Ventilation is critical in a well-designed bathroom and is necessary for moisture control and healthy indoor air quality, as also discussed in chapter 3, "Environmental and Sustainability Considerations." The designer’s responsibility is to plan a balanced and efficient system that does not compromise user comfort. Importantly, the designer needs to provide a ventilation system that will be used by the occupants.
Residential ventilation systems are generally designed with the assumption that indoor air is improved by mixing or replacing it with outside air. Air from outdoors is perceived to be fresher. Depending on the location of the home, this may not always be true. If the outside air is polluted, special ventilation systems may be needed that provide additional air filtration.
The minimum requirement for bathroom ventilation, as specified by the model 2012 International Residential Code (IRC), is an operable window of at least 3 square feet (0.3 square meters). One-half of the window area must be operable (1.5 square feet, .14 square meters). Within the IRC, an alternative to a window is an exhaust ventilation system (minimum 50 cfm, 23.6 liters per second), vented directly to the outside, provided adequate light is otherwise available.
Most experts recommend an exhaust system for bathrooms for more effective ventilation. NKBA Planning Guideline 26 states: "Plan a mechanical exhaust system, vented to the outside, for each enclosed area," of the bathroom, such as an enclosed toilet compartment (see Figure 7.7).
Although windows can be used to meet code requirements, relying on them for all bathroom ventilation can be a problem. It may not be practical or comfortable to open a window on cold or rainy days. An open window may compromise privacy or security. A single open window may not be enough to provide adequate air circulation to remove moisture.
Windows can be used in a bath not only for light and view, but also to supplement ventilation. If a window is being used for ventilation, place it high on the wall to take advantage of the natural
tendency of warm, moist air to rise. For example, operable skylights or roof windows can provide ventilation in nice weather. However, be aware that the placement of an operable skylight or roof window is dependent on maintaining building code requirements for a horizontal and vertical clearance from a plumbing vent.