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Leaky Bath Fans
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Minnesota Community Living May/June 2010

Leaky Bath Fans

By Mark Petersen. Access Builders Corporation

This winter we encountered several customers who had problems with water dripping from their bath fans. While leaks like these could be caused by an improperly installed roof vent, it has been our experience that water that shows up in and around bath fans is most frequently caused by condensation. This condensation usually occurs in bath fans that are infrequently used or run for too short a duration.

Bath fans are vented one of two ways, either out the side of the building or through the roof. The ones that are vented out the side of the building are typically not a concern. It’s the ones that are vented through the roof that can cause trouble. The problem occurs when warm, moist air from the living space travels through a cold duct that is located in the attic space.

Even though the ductwork in the attic should be wrapped in insulation, eventually an unused duct will assume the temperature of the surrounding air. So on those very cold days, the inside of a duct will be very cold, and when a fan is run for a short period of time, the warm air that is being expelled condenses on the interior of the cold duct. If a fan is repeatedly used for short durations during a particularly long stretch of cold weather, condensation will continue to build up in the form of frost. When the weather eventually warms up, the inside of the duct warms up as well, and the built up frost comes dripping back down into the house.

Condensation can even occur in a bath fan duct if the fan is never used. Just because a fan isn’t running doesn’t mean that air isn’t flowing through it. Most living spaces maintain a positive air pressure. This means that inside air is constantly trying to escape to the outside. There is a damper in most bath fans that allows air to flow in only one direction—out. This damper keeps the wind and water from being blown back into the building but does not prevent air from flowing out. It is likely that enough inside air could escape out through the fan to create frost on the inside of a cold duct.

We recommend running bath fans for extended periods during cold weather. If the fan is run for a longer duration (at least 15 minutes), the warm interior air has a chance to raise the temperature of the inside of the duct, thus avoiding condensation. If only a small amount of frost has built up, the warm air will dry it up before it has a chance to drip back down.

Inexpensive switches can be installed that let the fan run for a predetermined length of time. And if you’re at all like me, someone who would have no shot whatsoever to remember to regularly turn on a bath fan in a bathroom that is rarely used, there are 7-day programmable timers out there that might come in handy.

Mark Petersen, Access Builders Corporation

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