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Vinyl windows at high altitudes

Q: I want to buy a condo being built at an altitude of 9,000 ft. The contract calls for vinyl windows, but a local architect and the local lumber store both say that vinyl breaks down easily from the intense ultraviolet rays at this altitude and that I would much better off with wood-clad windows in the condo. Who’s right?

Allison Johnson, via e-mail

A: Paul R. Fisette, director of building materials and wood technology at the University of Massachusetts in Amherst, replies: Ultraviolet radiation can be destructive, and as you’ve already pointed out, the problem is exacerbated at higher altitudes, where the earth’s atmosphere, which filters out those UV rays, is thinner. Vinyl windows manufactured in the past degraded badly as a result of UV exposure. But the chemical composition of today’s vinyl windows makes them much better at resisting UV degradation.

For more than a decade now, the best vinyl windows (either clad or extruded) have contained UV-stabilizers. UV degradation is basically a chemical reaction, and UVstabilizers in vinyl link molecules in a way that makes them resist breakdown.

The American Architectural Manufacturers Association (AAMA) (847-303-5664) publishes a standard, listed as AAMA 303, that certifies the performance of vinyl windows. In addition to UV degradation, this standard also considers the performance of window frames when exposed to thermal stress and weathering. Be sure to specify that your windows have passed the standard established by AAMA 303.

By the way, wood and aluminum don’t get off scot-free in this department. UV rays decompose lignin, the natural glue that holds wood cells together. Without lignin, wood falls apart. Likewise, aluminum continually oxidizes under UV bombardment unless it is anodized or it is effectively protected with a durable, UV-resistant paint. Anodized aluminum is not supposed to be repainted if the anodizing fails, and repainting other types of aluminum frames when color fades is a difficult chore at best.

From Fine Homebuilding 121, pp. 20 March 1, 1999