Window quality and placement plays an important role in all homes. Thermal mass is another key element to the functionality of passive solar heating. Thermal mass, normally a floor or a wall, is a term for a material that can capture the sun’s heat cast through windows (usually south-facing) and store it to be released later into the home. Commonly brick, concrete, or tile are used either in flooring or interior walls. The percentage of thermal mass inside south-facing rooms needs to be greater than the area of the windows for that room and should be at least four inches thick but not thicker than six inches. Even a thicker drywall can increase the thermal capacity of regular walls in a home. Thermal mass is most common in direct gain systems, which utilize the living space as the solar energy collector. “South facing glass admits solar energy into the house where it strikes directly… thermal mass materials… [Utilizing] 60 to 75% of the suns energy striking the windows”(Greenbuilder). During the day the thermal mass absorbs the heat to reduce the temperature in the home and when the sun is no longer heating the home, the heat slowly releases into the home from the thermal mass. To be most effective, all thermal masses, including floors, should be left as bare as possible (without carpeting or paint) to best allow transmission of the sun’s heat. Another use for thermal mass is in an indirect gain system where the thermal mass is not a part of the living area. Trombe walls are located just inside south facing windows and use vents at the top and bottom of the surface to use natural convection to transmit heat through the home. Because the Trombe wall is constructed inside the house, it is not a source of heat loss on overcast days and requires little maintenance. Also, since thermal mass is basically any heat storing material, it is relatively the easiest passive solar system to design into a home and it can be added to already existing buildings.