In Cold Weather: Is Your Exhaust Fan Doing More Harm Than Good?

Building codes have just recently required an exterior discharge for exhaust systems. What if yours doesn’t discharge to the exterior?

The 2006 International Residential Code (IRC) for One and Two-Family Dwellings requires an outdoor discharge for exhaust systems. The first section in Chapter 15 reads, “The air removed by every mechanical exhaust system shall be discharged to the outdoors. Air shall not be exhausted into an attic, soffit, ridge vent or crawl space.” A mechanical exhaust system is anything with a fan. This requirement is carried forward through the 2012 version of the IRC.

Exterior discharge is an important requirement for two reasons. The first is safety. If you’re using a fan to exhaust air, it shouldn’t be exhausted into the spaces prohibited in the previous paragraph because it may find its way back into your home. We exhaust air to the exterior so that it’s diluted to the point that it’s not harmful.

This removes contaminants like┬ásmoke from cooking (grease and oil), cleaning fumes and VOCs. If you’ve ever set off your smoke detector because you let the oil in a skillet burn off, you’ve experienced how much your indoor air can be polluted… and you’ve realized how quickly it can be polluted.

The second reason for exhausting indoor air is to remove humidity. Your bathroom, kitchen and dryer exhausts all move humid air.


The image above is a bathroom exhaust fan taken in an attic space. The red arrow points to the opening in the side of the exhaust fan where a duct would be attached to carry the exhaust air to the exterior of the house. Since it’s left open, you can see the side of the fan – the black wheel.

The home owner turns on the fan before he|she takes a shower and the fan exhausts the hot, humid air from the shower below into the attic space shown in the image. This happens once a day, if not more.

What happens when you exhaust hot, humid air into a cold attic?

The hot, humid (steamy) air coming from the shower below is exhausted into the vented attic. When it’s very cold outside, the materials in the attic are very cold. The hot, humid air comes into contact with the cold surfaces in the attic and the humidity condenses onto the surfaces of those cold materials.


It’s just like humidity in the air condensing onto a cold glass of water. You’ve seen the water that pools at the bottom of your cold beverage in a restaurant. Sometimes there’s enough that you take care not to let it drip onto your clothing or you have to use a cardboard coaster to absorb the water. That’s what may be in your attic if your exhaust fan doesn’t have an exterior discharge. Moisture in an attic is an invitation for rot, mold and mildew.

In Conclusion
The 2006 version of the International Residential Code quoted earlier in this blog post is the first time many jurisdictions mandated exterior exhaust. It’s common to find bath fans, the occasional range hood and the rare dryer that exhaust into an attic, soffit or crawl space. It’s a good idea to verify whether your exhaust fans are all discharging to the exterior.

You should be suspicious of any long ducts to the exterior. Small fans like those found in bath fans probably aren’t strong enough to push air through a long duct. You should also be suspicious of multiple fans feeding into one exterior discharge because it’s probably not large enough to flow the air from all the fans operating at once. Both of these conditions will choke the fans – they’ll still make noise like they’re operating as intended but they won’t perform as intended.

You should occasionally check to be sure you don’t have any maintenance issues like loose fittings, collapsed/crushed ducts, rodent damage or condensation in the exhaust ducts. It’s a good idea to have regular service for your air conditioner so you can ask your air conditioning company to check your exhaust ducts and discharges when they service your air conditioner. The technician should be qualified to notice any potential issues.

Please leave your questions and comments below. Thanks!

Second image by OiMax used under creative commons license.

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