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Most skylight manufacturers test their products for heat loss, solar heat gain, and blockage of the sun’s ultraviolet (UV) rays. These measurements are expressed numerically. It’s helpful to know these terms before you shop:
R-value measures a material’s insulation value—the resistance a material has to heat flow. In reference to skylights, this value is usually placed on the glazing. The higher the R-value, the better it insulates. Skylights with the best R-values have low-e insulating glass with argon gas between the panes— these yield about twice the insulating value of standard dual-glazed panes.
U-value, which depends not only on the glazing but also on the frame and all related parts, measures the rate of heat flow (from either side) through a skylight. This measurement does not include solar heat gain. The lower the U-value, the better the product resists heat flow.
Shading coefficient measures solar heat gain through glass. This coefficient compares a particular glazing with a single pane of 1/8-inch-thick, double-strength glass. A low shading coefficient means low solar heat gain. UV blockage is a value approximated by glazing manufacturers, expressed as a percentage of ultraviolet rays blocked by the glazing.
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