GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Double dampers on bathroom exhaust fan

user-6651407 | Posted in General Questions on

We have already installed the bathroom exhaust fan’s, and they have their own backdraft damper towards the actual bathroom side.

We are using hard metal ducts, insulating them, and exiting them through the bird box soffit which extends out 5 feet from the closest vented soffit.

I want to make sure the duct extends through the soffit, and plan on using the Duraflo model suggested by Martin. This also has a backdraft damper.

Is two dampers too much? I don’t want to over think it, but would I be causing any air to become trapped between the two?

We have installed timers which automatically run the fans for 30 minutes.

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. GBA Editor
    Martin Holladay | | #1

    Don't worry -- the fan should be able to handle two backdraft dampers.

    That said, any duct system introduces static pressure (reducing cfm). A straight 6-inch duct with no elbows has significantly less static pressure than a 4-inch duct with two or three elbows. The smaller the diameter of the duct, and the more elbows, the greater the static pressure. Lots of builders discover (too late) that their poorly designed duct system has hurt the performance of their bathroom exhaust fan.

    -- Martin Holladay

  2. Otosan | | #2

    What is the Duroflo model that Martin suggested? A search does not return anything on it.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |