Direct gain is the heat from the sun being collected and contained in an occupied space. This heat can be retained by the building’s thermal mass, or can be avoided with reflective materials.
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 solar heat gain. Avoiding direct solar gain is also important in hot sunny climates.
Good direct gain is measured or predicted by determining how much heat energy the sun delivers to the interior space throughout the day and year. In many climates, more heat gain is desired in the winter, when the sun is low, while less or none is desired in the summer. Likewise, it is usually desired more in the morning, but less or none in the late afternoon. Direct gain is not the total amount of incident solar radiation, since not all of that will enter and be retained in the space.
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