An egress window is a window that is designed to be used as an emergency exit. The 2009 International Residential Code (IRC) established criteria for an egress window that include location and size. The IRC requires egress windows in basements and all rooms used as sleeping quarters. The egress window is to be constructed of a specific size.
Net Clear Opening
To meet the requirements of the 2009 IRC, the net clear opening must measure at least 5.7 square feet. The net clear opening is defined as the clear, free space that exists when the window is fully open. It is not the size of the glass pane, the rough opening size, but the actual opening through which a person would crawl.
In accordance with the IRC requirements, the minimum width of the egress window opening is 20 inches. The minimum height requirement of the opening is 24 inches. The window's sill must not be highers than 44 inches from the floor. The clear opening of the egress window is designed for access out of the house in an emergency situation. It is also designed for emergency access inside by a firefighter wearing an oxygen tank.
Basement Window Wells
Egress windows in basements have requirements that also pertain to the window wells. The window well must be large enough to accommodate the egress window when fully opened. A floor area that measures about 9 square feet or a minimum dimension of 36 inches in length and width is another requirement. The window well must possess steps or a permanently attached ladder for a depth that exceeds 44 inches. The width of the ladder must measure at least a foot and protrude at least 3 inches above the window well. It can in no way intrude or obstruct the fully open egress window in any manner.
Do not assume that a window that measures 20 inches by 24 inches will give you a net clear opening of 5.7 square feet just because they are the minimum requirements. It won't. You need to do the math and adjust the dimensions accordingly. The minimum requirements are guidelines to be followed in regard to the least amount of clear space allowed. Windows with hinges, such as casement windows as opposed to sliding ones, generally have a larger net clear opening in a smaller sized window. These work well as egress windows.