Passive solar design principles have been recognized for decades and are on display in Frank Lloyd Wright’s Solar Hemicycle designs, to cite one high-profile example.
Once lauded for providing “free heat,” passive solar design principles are now recognized as being less effective at maintaining indoor comfort than superinsulated buildings.
As a new generation of do-it-yourself builders got busy experimenting in the 1970s, passive solar design ideas proliferated. Builders claimed that attached sunspaces, solar collectors, trombe walls, earth ships, and many more ideas could keep a house comfortable with minimal or zero use of fossil fuels.
Superinsulated designs, likewise, saw intense development in the 1970s, and have come to the forefront of green building thanks to the Passive House standard. This demanding approach focuses on improving air-tightness and insulation, decreasing thermal bridging, and setting rigorous levels for energy use.
For a time, fans of both approaches debated their merits, but it’s clear that the superinsulated approach has won the battle. Hefty amounts of insulation coupled with rigorous air sealing result in a house that uses less energy and remains more comfortable than passive solar houses.
Martin Holladay, writing for Green Building Advisor, examined this topic and admitted that, though he sided with the superinsulation crowd, he still hung on to some passive solar ideas. Deliberate solar orientation and proper roof overhangs, for example, could yield energy savings if properly designed, he reasoned.
“Lately, however, I’ve begun to wonder whether there is any technical justification for these recommendations,” he wrote. “Do these design principles result in energy savings? Or am I just dragging around the stubborn legacy of my hippie past?”
Passive solar design must be optimized for both cold and hot climates. In hot climates, shading of all windows is important, and builders should skip the sub-slab insulation. Reflective roofing is preferred, and roofs should be insulated to R30. In cold climates, windows should be arranged for maximum solar gain, while roof overhangs should be designed to let in the sun during the cold months, and block the sun in the warm months. Orienting the house along an east-west axis allows for optimal window placement in both cases. Thermal mass is required to maintain a more even internal temperature in both hot and cold climates.
Many of these ideas have a certain instinctive appeal, but according to Holladay, their performance is lacking.
“For one thing, passive solar buildings never worked all that well,” he wrote. “Even back in the 1970s, they were cold on winter mornings and hot on sunny afternoons. But most solar enthusiasts were so excited by the idea of ‘free heat’ that we accepted uncomfortable conditions as a necessary part of the brave new solar future we were all busy creating.”
Windows, in particular, are not very energy efficient. More technologically advanced windows help, but they’re expensive.
“At night and on cloudy days, large expanses of south-facing glass lose significantly more heat than an insulated wall,” Holladay wrote.
A better approach would be to place windows where they’re most aesthetically pleasing, and invest in a photovoltaic array for on-site energy harvesting.
Holladay refers to cold-climate buildings, but hot-climate buildings are also better-served by the superinsulation approach. A superinsulated and airtight home uses less energy whether it’s in a hot or cold climate, and maintains a comfortable temperature with less variation than a passive solar home.
In addition, passive solar design requires a specific orientation to the sun, which is not always possible in an urban setting. Plus, in the context of attached dwellings, the superinsulated approach is much more feasible than the passive solar approach, especially in an urban neighbourhood.
That said, some passive solar design features may be desirable for other reasons. Windows are necessary in superinsulated homes just as in other homes, so designers have the freedom to place them for optimal views, daylighting, and breezes. Generous overhangs provide shading for windows and a feeling of enclosure, and an east/west orientation is friendly for photovoltaic installations.