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Passive Solar
House & Overhangs
Overhangs are very effective solar passive tools in many
climates: properly dimensioned they block the sun during the
summer months, when it is higher in the sky.
The concept behind overhangs are 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…
Note:
Overhangs only affects the amount of direct solar radiation that
strikes a wall, roof, door or other surface. They don’t affect
significantly other possible gains…
Image credit:
Infinite Power
Organization
Overhangs effectiveness
Overhangs are most effective at midday and for sunnier facing
walls, windows and doors (that is, 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 decreases a lot.
Overhangs with different depth produce different amounts of
shade, as showed by the images below:
Image source credits:
Austin Energy
Types of Overhangs
There are many types of overhangs (solid, vegetation-supporting,
louvered, fixed, operable, removable…) or devices and tools
equivalent to overhangs in its shadow-impact: shutters, eaves,
trellises, light shelves, awnings...
Sizing the overhang
There isn’t an universal way of sizing overhangs. Different
microclimate conditions and latitudes require different designs.
The general guidelines presented below may be useful, but they
are still general. They include three types of climates, and are
listed for solar noon (when the sun reaches its maximum height),
and are suggested by EERE, for Northern hemisphere countries:
- Cold climates (with more than 6,000 Heating Degree
Days, base 65°F/18°C): locate overhang shadow line at
mid-window using 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 shadow line at window sill using the June 21 sun
angle.
- Hot climates (above 2,600 Cooling Degree Days, base
75°F / 22°C): locate shadow line at window sill using the March
21 sun angle.
To know Heating Degrees Days and Cooling Degree Days for
particular climates, look for information in your regional
weather services.
See also:
Basics on
solar systems
Orientation and shape of the house
Natural ventilation
Energy efficient
windows, doors and skylights
Solar protection
Awnings and screens
Pergolas and House Shading
House Shading, Trees and
Shrubs
Shading and climate zones
Windbreaks for your
house
Where to plant trees and
shrubs
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