One of the things that a home inspector will typically check is to see that each bedroom has a means of emergency egress directly to the exterior of
the dwelling.  The egress allows the occupant to escape, but also allows emergency personnel access for rescue if needed.  If there is no door to
the exterior, then there should be an egress window.

What is an egress window?

An egress window is a window that is required in specific locations in a dwelling and is intended to provide an emergency means of exiting a
dwelling.  Egress windows are only required in dwellings.  Windows must meet specific size requirements to qualify as an egress window.

Where are egress windows required?

Egress windows are required in every room used for sleeping purposes (bedrooms) on any floor and in basements with habitable space.  If you
are constructing a new home, the code requires that you put an egress window in each bedroom.  It also requires an egress window in the
basement if habitable rooms will be finished in the basement.  If you install a basement bedroom or bedrooms, an egress window is required in
each bedroom but you need not provide another egress window if there are other habitable rooms in the basement. The bedroom window(s)
suffices for the habitable rooms.

If you have an existing home and you add a sleeping room in an unfinished basement, the code requires that you install an egress window in the
sleeping room or rooms.  Likewise, if you create habitable space in your basement other than a bedroom and you currently do not have an egress
window, the code would require that you install one as part of the installation of the habitable room.
Bedroom Emergency Egress

According to International Residential Code (IRC), an egress window must
satisfy four criteria:

  • Minimum width of opening: 20 in.
  • Minimum height of opening: 24 in.
  • Minimum net clear opening: 5.7 sq. ft. (5.0 sq. ft. for ground floor).
  • Maximum sill height above floor: 44 in.
  • The window must have a minimum net clear opening of 5.7 sq. ft.  Net
    clear opening refers to the actual free and clear space that exists when
    the window is open.  It is not the rough opening size or the glass panel
    size, but the actual opening a person can crawl through.

The window opening must be operational from the inside without keys or tools.  
Bars, grilles and grates may be installed over windows but must be operational
without tools or keys and still allow the minimum clear opening.

Do the math

At first glance, you might assume that a 20-in. by 24-in. window would be
acceptable for egress.  However, those dimensions would yield a net clear
opening of only 3.3 sq. ft. To achieve the required net clear opening of 5.7 sq. ft.,
a 20-in. wide window would have to be 42 in. high. Likewise, a 24-in. high
window would have to be 34 in. wide.

Nearly all window manufacturers specify which of their windows meet national
egress requirements right in their catalogs and list the net free opening of their
windows. This should take the guesswork out of selecting a window for you
addition or remodeling project. Also, the sales person who sells you the window
should be able to help you select a code compliant window provided that they
know you need an egress window.