A Light Discrepancy?

We received an e-mail from a reader regarding an article that appeared in our November 2013 issue:

I read the article titled “Rethinking Cool Roofing: Evaluating Effectiveness of White Roofs in Northern Climates,” by Craig A. Tyler, AIA, CSI, CDT, LEED AP. Although I agree with what I think is the article’s concept, the author repeatedly states ultraviolet (UV) absorption into the roof system contributes to heat gain in a building or that UV reflection helps to keep the roof cool. UV is not only not responsible for heat gain through absorption into the roof system, but it is also entirely the opposite end of the spectrum from the infrared (IR). UV exposure has long been known as a contributor to the breakdown of roofing materials.

When the author was contacted, he responded:

The reader brings up a valid point about infrared being at work here as well. Sunlight consists of infrared, visible, and UV electromagnetic radiation, known as ‘solar radiant exposure.’ Infrared, while commonly associated with heat radiation, is not the only component of sunlight that contributes to temperature rise. All sunlight will heat surfaces that absorb the solar radiation. Infrared consists of 49 to 53 percent of sunlight, with the remainder being mostly visible and UV light. UV exposure from sunlight reaching the earth’s surface can cause degradation to many types of construction materials, as well as people. Degradation can be observed as a reduction in physical properties, cracking/crazing, loss of gloss, and discoloration.

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