San Diego homeowners in the market for energy efficient replacement windows have certainly encountered the term low-e glass. We know low-e glass is a contributing factor in making the window energy efficient, but what does low-e mean and how does it work?
Let’s start with light, specifically sunlight, which we have no shortage of here in San Diego. There are three basic types of light: ultraviolet light, visible light and infrared light.
Ultraviolet light is the type of light that causes carpets, drapes and wall coverings to fade, while visible light is the part of the spectrum that illuminates things to the human eye. There are actually two types of infrared light, short and long-wave, but infrared light is basically the light that gives off heat energy. Infrared is the light form that heats your San Diego home through your sunlit windows and walls during the warm summer months.
The ability of a material to radiate heat energy is called emissivity. The “e” in low-e stands for “emissivity.” Generally, any material that is highly reflective of heat energy is said to have low emissivity and materials that absorb and radiate heat energy are highly emissive.
Low-e glass has been specifically engineered to minimize the amount of ultraviolet and infrared light that passes through the glass, while simultaneously allowing the maximum amount of visible light to also pass. This is accomplished with a microscopic transparent coating on the glass, thinner than a human hair, that reflects long-wave infrared light and significant amounts of short-wave infrared heat from the sun. And the opposite effect occurs in the cooler winter months — the low-e coated glass reflects heat back into your home.
The best analogy to how low-e windows truly work is a thermos. If you take a good thermos and pour hot coffee into it and close it, the silver coating on the inside thermos reflects the heat of the liquid back onto itself, while the outside of the thermos insulates the hot coffee from changes in the exterior temperature. Low-e glass works the same way, reflecting harmful types of light and heat away from the exterior in the hot summer months, and insulating and reflecting heat back into the interior of the home in cooler times of the year.
Low-e coatings are vital to the performance of energy efficient replacement windows and can significantly reduce energy consumption, especially in the summer months, when the hot San Diego sun is an everyday occurrence.