Is Your Bathroom Properly Ventilated?

Hamtil Construction LLC
by Hamtil Construction LLC
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.
This preset bath fan timer is a great solution for ensuring the bath fan runs for an adequate amount of time. The 30 minute preset is typically used for showering.
The duct shown is terminated to the exterior, which is required by code in most areas. Minimize the twists, turns, and fittings used in the ductwork for the best airflow.
The "paper test" in action.
Ensure proper make up air, because that is usually the only spot where it is drawn from during a shower.
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  • Hamtil Construction LLC Hamtil Construction LLC on Apr 28, 2014
    Hi Khol, thanks for the comment. I would suggest an 80cfm fan. Definitely wise to terminate to the exterior. I would try to keep the ducts separate if possible, just so you do not lose airflow, but I understand that it's not always possible. Also, one thing to keep in mind, is that a vertical pipe can sometimes leak condensation back downward (steamy shower, cold air in the walls/ attic). If you must do it this way, be sure to insulate the pipe, at least in the attic space. Definitely need to saw the door bottom too! Best of luck.

  • Hamtil Construction LLC Hamtil Construction LLC on Apr 28, 2014
    Hi @Cherie Dunkel-Hoysock - I am just revisiting this thread, saw your comments, and I was curious if you ever resolved your sewer/ vent issue?