Bathroom fans are great. They rid the home of excess moisture and allow you to see your face when trying to shave afterward, as long as they’re set up properly.

The down side is that when first installed, a hole is cut in the ceiling somewhat larger than the unit itself. Sometimes it is significantly larger, sometimes just a bit. That gap between the unit and the drywall allows conditioned air to escape whether the fan is blowing or not. That means warm air in the winter and cold air in the summer is leaving even if you finished your shower hours ago.

This is one of the easiest fixes around the house and it takes about ten minutes, including set-up and break-down.

1. Turn off the fan and the light within the unit (if it has one.)
2. Remove the light cover and light bulb.
3. Pull down the fixture cover (plastic piece that sits on the drywall.) In most cases you’ll now see that the cover is held in place with two metal spring clamps. You can leave it as is for now or make your life easier by pinching the clamp to remove it from the metal housing. (Tip: See where the clips attach BEFORE removing them.)
4. Now the fun part – see that gap all around the metal housing? Use an appropriate tape such as an aluminum tape or FlexFix to cover that hole. (See note about tapes below.)
5. As long as you have a CFL in that fixture you can also cover all the many holes inside the housing with FlexFix or similar tape. IF YOU HAVE A HEAT LAMP OR USE INCANDESCENT BULBS do NOT use FlexFix which will melt – use aluminum tape instead. As a rule we now recommend using aluminum tape.

Here’s a short video shot and edited by Martin LaMonica, featuring Cambridge HEET’s Audrey Schulman, explaining what to do.

Similar to a dryer exhaust, bath fans should vent outside. Venting a bath fan into an open attic is a recipe for rot, mold and other issues.

One great, and inexpensive, addition is a simple timer switch. Before entering the shower set the timer switch for about ten or 15 minutes longer than you’ll be in the shower. Now you don’t have to remember to shut the fan off – it will do its job.

There are nifty humidity sensing bath fans but if you don’t want the hassle or expense of a new fan, this switch is fast and inexpensive (just make sure you hire a competent electrician to do it.)