Friday, June 26, 2009

How efficient is my window

If your thinking of taking advantage of the current energy tax credits to replace your windows, you might want to consider just how much bang for the buck you will get.

Getting an energy efficient window may be one of the most complex and least understood improvements that can be made to a home. For example, what does it mean to have a low-E window? What about gas-filled, double-pane windows? How much will you save by replacing your old single-pane window if it's still in good shape? Unfortunately, your window vendor may not be familiar with what works best in your situation and how much you stand to save.

Before laying out big bucks, here's some factors you should be aware of and discuss with your contractor:


You should see the following factors included on the "NFRC label" (National Fenestration Rating Council) included with a particular window. You'll want one rated appropriately for the local area.
  • U-factor: this is a measure of heat transfer or insulation value. The Energy Star recommendation for the Cincinnati area is < .40 for windows and doors. If you are closer to Dayton, the suggested rating is < .35.
  • Solar Heat Gain Coefficient (SHGC): identifies how much solar heat passes through the window. The suggested rating can vary based on the U-factor, but in Cincinnati you'll probably want a rating < .40. Further north, the rating becomes something of a non-issue as long as you get the right U-factor (although a higher heat gain further north can be a plus for longer winters)
  • Visual Transmittance (VT) is a rating from 0 to 1 indicating how much light passes through (the closer to 1, the more light comes through).
  • Air Leakage (AL): Measures air passage per minute. The lower here, the better. The baseline standard is <= .3
  • Condensation Resistance (CR): Rated from 0 to 100 on how much condensation will build up on the window. This is another rating where the higher, the better.

When evaluating the type of window you want to use, here are construction factors to consider.
  • Low-E coating: a mettalic coating placed on the window surface that allows light to pass through, but is useful in reducing the U-factor of a window.
  • Gas fill: A typical double-pane window will use Argon or Krypton gas between panes to increase the insulating value.
  • Frames: Perhaps the most commonly marketed windows are either insulated vinyl or wood. Many prefer the look of wood, but from a maintenance and efficiency perspective the insulated vinyl may be a better bet. Other frame types exist including aluminum and wood clad.
  • Spacers: used to hold the window panes at a certain distance apart, manufacturers have made improvements in the type of spacer used to further increase the insulation value

Most homeowners automatically assume that replacing windows will translate into big savings, especially if they use an Energy Star rated window. However, the Energy Star office study shows that savings in our area equate to about $377 per year in a typical 2000 sq. ft. home when replacing single pane windows. As an energy auditor often tells me, this is one of the least efficient paybacks for a home. If you just laid out $10,000 for new windows, it could take over 25 years for the savings to pay for those windows - just about time to think about replacing them again.

The bottom line here is if you are replacing your windows in a typical home (and not for special applications like passive solar heating), it's good to consider the energy efficiency of the window, but be sure you are doing it for the right reasons. Replacing rotten or deteriorating windows and increasing the attractiveness of the home may be stronger considerations than the few dollars you'll save on energy bills.

Other resources:

You can find more detailed information about window technologies, ratings, and other energy saving tips at the following sites:

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