Older homes often lack bathroom exhaust fans. In the old days, if the bathroom was smelly or steamy, you were supposed to open a window to air it out.
This isn’t a very logical ventilation method, especially when temperatures are below zero, or when the weather is 90°F and humid. Yet this time-honored method of bathroom ventilation is still enshrined in our building codes. According to the 2009 International Residential Code (sections R303.3 and M1507.3), a bathroom with an operable window does not need to have a bath exhaust fan.
In spite of the code’s archaic loophole, builders should install an exhaust fan in every bathroom or toilet room — even when the bathroom has a window.
A bath exhaust fan can perform several functions:
Designing an exhaust-only ventilation system is a topic unto itself, and is beyond the scope of this article. For more information on exhaust-only ventilation systems, see Designing a Good Ventilation System.
When the bathroom door is closed and the fan is operating, where is the makeup air coming from?
If the bathroom has an exterior wall, some of the makeup air is coming from the exterior — for example, through leaks around the window or baseboard.
Some of the makeup air is probably coming into the bathroom from other rooms in the house, via the crack between the bottom of the door and the flooring. Of course, if the bathroom fan is exhausting 50 cfm, then 50 cfm must be simultaneously entering the building. If some of the makeup air is entering the bathroom through the crack under the bathroom door, an equivalent volume of exterior air must be entering other rooms of the house through a variety of random cracks in the home’s envelope.
Unfortunately, many exhaust fans pull some of their makeup air through a…