5 Factors Affecting Window Energy Efficiency Ratings

Some of the energy you use to heat and cool your home is lost through inefficient windows. During the summer, your air conditioner must work harder to cool hot air entering from drafty windows. Also, cold drafts coming into your home during winter are an important sign that it’s time to replace your windows.

Investing in energy-efficient windows will help you save energy and money.  Your exact savings will depend on climate, local energy costs, and your windows’ energy efficiency ratings. These ratings are important to understand when buying replacement windows.

The windows rating system informs homeowners on how much heat and light can be transmitted through a window.  The energy performance rating also shows how much air leaks through windows. Knowing the energy performance rating can help homeowners determine how they can expect their home's utility bills to change when they install new windows.

What Are Energy Performance Ratings?

window energy efficiency ratings

Energy performance ratings are the measurement of a window's energy efficiency. Energy performance measurements are determined by the National Fenestration Rating Council (NFRC), an organization that tests and certifies windows, doors, and skylights based on energy performance ratings.

The National Fenestration Rating Council has designed an energy-performance label to guide contractors and homeowners in their search for the perfect window. Rated windows are labeled to make it easy for consumers to compare products. Every ENERGY STAR-qualified window has an NFRC label.

What You Need to Know about Energy Efficiency Ratings

To be sure you’re buying energy-efficient products, it’s important to understand window efficiency ratings. Window efficiency is measured by the energy performance indicators displayed on the Department of Energy’s blue-and-yellow ENERGY STAR label, and the white National Fenestration Rating Council label, which is often found in the lower right-hand corner of the window.

  • The nonprofit NFRC organization is the industry-recognized certifying body for windows and doors.
  • The Energy Star label means the appliance has met strict energy efficiency criteria set forth by the U.S. government.

Energy performance ratings on windows are important because they can help the homeowner gauge his or her expected energy savings if they decide to install new windows. Ratings also help homeowners compare one window to another as they try to decide which window is right for them. The NFRC label typically lists five measurements that can help you judge how well a window will perform.

1. U-Factor

The U-factor is a measure of how well a window prevents heat from escaping your home or the rate of heat loss.  Resistance to heat flow means a window is insulated. The higher the resistance, the better the insulation. The entire window, including the glazing, spacers, and frame, are considered when measuring the U-factor. 

The U-factor range is between 0.15 and 1.20. More insulation produces a lower U-factor number. The lower the value, the more effective the product is in keeping heat inside a home or building. This measurement is particularly important in cold climates. The most energy-efficient windows will have a U-factor between 0.15 and 0.30.

2. Solar Heat Gain Coefficient (SHGC)

SHGC refers to the solar radiation a window allows inside. The range for SHGC is between 0 and 1. The lower the number, the less heat that comes through the glass. Find the lowest possible SHGC rating in warmer climates to minimize the use of air conditioning. Look for a slightly higher number in colder climates so that the sun can help heat your home in the winter.

Less solar heat gain is effective when temperatures are warm outside. Higher SHGC values usually come with higher U-factors, which can impact a home's energy efficiency.

3. Visible Transmittance (VT)

Visible transmittance is a measure of how much light a window lets in, ranging from 0 to 1. The higher the number, the more light is admitted into your home. A lower number means the room will be dimmer. With older window glazing techniques, VT and solar heat gain were the same; the brighter the room, the hotter it got.

New technologies allow windows to let in lots of light while the room stays cool.  Pay close attention to VT numbers if you're looking to fill your home with natural light.  Now with Low-E coatings and other break-through technologies, a window can have high visible transmittance with a low solar heat gain. This ensures that the room gets light without becoming hot and uncomfortable.

Homeowners who cut back too much on visible transmittance may find themselves using artificial light even during the daytime. Also, house plants can die without enough light.

4. Air Leakage

If you want to know how much air penetrates your home or building, this measurement is important. Air leakage typically ranges in value between 0.1 and 0.3, with 0.3 being the standard building code.  The lower the value, the more effective the window keeps air out.  Air leakage can change over time, as materials expand, contract, or become warped.

Energy Star standards don't consider air leakage because it's difficult to measure accurately and can change over time due to the changes in frame materials.  This measurement can help you compare similar products, especially if they'll be buffeted by the elements.

5. Condensation Resistance

Condensation resistance is a measure of how much moisture a window allows to build upon the surface. Condensation resistance is expressed between 1 and 100. The lower the number, the more condensation the window allows to build up. Too much moisture can cause mold or discoloration. This energy-efficient indicator is an optional rating.

When Is It Time to Replace Your Windows?

If it seems like your energy bill increases each month, it’s likely your windows are not functioning as they should. Replacing the windows in your home with energy-efficient replacement windows can help lower your heating and cooling costs, which will save you money each month. Replacing your windows can also improve the security of your home with stronger glass and advanced locking systems.

The best way to ensure that the windows you buy are best for your needs is to work with a professional window contractor. Your window expert can work within your budget to recommend windows that will be energy efficient.

With home improvement projects being costly and time-consuming, it’s common to put off replacing home windows until absolutely necessary. However, your old windows will only get less efficient as time goes on.  The only way to improve the situation is to replace them with premium energy-efficient windows.

Here are signs to look for that will let you know your windows are failing.

  • If your windows provide little protection from the sounds of the outdoors, they may not be sealed correctly or were poorly made or installed to begin with. Double- or triple-pane glass windows insulated with Argon or Krypton gas can decrease outdoor noise, which is especially helpful if you live in a busy neighborhood or on a high-traffic road.
  • When you feel a draft, even when your windows are closed, it’s a sure sign new windows are needed.  A drafty window can drastically change the overall temperature of your home, causing your HVAC system to work twice as hard to keep up with the fluctuating temperature.
  • If your window frames are soft to the touch and chipped, they may be already beyond repair. Soft window frames indicate rot and water infiltration. They may start sagging and warping.
  • Windows that were not installed correctly tend to develop balance issues, making the windows challenging to open and close. Rust or rotting can also cause difficulty when opening a window. If your windows do not close properly, you may not be able to lock them, which can impact the safety of your home.
  • If you notice that your windows are frosting between the layers of glass, it means that your seals have failed, allowing moisture to form between the glass panes. Once the seals fail, any insulating gas used to make the window energy efficient is now gone.

Choosing Your Replacement Windows

The typical process of choosing windows is to start with the types of windows. Once the type of window has been selected, the material of the window is next. It’s important to know what you need for your home, the design function for each window, and the amount you’re willing to spend. Choosing a window material that can withstand the climate you’re in is important; otherwise, you’ll be repairing or replacing them often.

It’s important to take your time when selecting the new windows for your home. Getting it right will add value to your home, improve its appearance and make it more energy efficient. There are six main materials that windows are made of. They include wood, vinyl, aluminum, fiberglass, wood-clad and composite. With all these choices comes a lot of pros and cons to weigh and consider.

The Window Professionals

Knowing the meaning of these energy-efficient indicators can help you decide which window is right for your home. Make sure you’re getting an energy-efficient window by checking with your windows installation contractor on these ratings.

Banner Construction offers quality windows, like Pella and Simonton. We are certified installation contractors for both and ensure performance in each of our installations. Our teams are experienced, licensed, and certified. If your windows need to be replaced, give us a call today to learn more about our products and services.

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