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Heat Loss and Gain: Windows, Doors, and Skylights Are Keys to Your Comfort

The heat loss and gain in your Chicago are home depends in great part on your doors, windows, and skylights due to thermal energy transfer. As a result, your home’s energy efficiency is directly related to the number, size, and materials from which your doors and windows are made. The more they can resist thermal energy transfer, the smaller is the amount of heat loss or gain for your home.

The energy efficiency of window and door materials is rated by the National Fenestration Rating Council (NFRC). The Council evaluates the efficiency, and reports its findings to the U.S. Department of Energy, which assigns the Energy Star rating to exceptionally efficient windows, doors and skylights.

Not all windows, skylights and doors measure up to the Energy Star label, but all must carry efficiency ratings to help consumers judge overall heat loss and gain from these products.

The NFRC evaluates the efficiency of windows, skylights, and doors based on the following criteria:

  • U-factor. The U-factor indicates how well the window prevents heat transfer from the inside of the building. A low U-factor indicates better efficiency.
  • Solar heat gain (SHG). This rating goes from 0 to 1 and it tells you how much the window resists transferring heat from the sun. Lower ratings indicate that less heat gets through the glass.
  • Visible transmission (VT). VT is another rating from 0 to 1 and tells you how much light the window lets through. A rating close to 0 indicates very little light transmission.
  • Air leakage. Doors, windows and skylights are rated by air leakage rates around their frames. This rating is expressed in cubic feet per minute for each square foot. Lower ratings are better.
  • Low-e. Although it’s not a rating, low-e (emissivity) may be a term you hear regarding window efficiency. Low-e windows have a coating that prevents infrared heat transfer inside and out. A low-e window will keep heat out and reduce the amount leaving your home through the glass.

Improving window, door, and skylight efficiency is easily accomplished by replacing inefficient products. While these upgrades often aren’t in a homeowner’s budget, other options are available that can improve energy efficiency. A professional energy audit will show you where you have air leakage and thermal losses. Then you can minimize these deficiencies by taking steps to save on energy.

Ways to improve energy efficiency


  • Seal the cracks around the window frames outside. Many homes have a layer of caulk between the home’s exterior and the window frame that dries out over the years. Silicone or acrylic caulk will tighten the frames and reduce air leakage.
  • Use storm windows in the winter. Storm windows reduce air leakage and serve as a thermal barrier to stop heat gain and loss year-round, They are also called storm panels and can be hung inside or outside.
  • Use thermal drapes. These are heavier than curtains and may be lined with foam or thermal fabrics to resist heat loss and gain. They should be hung close to the glass and fully cover the window.
  • Install reflective films. You can emulate the efficiency of a low-e window by applying solar window film. It will darken your windows slightly, and add privacy, similarly to tinted car windows.


Over time, the weatherstripping around exterior door frames can compress, which increases air leakage. It’s relatively easy to remove the old product and apply new weatherstripping that will tighten the seal at the top and the sides. A crack at the base of the door can leak air as well, but you can seal it with a door sweep screwed to the base or on the bottom. Draft stoppers made from fabric also will block air leaks, as well.


While skylights are an attractive option, if they’re placed on the southern side of the roof, they may create too much heat loss and gain winter and summer, unless you choose products that have high efficiency ratings. In the event you have an existing skylight and have noticed it generates too much heat in the summer or loses heat in the winter, consider adding a movable window covering.

These are specialized coverings, some of which have their own motors and open and close easily. If your skylight has a flat surface, you may be able to apply solar reflective film that will work efficiently year-round.

To learn more about heat loss and gain through windows and doors, please contact us at Comfort24-7.com to find a professional HVAC contractor in the Greater Chicago area, southwest Michigan or northwest Indiana.

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