2 - Btu. The higher a skylight's R-value, the greater is its resistance to heat flow, and the greater is its insulating value. Shading coefficient . It is expressed as a number without units between 0 and 1. The lower a skylight's shading coefficient, the less solar heat it transmits, and the greater is its shading ability.
Solar heat gain coefficient .
The portion of solar radiation admitted through a skylight, both directly transmitted and absorbed and subsequently released inward. The solar heat gain coefficient will probably replace the shading coefficient as the standard indicator of a skylight's shading ability in the near future. Spectrally selective coating . A low-E coating that blocks out much of the sun's heat while transmitting substantial daylight. A measure of the rate of heat flow through a material or assembly.