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Common Characteristics of the Most Energy Efficient Windows

| August 13, 2014

When you are replacing windows in your home, the best choice are the most energy efficient windows. In addition to providing you with the most obvious benefit of saving on your energy bills, energy efficient windows have many important characteristics that affect everything from the amount of sunlight in your home, to preventing the amount of air leakage around window openings.

Passive solar design

One of the most common characteristics of the most energy efficient windows is that you can select them according to orientation and size of glass to maximize the sun’s heat in winter and minimize the glaring sun in the summer.

For south facing windows in northern climates you will want to gain solar heat in the winter and use the roof overhang to protect windows from direct sunlight in hot climates in the summers. For preventing heat loss windows are treated with window glazing that is low-emissivity, which is another characteristic of energy efficient windows. 

Energy performance ratings

Energy performance ratings show how the most energy efficient windows can help you maximize passive solar design. There are three main energy efficient ratings for windows.

  • Solar heat gain coefficient (SHGC). The SHGC is the solar radiation that comes through your windows, measured in fractions. The lower the number, the less solar heat is transmitted through your window. When you select your windows ensure that the SHGC is greater than 0.6 to maximize heat in the winter.
  • U-factor is the rate at which your windows will conduct non-solar heat flow. Your windows should have a U-factor equivalent too, or less than, 0.35 so that the amount of heat transferred through conduction, or the transfer of heat between molecules through the glass, is at its lowest level. The lower the U-factor, the more energy efficient the window will be.
  • Air leaks is a rating that energy efficient windows have to show the amount of air leaking from around the window frame. The air leakage rating should be at the lowest possible level.

Ability to transmit sunlight

Windows will also allow sunlight to enter and to block sunlight which also contributes to the level of your home windows’ energy efficiency.

  • Visible transmittance (VT) is a term for measuring the among of light that comes into the home. The higher the window’s VT rating on a rating scale between 0 and 1, the more light is transmitted. You will determine the number rating you need depending on how much daytime light you require.
  • Light to solar gain (LSG) measures how efficient the window will be at allowing in sunlight while preventing the heat transfer that comes with it. Oftentimes when energy performance ratings are supplied, this measure is missing.

If you take into account these common characteristics of energy efficient windows, like their energy performance ratings and ability to transmit sunlight, you realize that with energy efficient windows, the benefits extend well beyond savings on energy bills!


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