ENERGY-EFFICIENT WINDOWS
Choose carefully in north-, south-, west-, and east-facing window sizing.

Glass conducts an enormous amount of heat to the outside. Windows can lose up to 50 percent of the heat at night.

Good window covers with insulated drapes or shades can easily add another R-3 to R-4 to a window. Also, install insulated window shades that seal around the edge of the window via magnetic strips to help block the flow of cold air from windows.

Exterior shutters can reduce heat loss, but must be opened and closed each day. To avoid this problem, consider rigid foam thermo-shutters on the inside, made from plywood glued to rigid foam, a lot more convenient than exterior shutters.

East & West Window Shading

Solar glazing in Direct-Gain Systems should range between 7 and 12 percent of the total floor space. More in "Glass to Mass Ratio"

In warmer climates, reduced south-facing glazing cuts down on solar gain while additional north-facing windows release heat. The net result, if properly done, is a cool interior. The greater the need for heat, the greater the solar glazing requirements….but it's not that easy!
Excessive glazing can result in overheating, while at night, too much glazing can lead to much heat loss and uncomfortably cold temperatures. Over-glazing can also lead to overheating in the summer, especially if the house is not properly aligned to the sun.

As a general rule, solar glazing should fall somewhere between the 7 - 12 % range of the glass-to-mass ratio. In a warmer climate, you may stay between 8 - 10 % glazing. In a colder climate, 12 % glazing might be needed.

SUMMERTIME COOLING
Window shading is a necessity for most direct-gain passive solar homes - particularly in the late summer and early fall. The warmer the climate, the more important shading becomes. In order to keep the hot sun off the window, and to avoid ultimately heating up the interior, external shades are more effective than internal shades.

Glaze for Ventilation Some of the solar glazing needs to be open-able for natural ventilation. Open-able windows should be on the north, east, and west sides of the house for cross-ventilation.

Create Sun-Free Zones Although newer glazings that limit UV penetration help reduce this problem, ultraviolet light can damage carpets, window coverings, and furniture. Also, be aware of where you plan to watch TV or use the computer regarding glare.

Special Requirements for Protecting East and West-Facing Windows

Due to the low sun angle, overhangs are ineffective on east- and west-facing windows.

Trees planted on the east and west sides of a home block the early and late sun which may contribute to summertime overheating. Vine-covered arbors and trellises against the side of a house also provide shade for east- and west-facing walls and windows.

Canvas awnings, roll-down blinds, and vertical louvers all prevent sunlight from striking the window. External shades are more effective than internal shades but may require more maintenance and repair.

Size and locate windows correctly in the first place to prevent overheating in summer. Use shades to keep the summer sun out.

Unobstructed access to the sun for about 6 hours is essential for passive solar heating. Trees on the east and west shade a home during the summer mornings and evenings.