Low-E stands for low emissivity, and these windows are constructed to minimize heat transfer through the glass. Since windows are essentially huge holes in the walls of a building, choosing a low-E window design that’s appropriate for local climate and architecture can greatly increase a structure’s thermal efficiency, while reducing energy use and utility costs.
How does it work? A low-E window uses a microscopically thin layer of metal or metal oxide as a coating on the glass, which prevents heat from being transferred through the window. This coating can either help to keep a building cool in hot weather, or it can prevent heat loss in the cold winter months.
Low-E glass can be customized for different amounts of solar gain, meaning heat from sunlight. For example, a structure in a cold climate would benefit from a window that prevents heat loss, but admits as much heat from sunlight as possible. Low-E glass is typically transparent to visible light, so it shouldn’t be confused with the tinted or mirrored glass commonly seen in commercial buildings.
The performance of a low-E window is usually described as its “U-factor” or “U-value”. Simply put, this is the inverse of the “R-value” that is commonly referred to in describing insulation materials. So, whereas a high R-value (resistance to heat transfer) is a good thing for insulation, the U-value (heat flow) will be a very low number in an energy-efficient window.
Low-E glass is often combined with other design elements to maximize a window’s energy efficiency. Two or more panes of glass create added insulation, and the gaps between panes may be filled with an inert low-conduction gas like argon or krypton. And remember, the installation is just as important as the design of a window. To enjoy the efficiency benefits of low-E windows, the window frames must be properly mounted and sealed to eliminate any drafts or leaks.