Frequently Asked Questions (FAQ)

Who should I buy windows from?

A consumer should select a window manufacturer who manufactures and sells energy-efficient products (i.e. meets the recommended U-factor and Solar Heat Gain Coefficient in your climate), has a reputation for service and stability, and provides a warranty on the unit. We recommend that you buy products from an Efficient Windows Collaborative member manufacturer since these manufacturers have made a commitment to participating in the National Fenestration Rating Council (NFRC) certification and labeling process and in the ENERGY STAR Windows program. Many manufacturers will also participate in the Fenestration and Glazing Industry Alliance (FGIA) and will have a permanent label on the product indicating their participation. Others will participate in the Window and Door Manufacturer’s Association (WDMA), also with a corresponding label or insignia. Membership in all of these organizations is an indication that you are dealing with a manufacturer who cares about the quality of their product and the product’s energy efficiency.

Where do I go to buy an efficient window?

Before you buy a window, we recommend that you do your research via the web and written material and then shop around. Windows are complicated devices, made more complicated because recommended window types vary by climate. Climates where heating is very important to have different product recommendations than climates where cooling is more important.

First, educate yourself about what kind of window is right for your climate and needs. Learn about the NFRC and ENERGY STAR labels and why they are important. We recommend using this Window Buying Guide to get started.

Once you have an idea of what you want, visit retailers or distributors. Look for the product you want within your price range. Ask questions of the sales staff and let them tell you about the energy efficiency features of the products. Remember to:

  • Look for the ENERGY STAR.
  • Look for the NFRC label.
  • Select a product that is right for your climate and your pocketbook.

Sometimes a homeowner feels more comfortable with a particular contractor rather than a particular product. Installation is a very important issue with windows. Ask your contractor to describe the installation process, and be sure that they are approved by the window manufacturer. Ensure your contractor knows as much about ENERGY STAR products and NFRC labeling as you do.

After the window has been installed, make sure that you keep the NFRC and ENERGY STAR label. You will need this to qualify for any rebates or tax incentives.

Why are my windows drafty and will new windows stop the draft?

Drafty windows and air infiltration could have several causes. Improper installation is often the culprit but not always. If the windows were improperly installed, the defects can normally be corrected without removing or replacing the window. Proper caulking and flashing should prevent perimeter air leakage. However, if the installation twists or racks the window frame causing the sash weatherstrip not to be able to seal the sash to the frame, removal of the window to correct the problem may be necessary. You can determine whether the air is coming in at the joint between the frame and the house or the joint between the sash and the frame. Hold a lighted candle or lighter in front of each joint when the wind is blowing and slowly move it along the joint. If the flame flickers strongly or goes out, the joint is probably leaking. CAUTION: BE CAREFUL NOT TO CATCH THE DRAPES OR BLINDS ON FIRE!!!

If the problem is installation, you will have to work with the builder or a contractor to correct the problem. If the leak is between the frame and the sash and the window has been installed square, plumb, level and is not twisted or racked, the problem may be construction of the window or window design. If your windows are not efficient in your climate and particularly if it does not have insulated glazing, the air may not be leakage but simply cold room air flowing down the surface of the window. As the window surface becomes colder, it cools the room air next to it. This cooled room air is then heavier than the warmer room air further into the room. Because the cooled room air is heavier it sinks toward the floor and is replaced by warmer room air, which is then cooled. This process can result in noticeable air flow but it is not air leakage from the outside. Replacement of the window by an energy efficient window is the solution here.

If the problem is the construction of the window in a new house or a recent retrofit, you will have to work with the builder or contractor and the window manufacturer to correct the problem. Your ability to get satisfaction here will depend greatly on how the window was specified in your contract or by the builder and your ability to negotiate with these parties. You certainly should be entitled to receive what you specified and paid for. However, if the specification was vague or non-existent, you must negotiate to improve your situation.

Will new windows eliminate condensation?

Condensation is a direct result of interior humidity and the difference between indoor and outside air temperature. If you keep the humidity in your house low, then the likelihood of experiencing condensation is also low. However, the efficiency of your window will also impact the temperature and humidity level at which condensation occurs. Energy efficient windows will help reduce condensation. Here’s why: high performance windows with low U-factors result in inside glass surface temperatures much closer to the room air temperature. Windows with warm edge technologies and non-metal frames are also less likely to have condensation on the frame or at the edge of the glass. Insulated “superwindows” with three or more layers will virtually eliminate condensation on the interior surface of the glass—even under extreme cold weather conditions.

Note that in certain conditions (humid mornings after a clear night sky), some superwindows may have dew on their outside surface. These windows are such good insulators, the dew is condensing there just like it does on an insulated wall.

Does the gas fill leak?

The rate of dissipation can differ greatly depending on the quality of the glass unit’s seals. Studies have shown that well-fabricated units can retain gas at a loss rate of less than 1 percent per year, resulting in only a minimal reduction of insulating performance.

However, failure of the insulating glass unit seals would release all of the gas. The quality of seal systems depends on the seal type and the quality assurance in the manufacturing process. Insulating glass in ENERGY STAR windows must be certified through recognized certification programs, ensuring that product samples have passed durability testing under extreme conditions and that manufacturers implement in-house quality assurance. Most manufacturers offer a warranty against insulating glass failure which varies from a limited period to the lifetime of the window. A range from 10 to 20 years is common.

What is the difference between low-E types?

All types of low-E coatings reduce the heat transfer from a warm window pane to a cold window pane. They thus reduce the window U-factor, which is particularly helpful for insulating the house during cold seasons. However, low-E coatings differ in how transparent they are to solar heat. Different types of low-E coatings have been designed to allow for high solar gain, moderate solar gain, or low solar gain. These differences are reflected in the solar heat gain coefficient (SHGC).

Windows with low-E glass generally have a lower U-factor and insulate better than windows without low-E. The SHGC of a window depends greatly on the type of low-E that’s used. Windows with moderate-solar-gain low-E typically have a SHGC of around 0.25-0.35, but higher or lower values are available with high-solar-gain or low-solar-gain low-E glass. What level of solar gain is most desirable depends on the climate, orientation, shading conditions, house design and personal preferences.

What is the conversion from US (imperial) to European (metric) units for the U-factor?

To convert the U-factor from English to SI units, multiply the English number by 5.678. For example, If U=0.35 Btu/hr-sf-°F in English units, then 0.35*5.678 = 1.9873. The U-factor in SI units will be 1.9873 W/m²K. The SHGC is dimensionless so it has the same value in both systems.

About Us

provides unbiased information and education to the public on the energy efficiency, technical, and human considerations that influence window selection and use. The EWC collaborates with allied organizations, educational institutions, and government agencies that promote energy efficiency in residential windows. We do not sell, service, install, or recommend products, or endorse manufacturers. Descriptions and specifications of the products in the Window Selection Tool are provided by the manufacturer. The EWC is not liable for any manufacturing defects or inaccurate product descriptions and specifications and does not guarantee the performance of NFRC-certified windows, doors, and skylights.

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