Research in the recent past led to the development of low-emissivity or "low-e," glass and films that control heat gain and loss, reduce glare, minimize fabric fading, provide privacy, and occasionally provide added security in wind, seismic, and other high-hazard zones. New construction and window replacement applications commonly use glazing with these coatings.
Some low-e coatings and solar control films reduce solar heat gain without impairing visible light transmission excessively. These include tinted glass and spectrally selective coatings, which transmit visible light while reflecting the infrared portion of sunlight.
Many spectrally selective coatings or glazings also have some low-e properties as well.
Types of Glazing
Modern window glazing falls into three categories:
- Chemically or physically altered glass
- Coated glass or films
- Multiple-layered assemblies with or without either of the first two items.
Chemically or Physically Altered Glass
Tinting – One of the oldest of all the modern window technologies. Under favorable conditions, tinting can reduce solar heat gain during the cooling season by 25 percent to 55 percent. Tinted glass is made by alteration of the chemical properties of the glass. Both glass and plastic laminate may be tinted.
The tints absorb a portion of the sunlight and solar heat before it can pass all the way through the window to the room. Tinted glazings reduce the latter by 25 to 55 percent. "Heat absorbing" tinted glass maximizes its absorption across some, or all, of the solar spectrum. Unfortunately, the absorbed energy often transfers by radiation and convection to the inside.
Coated glass or films
Spectrally Selective Coatings
Spectrally selective coatings or tints reduce infrared light (heat) transmission while allowing relatively more visible light to pass through (compared to bronze- or gray-tinted glass).
For buildings that use daylight for lighting, a spectrally selective window is a good choice. Spectrally selective glass also absorbs much of the ultraviolet (UV) portion of the solar spectrum.
In a multi-paned window, they function best as the outermost sheet of glazing. Thermal performance is increased when combined with a low-e coating. Spectrally selective coatings often have a light blue or green tint.
Low-e and reflective coatings usually consist of a layer of metal a few molecules thick. The thickness and reflectivity of the metal layer (Low-E coating) and the location of the glass it is attached to directly affects the amount of solar heat gain in the room.
Most window manufacturers now use one or more layers of low-e coatings in their product lines. Any low-e coating is roughly equivalent to adding an additional pane of glass to a window.
Instructions: Click on the hot spots in the image below to see the properties of Low-E coating windows:
Low-e coatings reduce IR heat transfer by 5 to 10 times. The lower the emissivity value (a measure of the amount of heat transmission through the glazing or coating), the better the material reduces the heat transfer from the inside to the outside.
Most low-e coatings also slightly reduce the amount of visible light transmitted through the glazing relative to clear glass. The table below gives the emissivity values for different types of glass.
|Type of Coating||Emissivity|
|Clear glass. Uncoated||0.84|
|Glass with single hard coat low-e||0.15|
|Glass with single soft coat low-e||0.10|
There are three types of low-e coating available: Soft, hard coatings, and Heat Mirror.
- Soft coat is applied to the surface of a glass at lower temperatures. It's not durable enough to be exposed to the elements, so it's only used on the inner surfaces of windows which are not exposed to the elements.
- Hard coat is produced by fusing metallic oxide to the hot surface of glass during manufacture and is found primarily on storm windows and removable energy panels. Hard coat is applied on the glass surface at a high temperature. One layer is about 1/10,000 the diameter of a human hair. Hard coat is not quite as energy efficient as soft coat, but is tough enough to be used on surfaces exposed to the elements. Both types of low-e coatings (within insulated glazing assemblies) typically last for 10 to 50 years.
- Heat Mirror is a proprietary product that's applied to a thin polyester sheet suspended between the two panes of dual pane window. The coating reflects radiant heat while the sheet decreases heat loss by splitting the air space in two.
The only spectrally selective coatings now available are modified soft coat low-e coatings. The selective properties of the coatings are determined by modifying the coating's thickness and number of layers. A spectrally selective tinted glazing with a pyrolytic hard coat serves a similar purpose. These spectrally selective hard coats are currently under development.
Aftermarket Films - "Aftermarket" films are available for application on existing windows. They are relatively easy to apply on glazing up to 36 square inches. They are often applied to the glass with a water-soluble adhesive. To reduce the possibility of bubbles and wrinkles on large windows, have the film installed professionally. Most films should be applied to the inside surface of the glass since they can be damaged easily by weather.
If you plan to install the film yourself, be careful to select the appropriate film for your needs, and understand all directions before beginning. Plastic films generally last about 8 to 10 years before they start looking worn.