clothes dryer vents. These dampers are definitely not suitable for clothes dryer vents.

    Range hood vents are also tricky (because range hoods get gummed up with cooking grease). The Cape backdraft damper should never be used with range hood vents. Instead, choose an exterior termination with a damper that appears sturdy. I'm sure that a few GBA readers will suggest a good model. You'll find a few recommendations on these two previous threads on the same topic:

    What's the best kind of damper to install on a kitchen exhaust fan?

    Looking for good quality 3.25" x 10" wall cap with backdraft damper for kitchen exhaust fan - recommendations?

  2. David Meiland | | #2

    The dampers in bath fans have a gap around them. You can get a nice wall cap and reduce infiltration if you install the duct and the cap carefully, but it's hard to deal with exfiltration caused by strong winds pulling air out of the house through the fan, because the dampers will open under that pressure. The only option I can think of is a motorized damper that opens when the fan is on. These are commonly used on outside air intakes to forced air systems.

  3. Paul Kuenn | | #3

    And whatever you end up using Nathaniel, don't use ANY flexible (coil type) exhaust tubing as it will destroy almost half of the CFM output of the blower. You'll get much less resistance with solid vent tubing and try to use 45 degree elbows instead of 90s. As for dryers, EFI sells an efficient outdoor exhaust cap/damper that has worked well in high winds and -20F.


  4. Nick Welch | | #4

    Seiho wall vents have pretty good butterfly dampers built in. But that doesn't help on a roof-terminated exhaust, or if you don't feel like changing your wall vent.

    The dampers built into bath fans and range hoods/microwaves do not inspire confidence. The one on my microwave is laughable -- it has huge gaps that are constantly open to airflow. I'm not even sure why it exists.

    After some searching I found a good damper. It's made by American Aldes and you can easily find it on Amazon in nearly any duct size. It is expensive, but so be it. I just installed two for my range and bathroom exhaust ducts that go through the roof. I located each roughly where the thermal boundary of the attic insulation is, as this makes logical sense to me. No complaints so far. They do the trick and have a nice sturdy rubber seal around the perimeter. However, the weatherstripping they use to mate with the duct is not that great -- I used foil tape to get a better seal between the damper and the duct, although mastic would be easier, given the curved geometry.

  5. Nate G | | #5

    Thanks Nick, that looks excellent!

  6. C Darwin | | #6

    Also need good damper.
    But, watched the Tamarack Technologies backdraft damper that starts with butterfly.
    At the end, they halted their video when their yellow flapper appears to almost blow completely out in 5 seconds more.

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