Let the winter sun in, keep the summer sun out
The sun climbs high in the summer sky and stays low in winter. A well-sized roof overhang, eave or awning above a south-facing window uses that difference: it casts a shadow over the glass in summer, when sunlight would overheat the room, and lets the low winter sun shine in to warm it for free. This is one of the oldest and cheapest techniques of passive solar design.
The sun’s height at noon
For a location at latitude φ in the Northern Hemisphere, the sun’s altitude at solar noon is:
- Summer solstice (around June 21): 90° − φ + 23.44°.
- Equinoxes (around March 20 and September 22): 90° − φ.
- Winter solstice (around December 21): 90° − φ − 23.44°.
In the Southern Hemisphere, the same formulas apply to north-facing windows, with the seasons reversed.
Sizing the overhang
To fully shade the window at noon on the summer solstice, the overhang must project far enough that its shadow reaches the bottom of the window:
Projection = (gap + window height) ÷ tan(summer sun altitude)
where the gap is the distance from the top of the window to the underside of the overhang. In winter, the shadow falls a distance projection × tan(winter altitude) below the overhang; if that is less than the gap, the whole window receives sun.
Worked example
A south-facing window 60 inches tall, with the overhang 12 inches above it, at latitude 40°N, such as Denver, Philadelphia or Columbus:
- Sun at noon: 73.4° in June, 50° at the equinoxes, 26.6° in December.
- Projection for full summer shade: 72 ÷ tan(73.4°) = 21.4 inches.
- Winter: the shadow reaches only 10.7 inches below the overhang, above the window: 0% shaded, full winter sun.
- Equinoxes: 23% of the window shaded.
- The deepest overhang that still lets full winter sun in is 24 inches.
By latitude
For the same window and gap:
| Latitude | Summer noon sun | Overhang for full summer shade | Window shaded at equinox |
|---|---|---|---|
| 30° (Houston) | 83.4° | 8.3 in | 4% |
| 35° (Albuquerque) | 78.4° | 14.7 in | 15% |
| 40° (Denver) | 73.4° | 21.4 in | 23% |
| 45° (Minneapolis) | 68.4° | 28.4 in | 27% |
| 50° | 63.4° | 36.0 in | 30% |
Practical points
- The method assumes the window faces true south; at 20 to 30° off south it still works reasonably well, but not for east or west windows.
- Solar noon is not 12:00 on the clock: it shifts with your position in the time zone and with daylight saving time. Before and after noon the sun is lower, so shading is partial.
- The hottest weeks of summer come after the solstice, when the sun is lower. Many designers add a few inches to the projection or use adjustable awnings, deciduous trees or vines for late summer.
- The overhang should extend beyond the sides of the window by about the same distance as its projection, to shade the glass in the late morning and early afternoon.