Vents should terminate outside

Q: Can you talk about venting bathroom/kitchen fans?

In the house I'm renting, I asked about where these two fans vent, and my landlord said they just vent into the attic, which isn't particularly well vented. He said that this is common, particularly in older houses. Having grown up in the Pacific Northwest, I'm always worrying about mold. I'd like to know a little more for personal reference when I own a house and so I can stop worrying about what might be going on in my attic.

A: The purpose of the bathroom fan is to ventilate the moisture normally generated by bathing or showering. When moisture is directed to a cool, closed attic, it will condensate back to a liquid and soak the wood framing and insulating materials. Mold and decay is a major concern in damp attics.

Normally, the bathroom vent fan is located near an outside wall of the home where the bottom of the roof system is closer to the attic floor. The fan will direct all the moisture to one spot on the roof's sheathing where a dark stain of decay or mold is often found. When an attic has loose-fill insulation, the airflow from the fan will create a void in the insulation, leaving a bare spot on the attic side of the bathroom ceiling. The non-insulated spot is now cooler than the remainder of the ceiling, and condensation naturally forms in the cooler areas. An unvented fan can cause as much damage as an unattended roof leak.

The kitchen fan is intended to remove odors and moisture from the kitchen, and it also needs to be vented to the exterior of the home. In modern homes, the kitchen vent fan is located in a hood above the gas or electric range. Over the years, grease and oils will accumulate inside the fan hood, which becomes a major fire hazard. If the fan is vented to the attic, a cooking fire can soon spread to the entire attic through the vent for the hood.

A range fan must vent through approved metal pipe all the way to the exterior of the home. Whether vertical through the roof or horizontal through a wall or foundation, the range fan must vent to the outside.

Dwight Barnett is a certified master inspector with the American Society of Home Inspectors. Write to him with home-improvement questions at d.Barnett@insightbb.com. Sorry, no personal replies.