The Sun Path
Knowing the sun’s position in the sky at various times of the day and year is the first step in controlling it. In the winter, 90% of the sun’s energy arrives between the hours of 9 am and 3 pm. In the summer this is expanded.2 This time frame is also known as the solar window, and controlling heat/light in this window is the key to an efficient building.
Depending upon the current season, you will either want to accept the heat in the design, or avoid it. At the summer solstice the sun angles are at their highest and the winter solstice brings the lowest angles. Typically, a building is easily shaded from high summer sun angles to avoid heat gain, while still allowing for winter sun penetration. Dynamic shading devices (including vegetation) that respond to seasonal changes have higher performance. In many climate zones, the March and September equinoxes have the same sun angle, but require different responses to heat.
Programs that can help determine local solar conditions include:
- Ecotect– Auotdesk Software, contact IS for licensing information.
Elongating a building along the E-W axis maximizes its exposure to the sun, which allows it to continuously harness thermal gain and daylight or consistently avoid it. Light and heat are hardest to control on the eastern and western facades, where sun angles are low. Minimizing their surface area and appropriately shading them is vital. To maximize heat gain, windows on the solar side should be within 5-15° of the solar direction (South in the Northern Hemisphere or North in the Southern hemisphere). Study the ratio of building width to ceiling height to optimize the amount of light. The ideal ratio is 5 to 6 (building width) to 1 (ceiling height).
Although generalized responses tailored to the climate are often effective, site specific responses are also necessary. Knowing how surrounding obstructions whether permanent, seasonal, or planned in the future affects the solar access on your site will help tailor a site specific response to light/heat.
After the building is oriented based on solar access, controlling the sun is the next step. Controlling heat gain is best done before it hits the building or enters into the building envelope. Up to 12% of solar heat will transfer through opaque walls, but up to 85% will transfer through glazing. Five strategies for controlling solar gains ranging from most effective to least are:3
- Landscaping and orientation
- Movable shading devices
- Fixed overhangs and fins
- Light shelves
- Glass type
- Internal shading