Guide to Energy-Efficient Windows

By Beth Krietsch

Upgrading to energy-efficient windows reduces your carbon footprint, gives the environment a much needed break, and can decrease energy bills by 10% or more. The NFRC (National Fenestration Rating Council) has created a chart that lets you see how much money you can save each year by swapping out old windows for something more efficient.

This guide is designed to help you make a well-informed decision based on specific factors related to your home, the local environment, and your personal preferences. Considering price, environment, design, frame and glass types, and ENERGY STAR and NFRC labels will allow you to narrow your options to find the energy-efficient windows that best fit your home.

What to look for in energy-efficient windows

When looking for new windows, choose an option that allows an agreeable amount of light and heat to pass through, protects against UV rays, and maintains a consistent temperature indoors. Additionally, you’ll want to think about the following factors.

1. Price

Energy-efficient windows are likely to lower your home energy costs significantly enough to outweigh the initial sticker shock of replacement. This is especially true if you’re currently working with drafty windows that are more than 10 years old. So even though the initial cost will be high, the overall savings may be worth it.

2. Design

A window’s design plays a huge role in energy savings, as some models are more susceptible to air leakage than others. Awning, casement, fixed pane, and hopper windows are solid, air-tight options. These are generally more efficient than single- and double-hung standard windows and single- and double-sliding varieties because the design of the latter windows does not allow them to fully seal.

3. Frame

A window’s frame plays an important role in energy efficiency, mainly because of its heat-conducting properties. Here’s what you can expect based on frame-type:

  • Aluminum frames conduct heat easily and thus tend to be poor insulators. For the most efficient metal and aluminum frames, make sure an insulating strip called a thermal break is in place.
  • Wood frames are pretty good insulators, but they can contribute to air leakage when they expand, contract, and rot in response to weather. When weather-appropriate, well-made, and installed properly, wood frames can last for many years.
  • Vinyl frames are often filled with insulation and contain UV stabilizers that prevent sunlight from deteriorating the material, making them more efficient than some aluminum or wood frames.
  • Composite frames are made of composite wood products. These are often more efficient than conventional wood frames, are structurally very sound, and are resistant to moisture and decay.
  • Fiberglass frames can be filled with insulation, making them more efficient than types without this insular filling.

Glass considerations

When buying new energy-efficient windows, consider these factors related to the glass.

  • The number of window panes, usually ranging between one and four, refers to the layers of glass that make up the window. Single-pane windows tend to allow more air leaks, whereas double- or triple-pane windows are more efficient. The space between panes is filled with a layer of insulating gas that prevents the transfer of heat between indoors and out. To learn more about double-pane windows, explore our guide.
  • Glazed windows can prevent the transfer of heat and UV rays between indoors and out. Most glazes cannot be seen yet still do an excellent job of containing heat or preventing a room’s temperature from skyrocketing as sunlight comes through.

Energy efficiency ratings

When it comes time to embark on your energy-efficient window upgrade, consider your local climate, the design of your home, and energy performance ratings such as those from ENERGY STAR, whose ratings indicate whether a window or skylight is energy efficient, and the NFRC, whose ratings evaluate a product across several criteria, thereby allowing you to compare products. You’ll find an NFRC label on all ENERGY STAR–rated products.

Here’s a breakdown of what you’ll find on an NFRC label.

U-factor (or u-value)

This looks at heat escape and how efficiently the window will contain heat within a space. The scores range between 0.2 and 1.2. Look for numbers on the lower end of the spectrum.

Solar heat gain coefficient

This indicates the window’s ability to resist heat gain. The range here is 0–1, and you’ll want to look for smaller numbers.

Visible transmittance

Visible transmittance scores how well a window will allow light into your home. The range will be 0–1, with higher numbers indicating that more natural light is let into the space.

Air leakage

This measures how much air is able to pass through a window and enter a room. The range here is 0.1–0.3. Look for numbers on the lower end.  

If you’re not heading out to the store, you can also search these online directories to see a product’s specific NFRC or ENERGY STAR rating.

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