How do they work?
Sunlight is composed of three different spectra of light: a visible spectrum, an ultraviolet (UV) spectrum, and an infrared (IR) spectrum. Infrared and ultraviolet rays make up more than 60 percent of sunlight and are largely responsible for the heat energy produced.
Low-E coatings work by reflecting heat-producing rays back to the source unlike normal glass, which allow these rays to pass through them. This means that in summer, sunlight is not allowed to enter the house, thus keeping it cool. In winter, the heat produced in your house is reflected back inside, thus requiring less energy to maintain a warm temperature in the house. These coatings, however, allow light to pass through unobstructed.
What are the different types?
Low-E coatings are available in two main types, i.e. soft coat and hard coat. In soft coat low-E glass, the coatings are applied to a glass sheet in a vacuum chamber. The chamber is filled with an inert gas, which is electrically charged. The combination of vacuum and electricity allows a thin layer of coatings to cover the glass. The coatings are then covered with another piece of glass. Sometimes, the space between the two glass pieces is filled with argon gas, which has insulating properties and prevents oxidation of the metal coatings. Soft coatings are the most energy efficient coatings available but they are not very durable. Therefore, soft coatings are applied on the side of the window glass that faces the interior of the house.
In hard coat low-E glass, the coatings are applied to the glass while it is not completely hard. A thin layer of coatings is poured onto slightly molten glass so that the coatings become fused to the glass sheet and are difficult to remove. Hard coatings are durable enough to be used on outer window surfaces but they are less energy efficient. These coatings are preferred for glass in storm windows and doors.
A third type of coating is called Heat Mirror. This product is only used for dual pane windows. The coatings are applied to a thin polyester sheet that is placed between the two glass panes. The coating reflects the outside heat while the polyester sheet minimizes heat loss by reducing the air space between the two glass panes.
What are the benefits of low-E coatings?
Low-E coatings have two main benefits as follows:
Energy savings: Low-E coatings provide energy savings both in summer and winter. By keeping the radiant heat from the sun from entering the house, low-E coatings reduce cooling costs. In winter, the coatings reflect radiant heat generated in the house and minimize heat generated to keep the house warm. There is a difference in the heat loss resistance values (R-value) provided by soft coat and hard coat glass though. While hard coat low-E glass has an average value of 2.5, soft coat low-E glass has an R-value of 3.5. For soft coat low-E glass insulated with Argon, the R-value goes up to 4.35.
Light view: Low-E coatings only obstruct heat. They allow visible light to pass through them. Increased lighting is an advantage over traditional heat obstruction methods such as window shades or tinted glass.