Low E glass (E stands for emissivity) was introduced in 1979 and is now a great favorite among contractors. Low E glass works by reflecting heat equally during the winter and summer, using a thin metallic coating on or in the glass. Low-E-windows can be installed in almost all regions of the world, but the main factor is related to the building design and window position. As a builder, you can use them for your new projects or even in major renovations, ideally for healthcare facilities, as maintaining temperatures is critical in some of these facilities. These windows can provide many benefits for the building owner, as they can save as much as 50 percent in energy use, provide greater operational efficiency, improved access to daylight, and natural views for occupants—without increasing energy costs for heating and cooling.
Choosing Low-E Windows Instead of Regular Windows
Low-E windows can provide aesthetic value to building occupants, and they will not reduce the amount of light entering the building while maintaining the natural look of windows. However, windows can also represent a large source of heat gain or loss, because when the right window is not used, it can increase heating costs and A/C usage, making it more costly to operate your building.
Similarly, windows with a poor ability to keep heat in allow warm air to escape the building in the winter, increasing the demands on heating systems. Window manufacturers have developed many new insulating and glazing techniques to improve the performance of windows.
The National Fenestration Rating Council defines five performance areas to consider when choosing windows most suited for your local climate:
- Look for Lower U-Factor Values: These numbers will represent how well a product prevents heat from escaping a home or building. Always look for U-values between 0.20 and 1.20, where lower numbers indicate a product better at keeping heat in.
- Lower Solar Heat Gain Coefficient (SHGC) is Better: SHGC measures how well a product blocks heat from the sun from entering the building. SHGC is expressed as a number between 0 and 1, with a lower SHGC indicating a product that is better at blocking unwanted heat gain.
- Visible Transmittance (VT) measures how much light comes through a product. VT is expressed as a number between 0 and 1, with a higher VT indicating a higher potential for daylighting.
- Air Leakage (AL) measures how much outside air comes into a home or building through a product. AL rates typically fall in a range between 0.1 and 0.3, with a lower AL indicating a product that is better at keeping air out.
- Condensation Resistance (CR) measures how well a product resists the formation of condensation. CR is expressed as a number between 1 and 100, with a higher CR indicating a product better able to resist condensation.
Not all Low-E windows are suitable for all climates, so be sure to choose the right window for the climate where you are building. It is important to note that facilities in warmer climates should install windows with a lower SHGC, and those in a cooler climate should install windows with a lower U-factor. Low-E coatings applied to exterior window panes prevent heat gains from exterior radiation, whereas Low-E coatings applied to interior windows prevent heat loss. Manufacturers often offer several Low-E coatings, with varying degrees of solar gain.
Low-E Windows Benefits and Costs
Windows manufactured with Low-E coatings typically cost about 10 percent to 15 percent more than regular windows, but they reduce energy loss by as much as 30 percent to 50 percent.
Furthermore, this improvement in the building envelope—particularly when coupled with other strategies that improve the efficiency of the building envelope—ultimately impacts the demands of building HVAC systems. These benefits should be included in evaluating the lifecycle costs of installing efficient windows.
Efficient windows are defined by the climate of the building in which they will be installed. Engineers and vendors are able to make recommendations based on local climate and building orientation. Efficiency improvements to the building envelope directly impact the heating and cooling needs of the building, therefore, HVAC systems should be adjusted accordingly to account for decreased demands on the systems.