Better Out Than In, I Always Say

Hands down, one of the most common “errors” I see on routine energy audits involves exhaust fans.

The first mistake is venting exhaust fans directly into the attic. The purpose of these fans is to remove heat, humidity and other objectionable vapors from indoors. Venting into the attic contributes to the buildup of humidity and dirt, which in turn finds its way back into the house through leaking ductwork and ceiling penetrations. Exhaust fans should always have ducting run from the fan outlet to the nearest soffit opening.

Before installing ducting, check the fan to make sure that dampers are in place and installed correctly. Correctly installed dampers remain closed when fans are not running. Better-quality dampers have interior rubber gaskets that create air seals when the damper is closed as well as exterior gaskets that create a tight seal with the ducting. Dampers are available through local Heating/Air Condition Supply houses.

The second-most common mistake is not USING the exhaust fans. Exhaust fans should be turned on every time moisture is created and allowed to run long enough to remove humidity. Towels, walls and cabinets retain moisture and then release it slowly, keeping room humidity high even when water is no longer in use.

The Home Ventilating Institute ( recommends that exhaust fans remain on for at least 20 minutes after room use. Using an automatic timer is a good way to achieve this, especially for households with young children. A number of timers are available including manual “countdown” timers, electronic delay timers and programmable timers. Other options include speed controls for varying air flow, occupancy sensors and humidity sensors (de-humidistat).

Switches and timers can be used in combination to provide the most effective ventilation for each room.
The key to selecting the right control is to consider how the space is used, at what times and by how many people.

This blog was written for Paul LaGrange’s BuildWrite website and was originally posted on February 13, 2009.


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