Because of masonry's high thermal mass, it is an excellent material for passive solar designed buildings that capture and use, rather than fight, the sun's heat. Thermal mass is the property of heavy construction materials, such as brick and block, that allows them to absorb and reradiate the sun's solar energy. Although most building materials absorb some heat, masonry's higher density and greater mass make it absorb heat more slowly and retain it longer. Passive solar buildings reduce the amount of electricity needed by using mass to store heat and natural radiation, convection, and conductance to control where it goes. DIRECT GAIN Direct gain is the simplest method of passive solar heating. In this system, sunlight directly enters and heats the interior living spaces. The masonry floors and walls store the heat and radiate it back into the rooms when temperatures drop. THERMAL STORAGE WALLS In regions with mild to severe winters, a thermal storage wall provides better performance than direct gain. A 10- to 18-inch-thick, load-bearing or nonload-bearing masonry wall is constructed on the south side of a building. The winter sun slowly heats the wall, which warms the interior. HYBRIDS Fans and blowers can be used in passive solar designs to help the natural flow of thermal energy. These mechanically assisted passive systems are often referred to as hybrid designs. COMPUTERS EASE DESIGN Determining the performance and efficiency of passive solar designs is complex. Computer programs can make the job easier by calculating solar loads, capacity of thermal mass, proper proportions of glass to storage wall areas, heating and cooling requirements, and overall thermal performance.