On a day like this last Sunday, a bitter cold day with strong sunshine, did you happen to notice steam rising off of any south-facing roofs? This perplexing phenomenon demonstrates how even when the temperatures drop well below freezing, solar energy is powerful enough to not just melt, but evaporate snow.
Liquid solar and photovoltaic arrays are on the rise around Bozeman, modules in which solar irradiance is transformed into energy for water heating and electricity. Of course, these examples harnessing solar power benefit from the same physical conditions as the rooftop evaporation.
What may not be as apparent as rooftop PV modules, however, are passive solar homes, which incorporate this same evaporative phenomenon to heat the home itself. Via a less mechanized approach, the Sun is treated as free-heating source that can be utilized in Montana, year-round, for heating.
In a nutshell, here is how a passive solar home works: The Sun’s angle in the sky is lower during the winter than in the summer, allowing more sunlight to enter homes—particularly through south-facing windows. The Sun’s energy, dubbed “incidental solar radiation” by the experts, provides about 1100 BTUs (British Thermal Unit, a measure of energy) per square foot of south-facing glass on an average Montana winter day.
In order to harvest the solar radiation, the energy can be captured and stored in a concrete slab floor. Over the course of a day, the slab is gradually heated by the Sun, then throughout the cold nighttime hours, the thermal mass of the concrete floor slowly releases this heat, warming the interior space above it. This process is passive; it requires no plumbing or wiring, just good design.
The ratio of window area to floor volume as well as quality window fabrication are important for controlling how much heat is stored. Typical homes of today are built with low emissivity windows that reduce solar radiant gain in a home. In a passive solar home, higher emissivity windows are used on south-facing areas and low emissivity windows on the home’s other exterior walls. As alluded to above, it’s critical to employ well-insulated windows to minimize heat loss.
In the summertime, the Sun is higher in the sky. In a passive solar home, roof overhangs are designed to the correct depth on the south-facing façades (the dimension may vary with latitude and specific climate), preventing sunlight from entering the home on those hot Montana summer days where heat retention is undesired. Simple shades can also be designed into the window systems to help reduce glare and unwanted heat gain.
To further prevent overheating, the placement of operable awning windows is configured into the home at high and low wall apertures to increase ventilation and airflow, respectively. High windows release the heat and low windows take in cool, fresh air. Just as airflow causes evaporative cooling on our skin, resulting in a cooling sensation, this passive cooling strategy reduces the need to run electric air conditioners and fans in the summer, an important factor to consider as energy costs are expected to increase dramatically within the lifetime of structures built today.
Unfortunately, passive solar homes have gotten a bad rap due to poor application of the design principles. However, passive solar design has benefitted through further study and experience––even past mistakes—and designers today are applying the lessons learned far more effectively. The biggest mistake has been the harvest of the Sun’s energy on not only the building’s southern exposure, but the eastern and western exposures as well. Due to low sun angles on the east and west sides, particularly in summer, roof overhangs cannot cut off summer solar radiant gain, causing the home to overheat. In our alpine environment, an array of west-facing glass can broil a room, even on a cool fall day.
Another common mistake with very tight, super-insulated passive solar homes is improper air exchange. We know now that designing an air exchange system into a home is critical in order to introduce fresh air and prevent moisture build-up during the winter when opening windows would be counterintuitive.
Living in a passive solar home is a unique and uplifting experience. With ample sunlight entering the home, the need to run energy consumptive lights during the day is eliminated. Residents report fewer symptoms of Seasonal Affective Disorder (otherwise known as the "winter blues”) than those who live in darker homes with fewer windows. Passive solar homes tend to favor open plans and spaces, and the occupants often report feeling less claustrophobic or compartmentalized. The views of the outdoors can expand a person’s seasonal awareness, too, by connecting the inhabitants to the beauty of the natural environment.
According to the Sonoran Institute, by 2026 Gallatin County will have added an additional 26,000 homes. As traditional fuel sources become more expensive and we seek a community response to the challenges of waste, we hope to see many new, passive solar homes intelligently harvesting the Sun’s energy. -TBM
Mark Pelletier and Emily Varmecky are co-owners of Greenovision Home Design, where we custom design beautiful, energy-efficient homes that stand apart from your neighbors. We believe that cookie cutters are for making cookies, not for home design!