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.
Why do we need exhaust fans?
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:
- It can exhaust smelly air, allowing fresher air to enter the bathroom.
- It can exhaust humid air, allowing dryer air to enter the bathroom.
- When operated for 24 hours per day or when controlled by a timer, it can act (in some cases) as the most important component of a whole-house ventilation system.
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.
Where does the makeup air come from?
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…