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Bathroom vent design

rjmhudson | Posted in Mechanicals on

The upper floor of my house has a cathedral ceiling constructed of 2.4″x6″ cedar planks. I’m putting a new roof on it this week with foam board under the metal so it will finally be insulated. One detail in the project is how to replace the bathroom fan. Currently, it is an exterior unit that draws through a 6″ duct. It draws plenty of air, but also leaks heated air. I’m planning to get a new rooftop unit so that I don’t have to build anything inside:

So, my questions are:

1) Is there some kind of (passive preferably) damper that could be put in to reduce air loss?
I’m thinking of something like this,, except oriented for exhaust, but I suppose an active damper could be tied into the switch as well. Which would you recommend?

2) Since the bathroom has a cathedral ceiling, I plan to stick with an external fan ( Fantech RE 54 or 6: What size would you recommend? I did a sizing calculation (LxWxH/8=8x9x8) and came up with 72 cfm, which is probably less than what I have now. I can get 116 cfm at zero static pressure with the 5″ duct-size fan. Or I can get the 6″ fan to stay with something like what I have now. Is the larger fan overkill? Can’t most motors be operated with variable speed control?

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  1. user-2310254 | | #1

    Robert. Better quality bath fans have a built-in damper, but it's a good idea to also install an additional damper somewhere in the exhaust line (inline or as part of the cap).

    I noticed that Fantech sells an inline damper as an accessory for its external fan.

    Out of curiosity, where are you located and how much rigid insulation are you installing on your roof? You have to go fairly thick to reach r-38 or r-49. Have you read Martin's article on insulating cathedral ceilings ( and installing rigid foam on roofs (

  2. rjmhudson | | #2

    Thanks for your response. So I should order the backdraft damper...but I'm not clear what you think about the fan sizing issue...I see that the brochure for the fan says that speed is 100% controllable, so I could get the bigger fan and put a rheostat like Fantech sells at the switch. Make sense? I'm thinking of using this with a whole house ventilation system

    Plans are for 8" of foam board insulation on top of the cedar planks. I've been over this with Martin previously.

  3. GBA Editor
    Martin Holladay | | #3

    For general information on sizing a bathroom exhaust fan, see these two articles:

    Bathroom Exhaust Fans

    Designing a Good Ventilation System

    Before you can choose the right fan size, you need to determine whether the purpose of the fan is to exhaust odors and moisture from the bathroom, or to be part of a whole-house ventilation system.

    An exhaust fan always incurs an energy penalty, so you never want your fan to be oversized.

    If you want a quality, quiet fan -- for example, one of the Panasonic models -- you need to remember that you aren't limited to roof-mounted units. You can mount your fan on your bathroom ceiling, and still vent the fan through the roof.

  4. user-1072251 | | #4

    ROBERT: While you're at it, check out the termination of your ceiling planks. If they extend to the outside of the framing, they could be leaking a fairly large amount of air through the V's. If they do, you'll need to air seal those ends.

  5. pbyar | | #5

    Robert, I have no idea how much you know, so forgive me if these questions are obvious. I offer them in the spirit of better safe than sorry. Is there anything behind the cedar planks providing air sealing? If not, do you have plans to address the problem? Have you sized the insulation to your climate zone?

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