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Thermal mass is a material's resistance to change in temperature. Objects with high thermal mass absorb and retain heat. Thermal mass is crucial to good passive solar heating design, especially in locations that have large swings of temperature from day to night.
Direct gain is the heat from the sun being collected and contained in an occupied space. Direct solar gain is important for any site that needs heating, because it is the simplest and least costly way of passively heating a building with the sun. Avoiding direct solar gain is also important in hot sunny climates.
The physical properties of the materials, coatings, and constructions that make up windows and skylights. Good glazing properties are important because they control the amount of daylight, quality of light, and amount of solar heat gain let into the building.