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Is Your Bathroom Properly Ventilated?

We've seen it probably more than 100 times. In an attic, the bath fan discharges directly into a pile of insulation. You would be surprised that it's more common than many people realize! If you don't know the condition of yours, it's worth a quick investigation. So, what are the indicators of a properly ventilated bathroom? Here are a few tips.

1. Select the correct size bath fan. Many have a square footage chart included at time of purchase, but there are tables available online as well, such as this one from HVI:

2. Ensure adequate make up air can reach the fan. As pictured, allow a minimum of 3/4" clearance under the door, and even more if possible. Use caution, however, that hollow doors only have a small solid rail at the bottom which could limit the cut off.

3. Always terminate the ductwork to the home exterior. In the photo, the duct is attached to a pre-cut galvanized sheet, and connected with a 4" starter collar. It is attached directly under a roof static vent which was installed solely for this purpose.

4. How long to allow a fan to run post-showering? The Home Ventilating Institute recommends that a fan should be left on for 20 minutes after a shower to thoroughly clear humidity and to ensure moisture and condensation in the fan body or ducting is minimized. A preset timer, such as illustrated in the attached photo, helps achive this.

5. Not sure if your fan is drawing air? As pictured, you can give it a quick (albeit not scientific) test using a sheet of paper. if your fan is blocked by insulation, the paper likely will not stick.

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  • Becky (J) P
    Becky (J) P Highland, IL
    on Nov 27, 2012

    my bathroom fan is only ventilated to the attic. I never understood why you would want moistness sitting around up in the insulation. We did get a new roof last year and I questioned the roofer about ventilating it to the roof, but he didn't seem too interested.

  • Douglas Hunt
    Douglas Hunt New Smyrna Beach, FL
    on Nov 27, 2012

    I love the preset timers for fans. A friend has one and that is definitely what I will do in the future.

  • Building Moxie
    Building Moxie Baltimore, MD
    on Nov 27, 2012

    very well laid out post Paul and super tips. I know myself when I recently installed a door on our bath that I wasn't necessarily conscious of a full 3/4" clearance (will check when I get home). Another thing we are guilty of in our house (all of us) is showering with the door open. I know that I upsized slightly when I selected my fan, but I did not take this into consideration. wish I would want have gone even bigger. I can't necessarily assume that we are in the minority, but I'd love to hear... and I'd be curious if there are any suggestions that address this use case specifically - other than "don't do it." As for a timer I do not have any installed in my house, but it is on my long to do list. You know SLSConstruction did a "How to Install" on our blog ... he calls a bath fan timer, "His number one piece of advice to improve indoor air quality, comfort, and to help save energy." (Number 1). cheers.

  • Building Moxie
    Building Moxie Baltimore, MD
    on Nov 27, 2012

    One other note on this, I always prefer to exit the house on a vertical surface if at all possible as opposed to through the roof (for obvious reasons) ... wonder if there there is more of practical reason for going up instead of out. cheers.

  • Miriam I
    Miriam I Bay Shore, NY
    on Nov 27, 2012

    Great tips, Paul! Thank you for the advice!

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