Spectrally Selective Windows
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| Other Names: | |
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| Spectrally Selective Glazing, Spectrally Selective Glass | |
| Description: | |
| A window glazing that permits some portions of the solar spectrum to enter a home while blocking others, used to reduce energy costs. |
Spectrally selective glazing is a tinted or coated window glazing that has optical or solar properties which vary across the solar spectrum. These glazings typically offer a low transmission of solar gains in conjunction with a high transmission of visible light, blocking unnecessary radiation and reducing energy costs.
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Spectrally Selective Windows
There are a wide range of spectrally selective window options but the newest generation of spectrally selective glazings exaggerate the differences between the visible and infra-red portions of the spectrum, two of the three components of solar radiation (visible radiation -- sunlight --, infrared radiation and ultra-violet radiation). In this way the spectrally selective windows ensure that infra-red radiation is blocked.
Description
Approximately 50% of the sun's energy transmits heat but does not transmit light. This occurs because the infrared and ultra-violet portions of the sun's light occur outside of the visible portion of the spectrum. Spectrally-selective glazings transfer a high percentage of the sunlight but screen up to 80% of the infra-red radiation. A low percentage of radiant heat transmits from the sun and reduces the need to cool building interiors.
Tints, Glass Types and Coatings
Spectrally selective glass may include different tints and types of glass.
Glass Tints
Spectrally selective glazings often feature a slight green or blue appearance. The tint is slight, however, because the visible light is transmitted relatively evenly over the visible spectrum. These tints don't affect the U-Value or thermal insulating value of the window. The most energy efficient spectrally selective replacement windows, are tinted so it is almost unnoticeable.
Tempered Glass
Large window units which have spectrally selective glass panels are often manufactured using tempered glass to strengthen the glazing unit.
Low E Coating
Low-emissivity (Low-E) coatings can be added to the spectrally- selective glazing to improve thermal performance.
Ratings
The effectiveness of spectrally selective glazings are measured by light-to-solar ratio (LSR). This is the ratio of visible light transmission divided by the solar heat gain coefficient for the glazing system. Clear un-spectrally-selective glazing units will have a value of close to 1.0 while a quality spectrally-selective glazing system will have a value of greater than 1.7. The highest possible light-to-solar ratio is approximately 2.0.
Considerations
Homeowners should review the benefits offered by spectrally selective replacement windows before making a purchase. These include the amount of visible light that the window transmits. The best spectrally selective energy efficient replacement windows transmit at least 70% of visible light, ensuring that the home benefits from proper daylighting while the windows block unwanted infrared radiation.
Selection
When choosing a spectrally selective replacement window homeowners should review the window's clarity, heat blocking rating, aesthetics and cost savings.
Clarity
A high quality spectrally selective replacement window should be clear yet able to block significant amounts of unwanted solar heat and reduce glare.
Heat Blocking Ratings
Spectrally selective windows that block acceptable amounts of heat and sunlight transmissions should display a Solar Heat Gain Coefficient of 0.51 or less and a Visible Light rating of 15% or less.
Aesthetics
A clear spectrally selective replacement window should provide a clear view of the outdoors.
Cost Savings
A spectrally selective window should, when used in combination with windows that offer sufficient U-Factor values, allow the homeowner to realize a return on the investment within four to seven years in reduced energy bills.
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