I have noticed that reflected light from windows with low-E glass seems to have caused areas of excessive fading on the composite decking of several decks in my area. And currently, I routinely maintain a treated-wood deck that will not hold a coating because of—I think—extreme heat that's due to light bouncing off glass on two opposing walls. One of the walls is at the rear of the house and is composed of a bank of sliding glass doors leading out onto the deck. The opposing "wall" is the deck railing, which has glass-panel balusters. For all intents and purposes, at certain times of the day the deck surface becomes a microwave oven, and the coating I apply evaporates or is absorbed, and seems to get cooked off the surface of the decking in a matter of months.
I've spoken with a window distributor, who says that the problem is an unintended consequence of low-E glass. Has anyone else encountered this problem or heard of light reflectivity creating issues like this?
Jim Grant San Diego, Calif.
Reflected light has caused problems with vinyl siding, so it's not surprising to hear that plastic decking can be affected too. Thermal distortion—the term used by the vinyl-siding industry—happens when temperatures exceed the 165°F point where the PVC substrate used to manufacture most vinyl siding starts to soften and melt. Insulated glass can develop a concavity that will actually focus reflected light, and it's not unusual to see temperatures exceeding 200°F in some of these sunspots (see "Low-E Windows Blamed for Melted Vinyl Siding," JLC).
There's no question that windows can dramatically raise the temperature on decking, but whether or not this is causing the composite decking to fade prematurely and the coating to "cook" off the surface of your client's treated-wood deck are good questions for the manufacturers of the decking and the coating.