Direct-Gain Passive Solar

Direct-gain passive-solar design was used extensively at the Cob Hill CoHousing project in Hartland, Vermont. A majority of the windows face south, with overhangs and window blinds that help block unwanted sun in the summer. Photo: Alex Wilson. Click on image to enlarge.

Over the past two weeks I've written about two relatively obscure passive solar heating strategies: isolated gain using sunspaces; and indirect gain using a Trombe walls. This week I'll cover a far more common and cost-effective approach: direct-gain.

Direct-gain passive solar systems rely on south-facing windows to bring solar energy directly into a house. That sunlight is absorbed by materials in the house (the floor, walls, furniture, etc.), which warm up, store some of that heat, and re-radiate it back into the room, warming the space.  

One way of looking at this is that the house itself serves as the solar collector and heat-storage system. There are no fans or pumps to move heated air or water around. The systems are silent, trouble-free, and easy to maintain--by washing windows and, in some cases, opening and closing window blinds to adjust incoming solar gain. The thermal storage function is most effective with high-mass materials, such as tile or concrete floors, fireplaces with brick or stone facings, and tinted plaster walls. These materials keep the space from getting too hot during the day, and they continue radiating warmth into the living space in the evening.  

The key to success with direct-gain passive solar heating is to provide the right amount of south-facing glass area and to couple that glass with adequate thermal mass. If too much glazing is installed, the space will overheat on sunny days. The better insulated the house, the less glazing can be installed before overheating becomes a concern.

Back when I was involved in passive solar energy in the late-1970s in New Mexico, there were lots of examples of houses being built with the best of intentions, but with way too much south-facing glass. They were like greenhouses on sunny days and, because the glazing was only double-glazed without low-emissivity (low-e) coatings, there was a lot of heat loss through that glass at night. The houses greatly fluctuated in temperature.

Another house at Cob Hill CoHousing showing south-facing windows with overhangs and window shades. Photo: Alex Wilson. Click on image to enlarge.

As we've learned more about the energy dynamics of homes, we've learned that it makes sense to use higher levels of insulation with reduced glazing areas. Better-insulated houses don't require as much solar gain to provide a significant fraction of the heating needs, and the careful balancing of insulation, glazing, and thermal mass can avoid those temperature fluctuations that were such a problem in the past.

To do this requires advanced energy modeling software. Fortunately, such programs are readily available today, including Energy 10, Energy Plus, and REM Design. These programs account for insulation levels, window area, glazing type, and thermal mass. Don't consider designing a direct-gain, passive-solar house today without using such a modeling program; make sure your designer has access to such capabilities. This is key to success.

In addition to this Energy Solutions blog, Alex contributes to the weekly blog BuildingGreen's Product of the Week, which profiles an interesting new green building product each week. You can sign up to receive notices of these blogs by e-mail--enter your e-mail address in the upper right corner of any blog page.

Alex is founder of BuildingGreen, Inc. and executive editor of Environmental Building News. To keep up with his latest articles and musings, you can sign up for his Twitter feed.