Low-E Windows and Doors
Glass is one of the most popular and versatile building materials used today. Great improvements have been made in the last decade to improve the solar and thermal performance of glass through the use of passive and solar control Low-E (low emissivity) coatings. ECS can help you choose the best Low-E windows and doors for your specific house orientation, configuration and budget. You can enhance your energy savings even further by selecting specific different Low-E windows for the different sides of your house. In the north Georgia climate, you do not want extra heat from the sun, so a low SHGC (Solar Heat Gain Coefficient) is important for windows that face South, East, and West. It is important to choose a low U-factor (the rate of heat loss) for all windows in warmer climates: in addition to minimizing heat loss, low U-factors also reduce your need for cooling.
In order to understand Low-E coatings, it’s important to understand the solar energy spectrum or energy from the sun. Ultraviolet (UV) light, visible light and infrared (IR) light all occupy different parts of the solar spectrum – the differences between the three are determined by their wavelengths.
- Ultraviolet light, which is what causes interior materials such as fabrics and wall coverings to fade, has wavelengths of 310-380 nanometers when reporting glass performance.
- Visible light occupies the part of the spectrum between wavelengths from about 380-780 nanometers.
- Infrared light or heat energy, is transmitted as heat into a building, and begins at wavelengths of 780 nanometers.
Low-E coatings have been developed to minimize the amount of ultraviolet and infrared light that can pass through glass without compromising the amount of visible light that is transmitted. Radiant energy is one of the important ways heat transfer occurs with windows. Reducing the emissivity of one or more of the window glass surfaces improves a window’s insulating properties. For example, uncoated glass has an emissivity of .84, while a top Low-E glass has an emissivity of .02.
This is where low emissivity (Low-E) glass coatings come into play. Low-E glass has a microscopically thin, transparent coating – it is much thinner than a human hair – that reflects long-wave infrared energy (or heat). Some low-e glass also reflects significant amounts of short-wave solar infrared energy. When the interior heat energy tries to escape to the colder outside during the winter, the low-e coating reflects the heat back to the inside, reducing the radiant heat loss through the glass. The reverse happens during the summertime. To use a simple analogy, Low-E glass works the same way a thermos does. A thermos has a silver lining, which reflects the temperature of the drink it contains. The temperature is maintained because of the constant reflection that occurs, as well as the insulating benefits that the air space provides between the inner and outer shells of the thermos…similar to an insulating glass unit. Since Low-E glass is comprised of extremely thin layers of silver or other low emissivity materials, the same theory applies. The silver Low-E coating reflects the interior temperatures back inside, keeping the room warm or cold.