Replacement Window GuideThe International Residential Code sets minimum sizes for emergency escape and rescue openings, commonly called egress windows. UpCodes publishes IRC Section R310, which requires a minimum net clear opening of 5.7 square feet, a minimum opening height of 24 inches, a minimum width of 20 inches, and a sill no higher than 44 inches above the floor. Confirm the exact code and edition your local jurisdiction has adopted before you order or install.
Where egress openings are required
The IRC requires an emergency escape and rescue opening in specific spaces where people sleep or spend time below grade. UpCodes publishes the text of Section R310.1, which names basements, habitable attics, and every sleeping room.
Because the requirement follows sleeping rooms, replacing a bedroom window with a smaller unit can put a house out of compliance. When you plan a replacement in any of these spaces, you check that the new window still meets the opening minimums that follow.
Minimum net clear opening area
The most misunderstood egress figure is the net clear opening, which is the actual free space a person can pass through when the window is fully open, not the nominal window size. UpCodes lists the general minimum as 5.7 square feet.
The code sets a smaller allowance for grade floor and below grade openings. UpCodes notes that grade floor openings have a minimum net clear opening of 5 square feet. Because sashes and frames eat into the clear opening, a window often has to be nominally larger than these numbers to qualify.
Minimum opening height and width
Meeting the area requirement is not enough on its own, because the code also fixes the minimum dimensions in each direction. UpCodes states that the minimum net clear opening height is 24 inches and the minimum net clear opening width is 20 inches.
These two minimums cannot both be at their limit at once, since 24 inches by 20 inches falls short of 5.7 square feet. In practice at least one dimension has to exceed its minimum so the total clear area still reaches the required 5.7 square feet.
Maximum sill height
The code also controls how high the opening sits so an occupant can reach and use it in an emergency. UpCodes states that the sill height is not more than 44 inches, measured from the finished floor to the bottom of the clear opening.
This 44 inch ceiling matters most in basements and finished lower levels, where a window set high in the wall may clear the area requirement yet still fail on sill height. When you replace a below grade window, you verify the finished floor to sill measurement along with the opening dimensions.