overhangs and shade

Overhangs are very effective in many climates: properly dimensioned, they can block sunlight during the summer months, when it is higher in the sky.

Working principle

The working principle behind overhangs is very simple: the higher and more vertical the sun is (which happens in the summer), the longer the shadow generated by the overhang along the face of the wall; the lower and less vertical the sun is (which happens in the winter), the shorter the shadow and the impact of the overhang…

Overhagns principle technique

Overhang effectiveness

Overhangs are most effective at mid-day for the sunny facing walls (south walls, in the northern hemisphere, and north walls in the southern hemisphere). If the building wall is more than 30° off true south/north, the effectiveness of the overhang will decrease proportionately.

Image credit: Infinite Power Organization

Depth

The depth of the overhang is an important design element, since it determines the amount of shade, as showed in the images below:

Overhang, shade and depth

Image credit: Austin Energy

Sizing the overhang

There isn’t an universal way of sizing overhangs. Different latitudes (and microclimates) may demand different designs.

However, the guidelines presented below may be useful. They involve three types of climates, and are a EERE suggestion for Northern hemisphere countries:

- Cold climates (with more than 6,000 Heating Degree Days, base 65°F/18°C): locate the overhang shadow line at mid-window; use the June 21 sun angle.

- Temperate climates (below 6,000 Heating Degree Days, base 65°F / 18°C, and below 2,600 Cooling Degree Days, base 75°F / 22°C): locate the shadow line at the window sill; use the June 21 sun angle.

- Hot climates (above 2,600 Cooling Degree Days, base 75°F / 22°C): locate the shadow line at the window sill; use the March 21 sun angle.

To know the Heating Degrees Days and the Cooling Degree Days in your region, look for information in your regional weather service.