Energy Efficiency
Air Leakage
Energy Analysis
New Homes
Window Coverings

 University of Delaware house

Photo Courtesy of DOE/NREL, University of Delaware house

New windows can add to the beauty, comfort and energy efficiency of your home. But they are a major long-term investment, so it’s wise to do your homework before deciding on replacements.

If energy efficiency is the main reason you’re considering a window replacement, be sure to consider the other measures your home needs as well. Insulating your home and sealing up air leaks while you have your windows replaced can dramatically increase the energy savings you will enjoy from your overall project.

Among the benefits of efficient windows:

Energy Savings
Inefficient windows can leak air and make homes uncomfortable. They can add 10-25% to heating bills and up to 75% to summer air conditioning bills. As with every energy upgrade, actual savings are dependent on many factors unique to your home.

Comfortable Living
Mountain home located in Sunshine canyon above Boulder, ColoradoWindows are a primary source of home comfort problems and energy-efficient windows will make a noticeable impact on your home living environment. Imagine being able to sit on the couch in front of a window and not feel cold. And efficient windows make a big difference in blocking out noise from outside..

Taking a whole house approach to energy efficiency helps the environment. New windows can lead to energy bill savings of more than 20 percent, with resulting decrease in greenhouse gas emissions.

Photo Courtesy of DOE/NREL, Mountain home located in Sunshine canyon above Boulder, CO

Checking out equipment and ratings

There are numerous variances in how windows are manufactured and installed, and some technical criteria by which their efficiency is measured ( and rebated). To fully explore window options, visit the Efficient Window Collaborative at (

  • Solar Heat Gain Co-efficient (SHGC) measures the amount of solar energy passing through the windows. Efficient windows can have a SHGC below 0.3, and the lower the number, the more efficient

    The U-Factor measures the amount of heat (in BTUs) that moves through the window. The lower the U-value, the better the overall insulating value. The most efficient windows have a U-Factor of 0.3 or lower.
  • Air Infiltration or Air Leakage measures the rate at which air passes through cracks in the window. It is measured in cubic feet of air passing through one square foot of window area per minute. The lower the value, the less air leakage. The best windows have an air leakage rating between 0.01 and 0.06 cfm/ft.

Other window considerations include:

2009 Solar Decathlon Gallery of Homes 2009 Solar Decathlon Gallery of HomesPhotos Courtesy of DOE/NREL, 2009 Solar Decathlon Gallery of Homes

  • The number of windows and where they point. Increasing the south-facing window area will let the sun provide more heat in the winter—but they should be shaded in summer. Reducing the east and west facing window area, or shading them, will keep your home cooler in the summertime.
  • Multiple layers of glazing. A double layer has twice as much insulating value as a single layer.
  • Thickness of air space. Many window manufacturers have increased the thickness of the air space in their double-glazed windows.
  • Low-conductivity gas fills. For gas fills like argon and krypton, ask where the gas is filled. If the gas-filled window experiences a high altitude variance in shipping, it is possible for the invisible gas to escape.
  • Low-e coatings. Thin, transparent coatings of silver or tin oxide permit visible light to pass through, but effectively reflect infrared heat radiation back into the room reducing heat loss
  • Edge spacers. The edge spacer holds the panes of glass apart and provides the airtight seal in an insulated glass window. Aluminum has been widely used for decades, but it has extremely high conductivity. Now, other materials that have less conductivity are being used and/or thermal breaks are incorporated into the window design.
  • Frames. Consider frames that are wood, vinyl or fiberglass.

Questions to ask contractors
Because windows are expensive, it is important to choose a quality company that will be able to stand behind its warranty long-term.


  • National Fenestration Rating Council ( is a non-profit organization that administers an independent rating and labeling system for the energy performance of windows.
  • American Architectural Manufacturers Association ( is the source for performance standards, product certification, and educational programs.
  • Efficient Windows Collaborative ( provides unbiased information on the benefits of energy-efficient windows, descriptions of how they work, and recommendations for their selection and use.

References, licensing, insurance
It is always important to speak with previous customers, check with the Better Business Bureau and ask for verification of the necessary licensing and insurance requirements. Download a home improvement hiring checklist from the Federal Trade Commission (


Get free estimates now.
301521_Be Green 728x90