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R Value - Learn About Window R Values

Last updated on 04/01/2022


When you start shopping for new windows, you’re going to pretty quickly run into a lot of numbers that can get confusing. You may see discussions about SHGC, U-value, R-value, and more. But what is R-value, and why should you care about it in the first place?

R-value is a measure of how much heat your windows keep in or out, depending on the season. Read on to learn more about this rating, how it’s calculated, and why it matters for your window selection process.

What Is R-Value? 

R-value, or resistance value, is a measure of how well a window insulates, or keeps heat where it’s supposed to be. While this term often refers to insulation, it can also apply to windows, doors, and even camping sleeping pads. In every case, R-value is a measure of how much heat the materials keep from transferring through it.

In the case of windows, R-value is the inverse of the U-value, which measures how much heat a window allows to pass through it. The lower a U-value is, the better the window is. But with R-values, you want windows with higher ratings, since that means they retain more heat in the proper place.

How Is It Determined?

So how is R-value for windows calculated, and what factors go into those calculations? R-value for windows is actually based on the U-value of the window. In order to get the R-value, you divide 1 by the U-value, using the equation R-value = 1/U-value.

U-values are calculated based on the insulation value of both the frame and the glazing on the glass, as well as the linear heat transfer coefficient for the window. This gives you the overall U-value for the window, or Uw. Once you have that number, you can calculate your R-value and determine your window’s insulation value.

What Raises an R-Value?

Now that we know a little more about how R-values are calculated, let’s talk about the factors that impact those ratings. You might be surprised to learn that larger windows tend to have better R-values than smaller windows do. Window glass does a more effective job of retaining heat than the frame does, so windows with a higher glass-to-frame ratio earn higher R-values. 

Adding more panes of glass to a window can also improve its R-value, as does filling the spaces between those panes with heat-resistant gases like argon or krypton. Certain spacers on the edges of window frames can help to keep heat in and can raise the R-value. You can also raise the overall R-value of your windows after they’re installed with blind, curtains, and the like.

Who Sets Window R-Values?

When you start looking into R-values for windows, you may wonder who sets those values in the first place? In many cases, window manufacturers will hire independent labs to test their windows and determine their R-values. This helps to remove bias from the testing process and gives homeowners peace of mind.

Window manufacturers hoping to earn ENERGY STAR certifications, however, will need to go through some additional steps. They’ll need to have their windows evaluated by the National Fenestration Rating Council, a non-profit organization specializing in ratings for windows and doors. The NFRC will assign the window a U-value which can then be used to determine the window’s R-value.

How Is It Tested?

The process of testing the U-value (and by extension, the R-value) of a window is relatively straightforward. Testers begin by placing the window in a tightly sealed, environmentally controlled chamber. This space is known as a hotbox and helps to eliminate any outside factors that may influence the rating.

Testing experts place a hot plate on one side of the window they’re testing and a cold plate on the other side. Then they monitor the temperature on the cool side of the window to see how long it takes to change. When the temperature on the cool side of the window is the same as the temperature on the hot side, the test is complete and the experts can use the data they’ve gathered to calculate the U-value.

Why Is R-Value Important?

So you may be wondering why you should care about what number some scientists in a lab somewhere assigned to your windows. But R-value has a very real impact on your life and, in particular, on your bills. Having windows with a higher R-value can save you a lot of money on your heating and cooling expenses.

The EPA estimates that replacing your old windows with ENERGY STAR-certified models could save you up to 15 percent on your energy bills each year. The average energy bill in the United States is about $115 a month as of this writing. That means that you could save, on average, as much as $200 each year just by replacing your windows.

Learn More About R-Value

R-value may seem like another boring number on your windows at first glance, but in reality, it’s a huge factor in your monthly expenses. R-value is calculated based on a window’s U-value, which is tested with hot and cold plates in a lab. Windows with more panes of glass, better frames, and larger sizes offer better insulation and more savings for your home.

If you’d like to learn more about R-value, check out the rest of our site at Window Joe Replacement Windows. We provide support to homeowners in selecting replacement windows and replacement window installers. Check out our guide to different types of window replacements today and start saving on replacement windows. 

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