Energy Saving Ideas

Replacing Inefficient Windows

Replacement | New Energy-Efficient Features | Recommendations

An inefficient window in a well insulated home is little better then a hole in a wall. In fact, heat gain or loss through windows can account for 15 to 35 of a home's utility costs. Fortunately, replacing windows with energy efficiency models can reduce heat gain and loss.

Candidates for Replacement
To improve a window's efficiency, either tighten up the window with caulk and weatherstripping, or replace it with an energy-efficient model. If the existing window has rotted or damaged wood, cracked glass, missing putty or poorly fitting sashes, consider replacing it with a more energy efficient window.

If possible, replace certain types of windows that are energy eaters, such as aluminum-frame windows. They are a common type of window that is cheap and easy to install. However, aluminum conducts both heat and cold, allowing unwanted hot and cold air into the home.

Similarly, if a home has single-pane windows, consider replacing a few of them at a time. Single-paned glass offers less protection from the outside elements than newer, more efficient double and triple-paned glass windows.

Cost
Be sure to spend the few extra dollars necessary to buy high performance windows. While the initial expense is high, new technology introduced over the past decade has increased the potential for new windows to actually lower energy bills. Some local utilities may offer rebates for purchasing energy-efficient windows.

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New Energy-Efficient Features
High performance windows with new glazing technologies not only reduce energy costs, but also make homes more comfortable by moderating temperatures and eliminating drafts. Glazing technologies have improved the window clarity and reduced heat gain from summer sunlight. It also reduces the solar radiation that leads to furniture fading.

This long-term investment has a large impact on a home's energy system. New technologies worth considering are multiple layers of glass, low-emittence glazing, and insulating gas filling.

In addition to energy saving features of the window, it is important to read the labeled energy efficiency rate. Many windows have two labels: a National Fenestration Ratings Council (NFRC) label and an Energy Star label. The NFRC is an independent ratings organization comprised of window manufacturers, retailers, distributors and non-profit energy organizations. NFRC labels provide product-specific performance ratings for technical qualities such as air leakage, visible light transmittance, solar heat gain and U-factor (how well the window transfers heat). In addition to the NFRC label, all high-efficiency windows also come with the familiar Energy Star label, which details energy efficiency and operating costs. Independent NFRC ratings provide the basis for the EEnergy Star window performance requirements.

The NFRC label includes several important technical ratings. First, the energy savings information is divided into two categories of residential and non-residential energy ratings. The label also describes the type of window and gives four energy ratings: U- factor, solar heat gain coefficient (SHGC), visible light transmittance (VT) and air leakage. The U- factor is a rating of how well heat is transferred by the entire window - the frame, sash and glass- either into or out of the building. The lower the U-factor number, the better the window will keep heat inside a home on a cold day. The SHGC rates how well the window transmits the warmth of sunlight. A lower number means less heat is getting through, which can help reduce air conditioning costs during the summer.

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Iowa Ratings Recommendations
Iowa is divided into two climate zones - north and central - by an imaginary line that runs across the state from Sioux City to Mason City to Dubuque. The northern zone needs windows that keep heat in the home, whereas homeowners in the central zone require windows that reflect away summer sun, but seal in winter heat.

Central Zone

• Look for a maximum U-factor of 0.40. A low U-factor will keep more heat inside the home and reduce excess work for your heating system.
  
• Look for SHGC values of 0.40 or less. Windows with a very low SHGC reduces summer cooling and overheating, however, they also reduce free winter solar heat gain.
  
• Select windows with a higher VT ratio to maximize the benefits of natural day lighting and view.
  
• Select windows with an air leakage ratio of .30 or less

Northern Zone

• Look for windows with a maximum U-factor of .35
  
• Select windows with a SHGC of .55 or less.
  
• A high visual transmittance ratio will help with passive solar heating in the
• winter and an air leakage ratio of 0.30 or less will help seal in warm winter air.

More Information and Additional Resources
The benefits of energy-efficient windows, descriptions of how they work, and recommendations for their selection and use can be found online at www.efficientwindows.org. Some of this information was adapted from the American Council on an Energy Efficient Economy's publication, Consumer Guide to Home Energy Savings, 7th Edition, the Energy Efficiency and Renewable Energy Network as well as Homemade Money by Richard Heede and staff of the Rocky Mountain Institute.

Additional information on home energy savings can be found in the Home Series: Home Tightening booklet available on the Iowa Energy Center’s Web site, www.energy.iastate.edu or by calling (515) 294-8819.

The Iowa Energy Center is a nonprofit organization dedicated to improving Iowa's energy efficiency and the use of renewable fuels.

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