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What to Look for in Energy Efficient Replacement Windows

Windows are the weakest link in a home’s thermal shell. For centuries homeowners have been battling drafts and heat gain. All homes, even the most modern, still suffer to some extent from the inherent inefficiency of windows and glass. At the same time home energy costs are becoming a major concern for families, spawning a renewed zeal for energy efficient building products of all types.

One of the first steps to improving a home’s energy efficiency is to install replacement windows. But it’s not as simple as walking into a building supply store and asking for a window. There are several different window types, each with individual characteristics such as glazing and frame type. Each type of window will have its unique thermal characteristics and, when searching for energy efficient replacement windows, there are a number of clues that reveal the window’s true level of efficiency.


Window glazing has a direct and profound impact on window quality. The term “glazing” refers to the panes of glass that fit into the frame. Windows may be single-, double- or triple-glazed, the numerical half of the words symbolizing layers of glass. The more glazing it has, the more energy efficient the window is.

In some cases manufacturers will hermetically seal air or gasses between each layer of glass to make it even harder for heat to pass through the glass. There are also coatings that are applied to the glass; usually between the panes although coatings exist which can be applied on the exterior of the window. The most energy efficient windows are likely to have a low-emissivity (low-e) coating that is barely visible, allowing plenty of light through while simultaneously blocking heat waves.

Reading the Label

When shopping for energy efficient windows, there are numerical clues that help in comparing one window type to the next. Four of these are grouped together on a label put there by the National Fenestration Ratings Council (NFRC).

  • U-Rating - The U-rating is a measurement of the amount of heat that is able to pass through the window each hour. The number ranges from 0.20 to 1.20—the lower the number the more efficient the window is.
  • Visible Light Transmittance - A measurement of the amount of visible light that is allowed to pass through the window, on a scale of 0-1. The higher the number, the more light transmitted.
  • Solar Heat Gain Coefficient - Measures how much heat the window lets in with the light; very important for energy efficient windows. Look for a low solar heat gain coefficient to ensure that you get the light without the heat.
  • Air Leakage - Also AL number, it signifies the strength of the window’s seal. A big issue for the notoriously inefficient windows of the past was air leaks; a big reason for the cold drafts that continue to inspire window designers and engineers. Air leakage is unwanted for obvious reasons, so search for a replacement window with a low AL number.

A window’s energy efficiency ratings should be blatantly posted, usually on a sticker stuck to the new window glass.

The Frame

Not all window frames are created equal. Wood, aluminum, vinyl (PVC), and fiberglass are common frame materials. Each has its own strengths and weaknesses. Because of its thermal properties, diversity, and comparatively inexpensive price tag, vinyl has quickly become the most popular of window frame materials.

The High and Low of It

Replacing old windows is no longer buying a simple piece of glass enclosed in a wood frame. Knowing what to look for in replacement windows can make a big difference, for both the shopping experience, energy efficiency, and lower heating and cooling bills down the line. It boils down to knowing the high and low of it; that is understanding the four ratings discussed above. All other aspects of a window’s makeup—glazing, frame, etc.—will be reflected in those four numbers.

Quality, energy efficient windows will also carry the Energy Star label—a quick and easy way to filter out the competition.

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