Energy Efficient Windows

How to Make Efficient Windows with Window Retrofits

Inch for inch, the average window loses ten times as much heat (or coolth) as the average wall. Yet, just as with insulation, energy efficient windows have the best payback during construction. Once the house is built, it takes a very long time to recover the cost of new windows from energy savings alone. However, if you need to replace windows anyway, and maybe your air conditioner or furnace, this is the time to pay extra for superior insulating value; it may allow you to downsize the A/C unit or furnace as well.


Ways to increase window efficiency

Try to “tune” your windows, to maximize energy efficiency and comfort based on the windows’ orientation:

  • North-facing window: high U-factor, to maximize insulation, high visible transmittance, to maximize visible light gain
  • East- and west-facing windows: low solar heat-gain coefficient to minimize summer heating, when the sun’s rays are horizontal during sunrise and sunset.
  • South-facing windows: high solar heat-gain coefficient, to maximize solar gain in winter, if you live in an area with winter-heating. Install an overhang or awning over south-facing windows to minimize summer heat- gain. This will not interfere with winter heat gain, since the angle of incidence of the sun is low, but will greatly reduce unwanted summer heat. It is a good idea for all windows to have infrared-reflective coatings, which keep heat out in summer, and heat in during winter.
  • Finding manufacturers making windows over R-5 is not easy. Reported extra costs for the windows range from 10 to 15 percent, to $1,000 dollars per window.
  • This is not an established market, and your negotiation skills may determine the price more than anything else. US:, (R-12). Canada: (R-14, 5 pane)


Window retrofits

Rather than replacing your windows, provided they are in reasonably good shape, it is cheaper to retrofit them for higher insulation value. Retrofitting your windows with a layer of acrylic may cost you around $25/sq. ft. (installed). It would take a while to pay for the retrofit out of the savings.


Window kits

Window kits, available at hardware stores, are sets of shrink foil with double-sided tape. You apply the double-sided tape to the window frame, apply the foil, and tighten it by heating the foil with a hair-dryer. The resultant film is crystal clear, and can be cleaned with a sponge (though it is susceptible to sharp objects).

The window kit will save you the equivalent of 2.5 gallons of heating oil per square yard per year if you have double-pane windows, five gallons on single-pane windows, in addition to saving you money on cooling energy.

Window kits that are mounted so as to get a lot of direct sun exposure may only last a single heating season. Otherwise, they can last many years. For example, a window kit I installed on our east side 13 years ago, additionally protected by an overhang, is still going strong. That little square yard of plastic film has saved us nearly $100 of energy in the last decade. Not bad for $1 of window kit and five minutes of my time.

New windows that open by means of a crank can usually receive the window kit as a permanent installation. In older slider or double-hung windows, it may only be possible to apply the kit for the winter season, if at all.

Carbon Buster/Miser Recommendation: Install window kits (“shrink foil”) on 50 percent of your windows.

5-year savings: $428, 3,300 lbs. CO2, 27,500 cu. ft. of natural gas.

Life-time savings: $171, 1,300 lbs. CO2, assuming life of two years.

New cost: $147; $23 for materials to cover 18.5 square yards of window, assuming 25 percent of foil is wastage, and four hours of your time at $10 per hour. You will have to apply the window kit 2.5 times over five years.

Payback new: 0.7 years. IRR: 101.9%. CROI: 20.9 lbs./$.


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