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Low-E Glass - Questions & Answers


  1. What is Low-E Glass?
  2. What does "Low-E" mean?
  3. What is radiant heat?
  4. How does Low-E work in winter?
  5. How does Low-E work in summer?
  6. How do windows with Low-E glass help keep me more comfortable?
  7. What about condensation?
  8. Why is Low-E Glass better than ordinary clear glass?
  9. What about fading?
  10. How should I measure the thermal performance of windows?
  11. Will Low-E windows really help the resale value of my home?


  1. What is Low-E Glass?
    Low-E Glass is clear glass with a thin, transparent metal-oxide coating which helps block the transfer of radiant heat. This results in a window that can keep any home, in any location, warmer in winter and cooler in summer.

  2. What does "Low-E" mean?
    Low-E stands for low-emissivity. Emissivity is a measure of how much a glass surface transfers radiant heat. The less radiant heat is transferred, the better.

  3. What is radiant heat?
    The sun radiates energy, which is received in shortwave rays that provide visible light. When the sun shines directly through your windows, shortwave rays enter your home, strike objects like the floor, walls and furniture, and warm them. These sun-warmed objects then re-radiate long-wave rays of invisible infrared heat. Likewise, long-wave radiant heat is provided within the home from other sources such as heating systems, fireplaces, light bulbs, appliances and even our warm bodies.

  4. How Does Low-E work in winter?
    Low-E works in two ways:
  5. First, the special coating on Low-E Glass is designed to easily transmit the sun's shortwave energy because it is transparent to that range of radiation. So it allows most of the sun's energy in through your windows, which helps heat your home in winter.

    Second, once that heat is inside your home and has been converted to long-wave radiant heat, the coating works to reduce the amount of heat that is absorbed by the glass, and the amount of heat that is transferred through the glass, by minimizing the emission of long-wave radiation. Since Low-E Glass does not absorb as much heat, that long-wave heat and the heat generated by your furnace, cannot be transferred back to the outside of your house through the glass in winter. Instead, the coating is specially designed to increase the insulating value of the glass. The coating works to reduce the amount of heat that is transferred through the glass to the cold exterior. That helps keep you warm in winter and reduces heating costs.

  6. How does Low-E work in summer?
    In the summer, heat enters your house through ordinary clear glass to the cooler interior in two ways: through direct sunlight and by long-wave heat radiating from the hot exterior environment (such as that heat which has been absorbed and reradiated by your driveway, sidewalk, bushes and shrubs). Low-E Glass will help reduce this unwanted heat gain, which reduces the amount of heat transmitted through the glass and into the cooler air inside.

  7. So how do windows with Low-E glass help keep me more comfortable?
    Heat always flows from warm to cool objects. When you pick up a can of soda, your hand feels cold because heat is transferring from your warm hand to the cold soda can.

    In winter, windows with Low-E Glass help keep the inside surface temperature of the glass warmer. So when you stand near windows in winter, the drafty feeling caused by heat leaving your skin and flowing toward the cold glass is reduced.

    In summer, direct shortwave sunlight transmission can best be minimized through overhangs, awnings, and window coverings. But Low-E Glass also helps keep you cooler by reducing unwanted heat gain.

  8. What about condensation?
    Condensation is created when moisture vapor in the air is cooled to its liquid state. This is most obvious on ordinary clear glass windows in the winter, because of the dramatic temperature difference between the heated air in your home and the inside surface temperature of the glass. Since the special coating on Low-E Glass keeps the inside glass surface of your windows warmer in winter, you can enjoy a more comfortable, higher humidity level in your home with greatly minimized condensation and fogging. In order for condensation to appear on Low-E Glass, humidity levels must be four times higher than on single-pane, clear glass windows.

  9. So why is Low-E Glass better than ordinary clear glass?
    Since ordinary clear glass has no coating, it absorbs and transmits heat very readily. In the winter, the clear glass absorbs heat from the inside of your home and transfers it to the cold exterior, causing your furnace to work harder. Low-E Glass insulating units are more than three times better than single-pane, clear glass at keeping heat inside in winter.

    In summer, clear glass absorbs heat from the outside and transmits it into the cooler interior, putting added strain on your air conditioners and fans. Since Low-E Glass helps stop this transmission, it helps you save money all year long.

  10. What about fading?
    The coating on Low-E Glass also works to reduce the early fading of your carpeting, drapes and upholstery fabrics caused by ultraviolet and other damaging rays, by blocking approximately two-times more ultraviolet light than clear, single-pane glass.

  11. How should I measure the thermal performance of windows?
    There are many factors which affect the thermal performance of windows. The climate you live in, the framing material, the window construction and the quality of installation will all affect the performance of the total window. Since glass represents up to 85 percent of any window, the quality and properties of that glass are extremely important in measuring overall window performance.

    R-Value or U-Value The R-value measures resistance to heat flow (you may have heard of this term with regard to fiberglass home insulation). Its inverse (1/R) is called the U-value, which measures heat flow, or thermal conductivity. It's one part of measuring how well your windows will perform. The lower the emissivity, the higher the R-value, and the lower the U-value (and the better). But solar heat gain is the other important factor to measure in evaluating a window.

    Shading Coefficient or Solar Heat Gain Coefficient The Shading Coefficient is a measure of how much solar energy is admitted through the glass. The higher the number, the better for passive heat gain. While ordinary clear glass allows more solar energy into your home than low-E glass does, clear glass has such a low R-value that it transfers that solar energy gain, as well as heat from your furnace, right back out of your home. If you spend more money heating your home than you do to cool it, the key is to have a high R-value and a relatively high Shading Coefficient. And that's exactly what Low-E Glass provides.

    Another way of describing the amount of solar heat gain through a window is the Solar Heat Gain Coefficient (SHGC). SHGC represents the percentage of the available solar energy which enters the interior of the home. Again, the higher the number the better for passive solar heat gain. SHGC will be included in the near future on window labels used by several states in order to allow the consumer to compare the solar heat gain between various window options.

  12. Will Low-E windows really help the resale value of my home?
    Ninety percent of consumers surveyed said they would like energy efficient windows in their homes. Typically, homebuyers are impressed with a great new kitchen or incredible bathroom, but now good windows are also a priority. According to a recent survey done by Remodeling Magazine, real estate agents nationally estimate a new set of energy-efficient windows returns 75 percent of your money invested.