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Reuben Saltzman

Bath fan duct: metal or plastic?

Should bath fan ducts be made from metal or plastic?

Metal bath fan duct

If you live in a cold climate like Minnesota, I recommend plastic. Why would someone want metal? It gives you a smooth interior, which means less air turbulence, which means more airflow. The end. That’s the only benefit that I can think of.

The downside to using a metal duct is that metal is an excellent conductor. The metal will be continuous all the way to the bath fan terminal, which means you’ll have a very cold duct during the winter. This leads to copious amounts of condensation, which leads to stains at the ceiling. It doesn’t matter if the duct is insulated or not.

Ceiling Stain bath fan

With a plastic, insulated duct, you’ll still get some condensation inside the duct, but not nearly as much as you’d get with metal. For this reason, plastic is just about all that’s ever used in Minnesota.

But, but, but… Airflow!

If you’re really worried about getting enough airflow, check with the manufacturer. Broan has an airflow calculator on their website, which can be found here: https://www.broan.com/Support/Specifier-Tools/Allowable-Duct-Length-Calculator

Broan duct length calculator

Just plug in the fan you’re using, what your desired flow rate is, and what type of duct you’ll use. They’ll tell you how long of a run you can have. The minimum required flow rate is 50 CFMs.

Author: Reuben SaltzmanStructure Tech Home Inspections

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5 responses to “Bath fan duct: metal or plastic?”

  1. Roger F Wachtl Jr
    March 4, 2019, 1:52 pm

    Hello, I really appreciate your blog. What is available to use as a plastic vent pipe for a bath fan? Regular PVC for drain lines? I wasn’t sure what would be best. I live in Upper Michigan. Thanks, Roger

  2. Reuben Saltzman
    March 4, 2019, 1:58 pm

    Hi Roger, I recommend something like this: https://www.homedepot.com/p/Thermaflex-KP-4-in-x-25-ft-HVAC-Ducting-R8-0523-0400-0001/203766679

  3. Roger F Wachtl Jr
    March 5, 2019, 7:50 am

    Thanks for the quick response. You guys have very good articles. I have learned a lot. Keep up the good work.

  4. Roger F Wachtl Jr
    March 6, 2019, 8:03 am

    Hello, I have read about putting a damper in the vent pipe to keep drafts from coming in. Are you familiar with these?
    http://www.ideal-air.com/shop/product/ideal-air-back-draft-dampers

    Thanks Roger

  5. Reuben Saltzman
    March 6, 2019, 9:26 am

    Hi Roger, there’s no need for those; bath fans are already supposed to have a damper at the terminal, and they also come with an internal damper right at the fan. See below.
    Bath fan with internal damper

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