Why does venting a clothes dryer into a garage, attic, or crawl space cause problems?
Friday, June 22, 2018
Home clothes dryers don’t start out venting into an enclosed space because the building code specifies that the termination of a dryer duct must be outside. Here’s what the Florida Building Code (FBC), Residential Edition, says about it:
But relocating the laundry to a different part of the house, or damage or clogging to the original dryer duct, may mean that a homeowner changes the vent termination to an enclosed space that is usually not air conditioned. It is often intended to be a temporary fix, but may last for years.
The three components of a dryer exhaust are heat, lint, and water as humidity in the air. When you think the about the weight of a load of towels when they come out of the washer, and then what they weigh when you take them out the dryer, it’s a lot of water. And it all goes into the air.
Mold grows best in high humidity at a temperature between 70º and 90º F. It also needs some sort of organic material to feed on. While a thin layer of dust is satisfactory, a coating of cotton lint on a surface is even better. So a dryer vent terminating in an enclosed and unconditioned space provides the heat, moisture and organic material to create an ideal environment for mold growth. Dryer lint is also highly flammable.
You can buy a box called an “indoor dryer vent kit” at home improvement stores, and it manages to control most of the lint, but not the water dumped into the air. Some homeowners use a much simpler system of venting down into a bucket of water, like in the photo at the top of this page. Either way, it creates an extremely moist and warm indoor environment that is conducive to mold growth, and we regularly find mold problems around indoor dryer vents.
The photo above shows the area above a dryer vent terminating into that same bucket in the garage. There is also extensive mold on the plywood that is not readily visible in the picture.
There is also a second issue with terminating a dryer in the garage: no ducts between the garage and house living area are allowed because they create a route for a fire in the garage to penetrate easily into the home. This code requirement is usually referred to when a homeowner wants to install air conditioning ducts from the house HVAC system to the garage, but also applies for dryer ducts.
The photo below shows an accumulation of dryer lint in an attic after years of the vent terminating in it. Venting a dryer into an enclosed space is never a good idea.
For more on dryer venting, see our blog posts What is the maximum length for a clothes dryer vent? and What are the code requirements for an outdoor dryer vent cover?
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