Understanding Window U-Factor

Energy efficiency is the number one concern for most homeowners when deciding what windows to install on their home. While there are many different features that contribute to a window’s overall energy efficiency, u-factor is one of the most important.


In order to truly have a good understanding of the windows that’ll be on your home, you must know the terms involved with them. A window’s u-factor is crucial to know because it can tell you if your windows will hold up well during the cold months of winter.

Luckily, we have you covered on everything there is to know about the u-factor of windows. Let’s get started!

What Is U-Factor?

Windows get rated by multiple factors that are important to its overall energy efficiency rating. U-factor is used to measure the insulation properties of windows but is not to be confused with r-value.

The u-factor system, also known as u-value, was created by the National Fenestration Rating Council (NFRC). The NFRC is a nonprofit organization that’s responsible for rating the energy efficiency of building components and is widely recognized by the window industry.

In fact, they’re so impactful that the US Department of Energy takes u-factor into consideration when they decide whether or not a window is deemed Energy Star Certified.

The u-value of windows describe the window’s ability to keep heat inside your home. If a window has a low u-factor, it’s more energy efficient compared to one with a higher u-factor.

Midwest homeowners need to keep an eye out for the u-factor when they’re moving into a home, installing new windows or replacing old ones. The winters get extremely harsh and you need every bit of heat to stay inside your home.

If your home is constantly losing heat in the winter, you’ll have to rely heavily on your furnace to keep it at the temperature you want. Your heating bills will sky-rocket in this scenario.

Do yourself the biggest favor by making sure your windows have a low u-factor. We’ll go over what determines the rating below.

How Is Window U-Factor Determined?

In terms of the actual formula to determine u-factor, it’s rather simple. However, as a homeowner you most likely will never have to worry about calculating anything yourself. The values are usually already labeled on the windows.

But, it’s better to know what actually goes into it so you have as much knowledge as possible when deciding what windows to go with. We mentioned r-value earlier and it comes into play when determining u-factor.

R-value measures a material’s resistance to heat. U-factor is the opposite of that and measures a window’s ability to keep heat from escaping. In order to find out a product’s u-factor, simply take the inverse of it’s r-value. ( U-factor = 1/R-value)

You can learn what goes into determining r-value in this article that we wrote.

Now that you know how to calculate u-factor, how do you know if that number is good or not? The best performing double or triple pane windows will usually have a u-factor of .30 or lower. The lower the number, the more energy efficient they are.

Why Is U-Factor So Important?

window u factor

You know what u-factor is and how it’s determined, but why does it matter so much for your windows? Since u-value is the measure of how well a window will keep heat from escaping your home, it’s one of the most important features of windows.

U-factor helps you save money in the winter when you need to have your furnace running. Windows with a poor u-factor will not be able to keep that heat indoors and will force you to crank the furnace up.

For those that live in states that get extremely cold like Illinois and Wisconsin, it’s imperative that your windows have a low u-factor.

Stay Comfortable with Low U-Factor Windows

Where can you get windows with low u-factor? The answer is Feldco. We’re the Midwest’s leading window company and all of our windows are made of energy efficient vinyl and are Energy Star Certified with a low u-factor.

With over 400,000 happy homeowners, it’s easy to see why people trust Feldco for the most energy efficient windows on the market.

If you find that your windows aren’t doing their job properly anymore or they don’t have a low enough u-factor, get a free quote right now for the best replacement windows in the Midwest.

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