THE RATE OF AIR INFILTRATION AROUND A WINDOW OR SKY LIGHT WITHIN A GIVEN PRESSURE DIFFERENCE. IT IS STATED IN CUBIC FEET PER MINUTE OF EITHER THE SQUARE FEET OF THE WINDOW'S AREA (CFM/SQ FT) OR PER FOOT OF WINDOW PERIMETER LENGTH (CFM/FT). THE LOWEST LEAKAGE RATES ARE THE BEST.
The air leakage rating is a measure of how much air leaks through the crack between the window sash and frame. The rating reflects the leakage from a window exposed to a 25-mile-per-hour wind, and is measured in cubic feet per minute per linear foot of sash crack.
A measure of the rate of infiltration around a window or skylight in the presence of a strong wind. It is expressed in units of cubic feet per minute per square foot (cfm/ft2) of window area or cubic feet per minute per foot (cfm/ft) of window perimeter length. The lower a window's air leakage rating, the better its airtightness.