Direct gain is solar radiation that directly passes through the home’s windows and is traped in the living space. Direct gain uses classic passive solar design strategy – the sunlight falls directly into the space and is absorbed by an abundance of thermal mass materials.
Indirect gain collects, stores, and distributes solar radiation using some thermal storage material (e.g., Trombé wall or a thermal storage wall). Conduction, radiation, or convection then transfers the energy indoors. Sunlight is absorbed by the wall, which heats up slowly during the day. Then as it cools gradually during the night, it releases its stored heat over a relatively long period of time indirectly into the space.
Isolated gain (e.g., sunspace) collect solar radiation in an area that can be selectively closed off or opened to the rest of the building. That heat than can be distributed into the living area in a variety of ways. The sunspace has the same characteristics as a direct-gain system – extensive south-facing glazing and thermal mass, and it should be well constructed, with low infiltration and high insulation levels.
In the case of passive solar space heating the whole house operates as a solar collector (passive solar home). A passive solar home is designed to let in as much sunlight as possible. It is like a big solar collector.
Passive solar heating features can reduce heating bills by almost 50 percent and it requires little or no investment of external equipment. Building a passive solar home may even cost the same as building a conventional home, especially if you’re working with a builder who is familiar with the processes of passive solar heating systems.