Why codes require windows
Windows bring daylight and fresh air into habitable rooms. The International Residential Code (IRC), adopted with local amendments in most US states, sets minimums for both in section R303.1:
- Natural light: glazing area of at least 8% of the floor area of each habitable room.
- Natural ventilation: openable area of at least 4% of the floor area.
Habitable rooms are bedrooms, living rooms, dining rooms and kitchens; bathrooms, closets and hallways are not.
Worked example
A bedroom measures 14 × 12 ft and has two windows with 36 × 48 in of glass each. They are double-hung windows that open about 50%.
- Floor area: 14 × 12 = 168 sq ft.
- Glazing: 2 × 36 × 48 ÷ 144 = 24 sq ft, or 14.3% of the floor area.
- Minimum glazing: 168 × 8% = 13.4 sq ft, so the room passes.
- Openable area: 24 × 50% = 12 sq ft, above the 4% minimum of 6.7 sq ft.
Exceptions
The IRC allows less glazing if the room has artificial lighting providing at least 6 footcandles of average illumination, and less openable area if a whole-house mechanical ventilation system is installed. Rooms can also borrow light from an adjoining room through a large opening.
Egress windows in bedrooms
Every sleeping room, and basements with habitable space, need an emergency escape opening. Under the IRC:
| Requirement | Minimum |
|---|---|
| Net clear opening | 5.7 sq ft (5.0 sq ft at grade level) |
| Clear opening height | 24 in |
| Clear opening width | 20 in |
| Sill height above the floor | 44 in maximum |
A window can meet the 8% light rule and still fail egress if its sash does not open wide enough, so check both.
Daylight beyond the minimum
Designers often aim for glazing of 15 to 20% of the floor area for bright rooms. Orientation matters as much as size: south-facing windows bring winter sun, while large west-facing windows can overheat rooms in summer. Tall windows send light deeper into a room than wide, low ones.
Energy considerations
Windows lose more heat than insulated walls, so more glass means higher heating and cooling loads. ENERGY STAR windows rated for your climate zone, with a low U-factor and an appropriate solar heat gain coefficient, limit the penalty. Check with your local building department, since adopted code editions and amendments vary.