How To Look At A House
McGarry and Madsen's home inspection blog for buyers of
site-built, mobile/manufactured and modular homes
What is the code for bathroom ventilation?
Monday, August 6, 2018
A switched exhaust fan that is rated at a minimum of 50 cfm (cubic feet per minute) or a continuous fan that produces 20 cfm or more is necessary to meet International Residential Code (IRC R303.4, M1507.4) requirements. Although the continuous fan is an option, we never see it in residences.
Also, the code requires that the fan must terminate outdoors. Because the purpose of the fan is to get bathroom odors and the humid air from bathing out of the home, the fan cannot dump air into the attic, which would only move the high-humidity air to a new problem area. It should not be ducted into the attic or under the floor. Also, duct termination at the edge of the attic near a soffit vent does not count.
As an alternative, a window with a minimum opening of 1.5 square feet is also acceptable by code (IRC R303.3). The requirement refers to openable area, not the total area of the glass. A single-hung window, for example, will have an openable area of slightly less that half of the total surface of the window.
Opening a window was the way bathrooms in older homes were ventilated, but it is not acceptable to most homebuyers today. The average family lives in a closed, air-conditioned environment, and opening the bathroom window—especially when it’s 95º outside in August or 20º in February—just doesn’t happen.
Wall switches are available for exhaust fans with timers, like the one shown below, so you can set the fan to run for several minutes after you leave.
Also, a newer generation of wall switches uses a humidity and temperature sensor that leaves the fan on until the humidity and temperature has dropped to an acceptable level. An example on one made by Panasonic® is shown below.
Also see our blog post My bathroom is stinky and humid even though it has an exhaust fan. What can I do to fix it?
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To learn more about heating and air conditioning systems, see these other blog posts:
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