Isolated solar gain is one of three types of passive solar heating. The other two, direct and indirect gain, take place directly within or in close proximity to the space being heated. Isolated solar gain happens when solar heat is collected in one area and used in another. It is considered passive because it can be accomplished without the use of mechanical devices, such as pumps or fans.
Sometimes, mechanical fans are used to transfer the heat from an isolated space to the rest of the house. This would translate into an active system. Most of the time, though, heat moves from a sunroom or sunspace through conduction.
The most common manifestation of isolated solar gain consists of sunspaces. Sunspaces, such as sunrooms, utilize thermal mass, such as a masonry wall, a masonry floor, or water containers, to absorb solar heat. Sunspaces are usually located on the south side of a home. Low-emissivity windows and vertical windows with no overhead glazing help regulate room temperature. These well-insulated, self-contained spaces heat to high temperatures during the day. At night, heat can be distributed to the home through ceiling and floor vents, windows, doors, or via conduction through a shared thermal wall.
Greenhouses are often placed in the isolated solar gain equation. However, greenhouses, designed to grow plants, are not necessarily comfortable or efficient living spaces. Still, heated air from a greenhouse can feasibly flow into the rest of the house.
For a smaller variation, try a solar closet, an insulated cube filled with water, with a passive solar air heater attached to one side.