Windows perform more functions than any other component of our buildings. They provide daylight, natural ventilation, and glare control. They control fading, sound intrusion, water tightness, and thermal buffering. They determine condensation, durability, clarity of views, and maintenance schedules. And, of course, they need to be attractive and affordable.
Because of all these functions, it is advisable to purchase the highest-performance windows you can afford. Most windows now carry rating labels developed by the National Fenestration Rating Council (NFRC). They tell the U-value, which reflects insulating effectiveness; the solar heat gain coefficient (SHGC), which tracks how much heat radiation passes through a window; and the visible transmittance, which covers daylight, views, glare, and fading.
California building codes now require that all windows be dual glazed (have two layers of glass) with a U-value no higher than .32. (The lower the number, the better the insulating performance.) The SHGC varies between 0 and 1. A lower number reduces cooling costs. Usually, U-values and SHGC numbers move up or down together, but for passive solar design — winter heating from the sun being essential — it is important to get separation, still seeking a low U-value but getting as high a SHGC number as possible to let in more heat radiation.