What Are U-Values for Windows? Why Do They Matter?

Alright, so you’re browsing your local home improvement store looking for new windows for your home. There’s so many windows, how do you know which one is better than the other?

Well, energy efficiency, of course! When homeowners think of new windows, they always think energy savings or energy efficiency.

One way to determine the window’s energy efficiency is by looking at its U-value, which is also called U-factor.

U-values are used on window energy ratings to tell you about the window’s ability to insulate against heat.

window specifications at home depot

What Do U-Values Measure?

So U-values are a good number to know when buying windows, but what exactly does it measure?

U-values measure the amount of heat loss and how fast it’s lost. It’s measured in watts per square meter Kelvin (W/m²K). Now this may sound super confusing, and it is.

You can find U-values for walls, roofs, windows, doors, floors, and more!

Luckily, you don’t need to do any math because U-values are given to you in a window’s specification sheet. If you’ve bought windows in the past, you’ll see this number on the sticker that comes attached to your window.

All you need to know is that a lower U-value equals better performance for your window when it comes to retaining heat in your home in the winter and not letting heat enter in the summer.

Why Do U-Values Matter to You?

Well, not all windows are made the same. Some are better than others when it comes to managing your home’s ideal temperature.

If your home is old with single-pane windows, the U-values are probably higher than 5 W/m²K. Newer double-pane windows have U-values that are less than 1.6 W/m²K. This is a large improvement when it comes to energy efficiency.

Knowing this number helps you make purchasing decisions that made you confused before.

It also gives you more confidence in your new window investment.

How to Find the U-Value of a Window

window label
Image via NFRC

If you’re just browsing the window aisle of your local Home Depot or Lowe’s, you’ll find the U-value or U-factor labeled on an attached sticker or on the specifications sheet underneath the window.

If you’re working with a window manufacturing company, you can always ask them. Make sure that you are looking at the U-value for the entire window, and not just the center pane, which might give you a better number.

The entire window includes the frame and edges, where most of the heat is lost.

R-Value Vs U-Value: What’s the Difference?

U-values and R-values are thrown around everywhere in the window world. Are they the same? What exactly is the difference between them?

Both U-values and R-values measure a window’s performance related to heat transfer, but they measure different aspects.

However, U-values material how much heat is transferred and the speed. Basically, it measures heat flow through the material you’re testing. A lower U-value has better energy efficiency.

R-values, on the other hand, measure the material’s resistant to heat. What is its ability to stop heat flow? A higher R-value means it has better energy efficiency.

U-values are stated in the units W/m²K, whereas R-values are expressed as m²K/W.

U-values and R-values are inversely related. To get the R-value from the U-value, divide 1 by the U-value.

I’ll show an example below.

calculate u value and r value

What Affects U-Values?

There are many ways that U-values can be improved. Look for these features in new windows:

  • Double-pane or triple-pane glass – Triple-pane windows will have better U-values but also will cost more than double-pane windows.
  • Gas-filled windows – Using an inert gas such as argon will decrease the amount of heat that your windows transfer.
  • Low-E coatings on the surfaces of your window glass – These microscopic coatings on your windows will reflect more heat off the glass’s surface.
  • Frame material – Wood, fiberglass, and vinyl frames have better insulation than materials such as aluminum, which transfers heat quickly.
  • Thickness of glass – Thicker glass transfers heat slower.
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