Natural light and ventilation are measured against floor area: the US IRC asks for glazing of at least 8% of the floor and openable area of at least 4%. Two 1.2 × 1.2 m windows in a 20 m² room are 14.4% glazing: 7.2% openable if half opens. Each window lintel is the opening plus 150 mm bearing each end (1.5 m).
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Glazing as % of floor area
14.4%, meets 8% (IRC R303.1)
Openable area as % of floor
7.2 %, meets 4%
Total opening area
4.77 m²
Window lintels
2 × 1.50 m
Door lintels
1 × 1.20 m
Frame perimeter (sealant / trim)
14.70 m
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Glazing = 2 × 1.20 × 1.20 = 2.88 m² = 14.4% of 20.00 m²
Openable = 2.88 × 50% = 1.44 m² = 7.2% of the floor
Lintel = clear width + 2 × 150 mm bearing
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Formulas
Glazing % = window area ÷ floor area × 100Openable % = window area × openable share ÷ floor area × 100Lintel length = clear width + 2 × bearing
Worked Example
20 m² room, one 0.9 × 2.1 m door, two 1.2 × 1.2 m windows, half of each opens:
Glazing = 2.88 m² = 14.4% of the floor (≥ 8%)
Openable = 1.44 m² = 7.2% (≥ 4%)
Lintels: windows 2 × 1.5 m, door 1 × 1.2 m
Lintel Lengths (150 mm bearings)
Opening
Lintel
0.9 m door
1.2 m
1.2 m window
1.5 m
1.8 m window
2.1 m
2.4 m opening
2.7 m (check bearing)
Frequently Asked Questions
How much window area does a room need?
Under the US IRC, glazing of at least 8% of the floor area for natural light, with at least 4% openable for ventilation (unless artificial light and mechanical ventilation are provided). Other codes use similar ratios.
How long should a lintel be?
The clear opening plus a bearing at each end: commonly at least 150 mm (6 in) on masonry, more for wide openings or heavy loads. Follow the lintel maker's tables.
What is the frame perimeter used for?
Ordering sealant, trims, flashing tape or architraves around doors and windows.
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