How the Sun's Path Can Inform Design

Sun shining into a building provides free heat and natural light. It can also create glare and, when the heat isn?t needed, discomfort and added demand for cooling. Using sunlight when it?s needed and deflecting its power when it?s not are two of the most important tasks in building design.

The sun rises in the east and sets in the west. Because the earth rotates on an axis that is tilted, however, the sun doesn?t stay exactly in the east or west. In summer, it rises north of east, travels high across the southern sky, and sets north of west. In winter, it rises south of east, travels low across the southern sky, and sets south of west. How high it gets in the summer and how low in the winter depend on how far you are from the Equator. (If you?re reading this in the Southern Hemisphere, north and south are reversed.)

Passive-solar design suggests putting most of the glass on the south fa?ade and shading it with overhangs that keep the sun out in summer but allow it to penetrate and provide free heat in winter. The need for cooling tends to dominate the energy profile of large buildings everywhere and almost all buildings in the southern U.S., however, even in winter, so in these places shading the glass year-round is beneficial, and putting more of it on the north is an option. In almost every case glazing on the east and west fa?ades is problematic, because it?s hard to control glare and overheating from the low morning and afternoon sun. That?s why energy-efficient buildings are ideally elongated on an east-west axis, with longer fa?ades facing south and north.

Artists and others who are sensitive to light quality tend to prefer indirect light from northern windows. Modern energy-efficient glazing technologies make it possible to provide this light without compromising thermal performance as much as was once the case. It is still advisable not to overglaze on the north, however, and to plan for some direct sunlight entering through north-facing windows during the summer in early mornings and late evenings.

To maximize the benefit of the sun?s rays while mitigating problems, some designers specify different glazing for different orientations. Low-emissivity (low-e) coatings can be adjusted to create glazing systems with different levels of thermal transmittance (U-factor), solar transmittance (solar heat gain coefficient, or SHGC), and visible light transmittance (Tvis). On the north, glazing should have a low U-factor to minimize heat loss and a high visible light transmittance. On the south, if solar gain is desirable in winter, glass should have a high SHGC, while the desired U-factor and Tvis depend on how the spaces will be used. And on the east and west the goal is to minimize solar gain, so a low SHGC is the priority.

For more information:

?Getting to Know a Place: Site Evaluation as a Starting Point for Green Design,? EBN Vol. 7, No. 3

Heating, Cooling, Lighting: Design Methods for Architects, 2nd Edition, by Norbert Lechner (Wiley: 2000)

Sun, Wind, and Light: Architectural Design Strategies, 2nd Edition, by G. Z. Brown & Mark DeKay (Wiley: 2000)