Understanding Passive Solar Heating and Cooling

Image Gallery Passive solar design has come a long way since the ’70s. Modern solar houses are attractive as well as energy efficient. WHY NOT GO SOLAR/MOTHER EARTH NEWS CU

Being passive isn’t always a bad thing, especially when it’s passive solar heating and cooling. The idea behind passive solar is to design buildings that take advantage of natural heat from the sun in winter; and shade and wind and in the summer. Although the concept has been used in many cultures for centuries, passive solar design principles recently have been refined a great deal, even since the 1970s.

Passive solar, on it’s most basic level, works like this: Rays from the sun enter a building through windows, heat the air and are absorbed by floors, walls, furniture, etc. Some materials, such as stone, brick and plaster, more effectively absorb the heat. As the air cools at night, the absorbed heat releases into the building and maintains a comfortable temperature.

How Passive Solar Works

Kelly Lerner, architect and author of Natural Remodeling for the Not-So-Green House, says there are four things to consider when designing or remodeling a building to most effectively utilize solar energy: south-facing glass (glazing), shading, insulation and thermal mass.

“In most North American climates, the right amount of south-facing glass is 7 to 12 percent of the floor area of the building — a lot less than you might think if you’ve seen solar home designs from the ’70s,” she says. “In the ’70s, we really used too much glass with too little thermal mass, so instead of passive solar heat, we had something more like ‘passive-aggressive’ solar heat, creating buildings that were too hot during the day and too cool at night. Too much glazing can be a detriment on cold winter nights, allowing heat to escape.”

Longtime passive solar expert Ron Judkoff, director of the Buildings and Thermal Systems Center at the National Renewable Energy Lab, says, “It’s the thermal storage part that’s kept passive solar from becoming more widespread. It’s also the biggest additional expense: adding brick or another internal thermal mass instead of drywall.”

Judkoff points out that production builders are accustomed to using lightweight materials not suited to storing (and releasing) heat gathered through south-facing glass. “But you need adequate thermal storage mass for a passive solar thermal building to work as it’s supposed to,” he says.

Insulation is another important aspect of passive solar design. “There’s no point in getting heat from the sun into the house if it simply escapes through air or the walls,” Lerner says.

A well-designed passive solar building is warm in winter and cool in summer. The design principles, though somewhat variable with location, can be utilized to reduce cooling costs — even in warm areas of the South. “Good shading is essential for the summer months to prevent overheating,” Lerner says. “On summer solstice, south-facing windows should be completely shaded from 9 a.m. to 3 p.m. Check Sustainable by Design for sun angles at your latitude and draw a diagram to calculate shading for your home.”