Low-E Glass and Spectrally Selective Glazing - 11/22/2004 - Home Condo Doors Windows

Low-E Glass and Spectrally Selective Glazing

Windows with spectrally selective glazing allow more natural light into homes or other buildings, while controlling radiated heat.

Windows are no longer simple panes of glass, but have evolved into complex engineered systems. Spectrally Selective Glazing incorporates technical advances in low-e coatings that filter out the heat producing portions of the solar spectrum, but still allow the greatest possible visible light transmittance. Windows with spectrally selective glazing allow more natural light into homes or other buildings, while controlling radiated heat, providing maximum energy efficiency, and reducing heat loads in areas where cooling costs are high.

Low-emissivity (low-e) coatings were designed to help control radiated energy, which cannot be controlled by normal gas-filled double pane windows. Low-e coatings consist of microscopically thin metal or metal-oxide layers deposited on the glazing surfaces to diffuse radiated heat by reflecting it, rather than allowing it into the air layer. Spectrally selective glazing is an innovation that uses special low-e metallic coatings in "sputtered" application process. As light passes through the window, these coatings filter out the infrared (or heat-producing) waves from the solar spectrum, reducing both radiated heat (as do other low-e coatings) and the heat-producing element of sunlight. Spectrally selective glazing allows windows to provide bright, natural light while still achieving low heat transfer and high energy-efficiency. This technology is usually best suited to "cooling dominated" climates, or areas where cooling and heating costs are about equal. Spectrally selective glazing would not allow solar heat gain in "heating dominated" climates.

Selective low-e coatings may add $1.00 per square foot or more to the cost of glazing materials.

Spectrally selective glazing cuts costs by reducing energy bills and HVAC loads. Examples from Boston, MA and Tuscon, AZ on the Efficient Windows Collaborative website demonstrate that significant savings are possible in both climate types.

The Model Energy Code and some local building codes require energy rated window assemblies for new construction.

Not Applicable

The National Fenestration Rating Council (NFRC) has established a labeling system to aid in the process of window selection. For complete information on these labels, see the contact information below. Choosing the best type of window/glazing for a given project involves many factors, including directional orientation of the window, shading, and climate conditions. There may also be trade-offs between heating and cooling season performance, not to mention cost considerations. Many builders, designers, and architects use computer software programs such as RESFEN to assist them in making the best choice among many options. (See the Contacts section for more information.)

Energy performance data on NFRC window labels includes a listing of the Solar Heat Gain Coefficient (SHGC), expressed as a decimal factor between 0.0 and 1.0. The arrangement of the low-e coatings within the window assembly largely determines this rating.

In climates where the heating and cooling costs are roughly equivalent, or cooling dominated, a low SHGC rating (under 0.45) is desirable to reduce the effect of solar radiation on air-conditioning costs. While this assembly does not allow solar gain during the heating season, it is still effective at reducing heat loss from warmed interior space to cold outside air.

NFRC labels also provide a decimal factor rating of Visible Light Transmittance (Tv), the amount of the visible light spectrum that passes through the window assembly. Most advanced spectrally selective glazing in double pane assemblies transmit 75% or more of visible light (Tv=0.75).

In general, spectrally selective glazing is indicated by a low SHGC rating (0.45 or less) and a high Tv rating (0.50 or greater). Some manufacturers allow customers to special-order windows with whichever glazing material is best suited to the application.

Not Applicable

Spectrally selective glazing allows higher levels of natural light to enter the home, reducing the need for supplementary artificial lighting during the daytime, and allowing occupants to feel more attuned to the outside environment. Nighttime interior surface reflectivity, which can prevent occupants from seeing outside during darkness, is also reduced or eliminated. In some cases, use of low-e spectrally selective glass can allow a reduction in the size of the cooling system needed, or allow more glass area without increasing cooling loads. In other words, homes may be designed with larger windows without increasing heating and cooling costs, or sacrificing comfort. Spectrally selective glass offers the biggest advantages in areas where cooling costs are higher than heating costs.

Selective low-e coatings may add $1.00 per square foot or more to the cost of glazing materials.