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Community and Q&A

Bathroom Exhaust Fan drafts…

mscott2013 | Posted in Energy Efficiency and Durability on


I was wondering if folks had any suggestions as to how to reduce cold air infiltration/drafts from bathroom exhaust vents.

I’ve found that in some of my bathrooms, cold air is coming into the bathroom from the overhead ceiling exhaust fan. When I go outside, I see that the exhaust pipe has a simple damper on it. I imagine that when the wind blows, it can easily get up and into the exhaust duct and the damper doesn’t stop it.

Are there products that can help resolve this issue?


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  1. GBA Editor
    Martin Holladay | | #1

    If air is leaking backwards through your bathroom exhaust fan, it's possible that your house is under negative pressure with respect to the outdoors.

    This can happen (a) when you operate a powerful exhaust fan (like a range hood fan) elsewhere in the house, or (b) if you have an unbalanced forced-air heating system with leaky ductwork (especially if there is a supply system leak in a duct located outside of the thermal envelope -- for example, in a crawl space or attic).

  2. MJDesigns | | #2

    Please take a look at the following link as it might prove helpful.

  3. davidmeiland | | #3

    OK, a quick look at that site.... claims some healthy energy savings just by blocking fan drafts. They must have done some tightly controlled studies to come up with those numbers, eh?

  4. davidmeiland | | #4

    Morris, Martin is correct about some of the possible causes, and it can also happen because of wind blowing against the wall where the wall cap is installed. You can install a better wall cap like a Seiho, or you can install an inline damper in the duct (I've used butterfly dampers for this with good success). The inline damper will reduce your airflow somewhat, so you don't want to do that if the airflow is already marginal.

  5. heidner | | #5

    David, If you mouse over the "larsonfan" example -- it appears to be a popup lid that seals the vent. Just a different form of a damper.

    I'd also suggest that sometimes the simple dampers work pretty well - if they are cleaned of the grease, lint and muck that collects on them over the years. If they don't swing freely - they will never seal. If you have access to the damper clean it...

    If it is an old shaded pole motor (like the motors from old phonograph players -- remember them), then it might be time to replace the bathroom vent with a newer more efficient vent like one of the Panasonic Q series fans.

  6. Oak_Orchard | | #6

    Can someone explain how the Larson fan works, the web site is not self-explanatory.

    The damper seems to be a pressure activated "plug" as opposed to a gravity and/or a spring mechanism.

  7. mscott2013 | | #7

    Thanks Martin. I will check my home for the possible causes you mention!

    David, thank you for your suggestions as well. The vent in question is on the side of the house that gets a ton of wind pushing against it. To that end, does Seiho make a version that can go under eaves?

  8. homedesign | | #8

    Morris, you mentioned that the problem is with "some" of your bathrooms...
    Is your house 2 story?
    If so....are the "problem" bathrooms on the first floor?
    The reason I ask .... perhaps your house has a high Neutral Pressure Plane.

  9. davidmeiland | | #9

    Morris, the stuff that Seiho makes for exterior use is mostly or entirely sidewall...

  10. mscott2013 | | #10

    David- Thanks for clarifying on Seiho. The exhaust vents in question are under the eaves and so I'd need to find something appropriate for that. I will keep looking!

    John- To answer your question, the house has a basement, ground floor, 2nd floor and finished attic. The problem bathroom vents are on the first/ground floor and a bit on the 2nd floor.

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