What’s the best way to capture solar heat from south-facing windows?

Cassandra Adams is an architect with more than 22 years of experience on public and private-sector projects. She has taught in the Department of Architecture at UC Berkeley, where her research focused on construction and environmental issues.

Question: We will be building a post-and-beam home in the next 12 months. Our plot has great potential for passive solar design and the house will have considerable window space on the south side, with a three-foot overhang. What is the best way to capture that heat? I need help selecting windows and floors.

– Jeff Kushner, Victor, MT

Answer: For the windows, use double-paned low-E glazing that has a SHGC (Solar Heat Gain Coefficient) greater than 0.70. The low-E glass should be manufactured with a pyrolitic or hard-coat glass-coating process, which allows solar heat gain. (Low-E glass products manufactured with a sputtered or soft-coating process reduce solar heat gain.)

The floor exposed to the sunlight near the window becomes a heat sink—it soaks up heat during the day and releases it slowly into the room at night. Dense materials such as concrete, earth, brick, ceramic tile or stone pavers over a concrete substrate are the best heat sinks. Also, dark colors absorb more heat than light colors. A dark-colored masonry or concrete floor, or a small, dirt-filled indoor garden located just inside the window, will provide excellent heat sinks (don’t forget to insulate under the floor and around the slab perimeter). You could also construct a dark-colored masonry wall inside the window (with small “view windows” pierced through it) to capture the heat.

Water is another excellent heat sink. Back in the 1970s some people placed black-painted drums of water inside their south-facing windows. This worked well, but the aesthetics are not for everyone. I do not recommend placing an open pool inside near the window. The humidity that it brings into the house, and the likely algae growth, could cause problems.

No matter what you use for a heat sink, it is important to reduce heat loss at night. Ideally, you would use an insulative covering on the windows that provides a good seal. Look for insulating blinds or shutters that fit tightly into or around the window opening. Floor-to-ceiling lined drapes are a less-effective alternative, but are better than bare windows.

For more information:

Read GreenHomeGuide's Ask A Pro article "Is Concrete Flooring Eco-Friendly?" for more information about concrete floors.

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