Indirect solar gain is one of three methods of passive solar heating, in addition to isolated and direct gain. When solar heat is collected in one area and transferred to another through the process of convection, this is the indirect method.

Here's a rundown of how it works. As with most passive solar heating systems that don't require the use of mechanical pumps or fans, heat is invited indoors through a series of south-facing windows. It is then absorbed through building materials with thermal mass, or the natural capacity to absorb heat. These materials include masonry walls or floors. When placed in a container, water can be even more effective than masonry or concrete in that it exchanges heat more quickly. Water walls and roof ponds are examples of how water containers can be used for indirect solar gain. These are described below.