Low-E glass controls heat transfer while allowing visible light to pass through. The U factor measures heat transfer, with lower U factors indicating higher energy efficiency. The glass reflects long-wave radiation energy to keep heat in desired locations. It is available in high, moderate, and low gain panels for different climates. Low-E glass is coated with an ultra-thin metallic coating and costs 10-15% more than standard windows but reduces energy loss by 30-50%. Early versions had visibility issues, but technology has improved to allow for better visibility while still filtering heat.
Low-E glass is a type of treated glass that conducts visible light while controlling the transfer of heat. Low-e stands for low emissivity. This glass is an energy efficient way to control the heat that is allowed into and out of a home or space.
The heat transferred through the glass is measured by the U factor. This is the rate at which the glass conducts non-solar heat passing through it. The lower the U factor, the more energy efficient the glass is. Low-E glass allows visible light to pass through but blocks heat-generating ultraviolet light, requiring less artificial heating or cooling to keep a room at a desired temperature.
This glass works by reflecting heat back to its source. All objects and people give off different forms of energy, affecting the temperature of a space. Longwave radiation energy is heat, and shortwave radiation energy is visible light from the sun. The coating used to make low-e glass works to transmit short-wave energy, allowing light in, while reflecting the long-wave energy to keep the heat in the desired location.
Low-E glass is available in high, moderate and low gain panels. In particularly cold climates, heat is stored and reflected back into a home to keep it warm. This is achieved with high solar gain panels. In particularly hot climates, low solar gain panels work to repel excess heat by reflecting it back out of the space. Moderate solar gain panels are also available for areas with temperature fluctuations.
The low-e glass is enamelled with an ultra-thin metallic coating. The manufacturing process applies it with a hard or soft coating process. Soft-coated low-E glass is more delicate and is easily damaged, so it is used in insulated windows where it can be between two other pieces of glass. Hard coated versions are more durable and can be used in single pane windows. They can also be used in retrofit projects.
Generally low-e windows cost between 10 and 15% more than standard. The reduction in energy loss can range from 30 to 50 percent. Windows made with this type of glass are a larger investment initially, but will pay for themselves in lower heating and cooling costs.
Visibility was an issue with some early low-E glass, as the original glass was said to have a brownish tint. Technology and manufacturing have continued to improve its quality resulting in a spectral selective glass that allows for the best possible visibility while still filtering heat.
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