Can I Run a Bathroom Exhaust Fan Into a Basement?

Venting Exhaust Fans

• Bathroom exhaust fans must be vented to the outside, either through the wall or run through the ceiling to the soffit so moisture can be vented outside. According to Lowe’s installation instructions, you can connect a flexible plastic duct vent to run from the fan to the soffit vent outlet. The fans that are vented through a wall need gravity-hinged dampers on the outside wall and an exhaust fan on the inside wall.

Basement Venting

• According to the Canada Mortgage and Housing Corporation, exhaust from bathrooms should never be vented to a basement, attic or anywhere else in the house where moisture can cause mold to grow. The ideal is to remove the moisture from the bathroom so that when you open the door, the steam is removed and will not move through the house where it can raise its humidity. During the summer, when basements are damp and cool, any added humidity increases the chance of mold growth and a mildew smell.

Ideal Humidity

• The ideal humidity of a home during the colder months ranges between 45 percent to 55 percent for a person, according to livingwithmyhome.com; and for furniture and woodwork, the ideal humidity is 40 percent to 45 percent. Not venting the bathroom to the outside increases the relative humidity in the home every time someone uses the shower or bathtub. Higher humidity levels in the home can cause condensation on windows and walls. If the humidity level stays high, mold can grow on walls and other surfaces.

Testing the Fan

• A good bathroom exhaust fan quickly removes the humidity from the room and brings in cool air from the room outside of the bathroom. When the exhaust fan is on, you should feel new, cooler air coming into the room from under the closed door. If the air isn’t coming in from under the door or you can hardly feel it, the fan is not pulling hard enough. This means that it could take longer to remove the humidity from the room before you can leave the door open.

  Another way to test the exhaust is to place a piece of toilet paper up to the grille and turn on the fan. The paper should be tight to the grille. This indicates that the fan has a strong pull to remove the humidity, according to Canada Mortgage and Housing Corp.