Passive Solar Design

south light

Passive Solar Design takes advantage of site, climate, and the energy of the sun to provide thermal comfort through heating and cooling. The shape and orientation of buildings, as well as its details and systems are all keys to optimizing passive performance. The strategies vary depending on the specifics of the site, meaning that passive solar buildings are integrated into and appropriate for their locations.

The site’s location and microclimate impact the building’s form and orientation. Important factors include solar access, harsh wind, weather or fog, relationships to slopes or existing vegetation, and diurnal temperature swings. Generally buildings should have the majority of their glazing facing within 30° of due south, and we find that in most instances the optimal orientation is roughly 17.5° east of due south. This creates a building with more morning gain and less in the afternoons.

In addition to south-facing glazing, passively heated buildings typically feature high insulation levels and tight construction, shading elements at windows, and thermal mass. Properly sized shading over windows and doors helps to control unwanted solar gain during the hot months. While it is usually best to limit east, west and north facing glazing, modern glazing systems minimize high energy loss or gain penalties in these locations. Thermal mass is defined as a heavy, dense material that can absorb the heat of the sun, and then radiate it back to the space during the evening. Many elements in a building can act as thermal mass, including slab-on-grade floors, thick soil or plaster wall finishes, thick or double gypsum board, and masonry elements such as fireplaces, masonry heaters, or planters.

Passive cooling similarly features proper shading and thermal mass within the building envelope, as well as operable windows placed to take advantage of natural ventilation. Here in California where nighttime temperatures are lower, night flushing via fans or natural convection (warm air rising) can be used to remove heat stored in the thermal mass from the building. Windows or fan openings are then closed in the morning and the mass helps keep the building cool and comfortable.

Other passive design features that can reduce the active energy needs of a building include day-lighting, air-to-air heat exchange, radiant barriers, and ventilated roof systems. Employing these passive strategies can reduce or eliminate the mechanical systems, saving both direct costs and long-term energy costs.

In most climates, buildings can achieve passive comfort for a majority of the time, with needs during extreme periods of weather being met through supplemental systems, ideally from renewable energy resources.  Integrating these passive strategies will continue to be important in the future, as energy codes become more restrictive, and we strive to reach a carbon-neutral built environment.