A passive solar home requires five elements to take full advantage of the sun’s free heat: apertures to let in the sun’s warming rays; a means of preventing too much solar gain in the summer; an absorber surface that minimizes reflection; thermal mass to store the heat until it’s needed; and a distribution system to move the heat to where it’s required.
For a truly passive house, each of these elements should operate without mechanical power or occupant intervention. As examples, summer solar gain is handled by properly designed overhangs. The distribution system would be natural convection within an open floor plan, with storage and release handled by concrete—a massive and dense material with high specific heat (heat storage capacity per unit volume) and moderate thermal diffusivity (the propensity of heat to dissipate to all areas of the mass).
Perhaps the least-understood elements of passive solar design, and the ones that plagued the early passive solar pioneers in the 1970s, are the ratios of south glass area to floor area and of south glass area to thermal mass. Without proper balance—and an appropriate absorber and mass storage—an otherwise well-designed house can be unlivable.
Too much glass can mean:
But without sufficient thermal mass, even the proper glass-to-floor ratio can lead to daily or even hourly temperature swings and heat stratification that can make a home uncomfortable. The south-facing glazing design standard for today’s passive solar homes is a window area between 7% and 12% of floor area. (For example, a 1,000-square-foot space would have between 70 and 120 square feet of south glazing.) That ratio can apply to the entire house if all stories are to be passive solar designed, or just to the primary living floor. It’s often more appropriate to design a bedroom floor to be sun-tempered, with south glazing of 5% to 7% of the floor area, which doesn’t require any additional thermal mass beyond normal building materials and has the benefit of providing more privacy. Beyond 12%, we enter the active solar range in which direct-gain thermal mass is not sufficient to maintain a uniform and comfortable indoor temperature without fans or pumps to move the heat to remote storage and retrieve it on demand.
The goal in designing a passive solar home’s thermal mass is to be able to store midday solar heat until the early evening, when it will passively return to the living space. Thermal mass operates like a flywheel that dampens any sudden changes in acceleration or, in this case, changes in insolation—the amount of solar energy entering through the apertures—which would otherwise raise indoor air temperature.