What Is a Good U-Value for Windows?

Oct 13 2022 Posted By Ontario Window Reviews

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White French windows facing a garden

Before you have new windows installed in your home, it is important to check their U-value

U-value for windows pertains to how well they insulate temperature. This simple number defines the efficiency and overall quality of your new windows. The lower the number, the better the insulation levels. The U-value varies widely, but there is a standard measurement that provides the best insulation—it is the rate by which ambient heat is transferred through a structure (e.g. windows, doors) from a warm area to a cold area.

In Canada, if it ranges between 0.6 to 2.0—U-factor (W/m² – K)—that’s the perfect value. In the States, 0.20 to 1.20—U-factor (Btu/h/ft² – °F)—is a good measurement.

However, there is more to it than just that. This guide will delve into the importance of window U-value and energy efficiency

Three black windows and a radiator in an apartment unit

Benefits of Having a Good U-Value for Windows   

Having a good U-value for windows entails a steadier and cooler temperature during the hot summer season, especially when you have your air conditioner on. This also applies to maintaining indoor warmth during the cold winter months

By keeping the temperature at optimal levels, you also save exponentially on energy expenditures. 

3 Steps to Determine If a Window Has A Good U-Value Rating

1. Does it Have Multiple Panes?

Check if the window has more than one pane. If it’s double or triple-glazed, it’s bound to provide proper insulation. 

2. Check the Spaces Between the Panes

Windows with a good U-value have argon or krypton gas filling in between the glass panes, and not just air. Argon and krypton are colourless, odourless, and non-toxic gasses that help reduce the transfer of heat through window glass. 

3. The Window Frame

A reliable window has an equally reliable frame made of high-quality vinyl. Wood frames insulate just as efficiently as vinyl does, but the former tends to require a lot more maintenance. Hence, vinyl is a great alternative to wood frames

Other Energy Performance Metrics Associated with U-Value 

SHGC (Solar Heat Gain Coefficient)

You will find the SHGC measurement on nearly every window label, most likely right next to the U-value. 

The SHGC is a highly important metric that represents the fraction of solar radiation that passes through a window. This factor differs from the U-value, as it measures direct solar heat and not the transfer rate of ambient heat.


Another metric to take into account is the R-value. This number represents the amount of thermal resistance. Supplementing the U-value, the R-value offers further insight into how well a window can insulate.

While they may sound like they have a similar purpose, their differences lie in the fact that R-value determines resistance to heat transfer, whereas the U-value of windows measures the rate of heat transfer. 

So, when purchasing new windows, look for a lower U-value and a higher R-value.

Air Leakage

This element measures the air movement that occurs in situations wherein a difference in pressure is detected on either side of the window glass. You can find this information on the NFRC label.

Energy Rating

The energy rating details three energy performance metrics combined:

  • U-value;
  • SHGC; and
  • Air leakage. 

Visible Transmittance  

Visible transmittance is the amount of visible light and glare that can travel through window glass. This energy performance metric can be located on most labels alongside the SHGC and U-value.

Two people dressed in uniforms installing a window with a white frame

Get Great U-Value for Windows  

This concludes our comprehensive guide on U-value of windows. So now, the next time you go “window shopping,” you’ll be able to better understand the U-value/U-factor ratings on NFRC or CSA labels, along with the other related energy performance metrics. 

However, aside from superb energy performance ratings, you also need to make sure that your windows are durable, secure, and long-lasting. So get the best windows from trusted and acclaimed brands—browse through our list of trusted window companies today!

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