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The IRC egress minimums
Under IRC R310, basements with habitable space and every sleeping room need at least one emergency escape and rescue opening. City handouts from St. Paul, Boulder County and Lincoln all print the same numbers:
| Requirement | Minimum or maximum | IRC section |
|---|---|---|
| Net clear opening | 5.7 sq ft (5.0 sq ft at grade floor) | R310 |
| Clear opening height | 24 in minimum | R310 |
| Clear opening width | 20 in minimum | R310 |
| Sill height above floor | 44 in maximum | R310 |
| Window well area | 9 sq ft minimum | R310 |
| Well width and projection | 36 in minimum each | R310 |
| Well deeper than 44 in | Permanently attached ladder or steps | R310 |
Subsection numbers differ between IRC editions; the values are the same in the 2012–2021 city handouts cited below.
Width and height combinations
| Clear height | Width needed | Area |
|---|---|---|
| 24 in | 34.2 in | 5.70 sq ft |
| 28 in | 29.3 in | 5.70 sq ft |
| 30 in | 27.4 in | 5.70 sq ft |
| 34 in | 24.1 in | 5.70 sq ft |
| 36 in | 22.8 in | 5.70 sq ft |
| 41 in | 20.0 in | 5.70 sq ft |
Widths below 20 in are raised to 20 in. 5.7 sq ft = 820.8 sq in.
How to measure an egress opening
- Open the window as far as it goes in normal use.
- Measure the clear width between the stops or sashes, not the outside of the frame.
- Measure the clear height from the sill to the underside of the head or the raised sash.
- Multiply width × height in inches and divide by 144 for square feet.
- Measure the sill from the finished floor, after any new flooring is in.
Using both minimums together does not work: 20 in × 24 in is only 3.33 sq ft. A 24 in tall opening needs to be about 34.2 in wide, and a 20 in wide opening about 41 in tall, to reach 5.7 sq ft.
Worked example
A casement with a clear opening 24 in wide and 41 in tall gives 984 sq in, or 6.83 sq ft, which passes 5.7. Set in a 40 in sill, the opening's bottom is 4 in under the 44 in limit, so a 2 in subfloor and flooring still leave room. The same opening in a slider would need a frame roughly twice as wide because only one sash moves. Outside, a 40 × 38 in well gives 10.6 sq ft and passes; at 52 in deep it needs a fixed ladder.
Where basement inspections usually fail
- Bedroom windows that are too small or too high, especially sliders and double-hungs measured by the frame instead of the opening.
- Window wells under 36 in deep from the wall, or deeper than 44 in with no ladder.
- Ceilings that dip under 6 ft 4 in at a duct or beam once drywall and flooring go in.
- Stairs with a short top or bottom riser after new flooring changes the floor levels.
- Missing fire blocking at the top of furred walls and around soffits.
Many cities publish one-page handouts on these items, like the St. Paul, Boise and Des Moines sheets cited on this site. Ask for your city's version before you frame.
Before you call for an inspection
- Measure every egress opening with the sash open and write the numbers on the frame.
- Measure ceiling height at the lowest duct or beam after the ceiling material is up.
- Check riser heights top and bottom after flooring.
- Leave insulation and vapor details visible until the insulation inspection if your town does one.
- Have the permit card and approved plans on site.
Common questions
What size does a basement egress window need to be?
At least 5.7 sq ft of net clear opening (5.0 at grade floor), 24 in high and 20 in wide when open, with the sill no more than 44 in above the floor.
How big does an egress window well need to be?
At least 9 sq ft in area, with both the width and the projection from the wall at least 36 in. Wells deeper than 44 in need a fixed ladder or steps.
Related calculators
Sources
- https://www.stpaul.gov/sites/default/files/Media%20Root/Safety%20&%20Inspections/DSI.Bldg_EgressWindow.pdf
- https://bouldercounty.gov/?p=218003
- https://www.lincoln.ne.gov/City/Departments/PDS/Building-Safety/Residential-Charts-and-Diagrams/Typical-Egress-Window-and-Well-Detail