Energy SavingsWhat is U-value and Why is it Important?

“U-value?” we hear you ask. “What’s that?” Don’t worry! The Window Geeks are here to tell you.

This is Part 1 of a 2 part series on window U-values. 

Let’s begin with understanding the concept behind a window U-value. It's all about energy efficiency and heat, and how easy it is to escape from your window. Like the R-value is the thermal resistance of a wall, the U-value is the thermal resistance of a component such as a window. Technically they are the same thing, however, the U-value is the inverse of the R-value. To put it in simple terms it is 1 divided by the R-value. But be careful, there are imperial and metric units to watch out for. R-Value is Imperial, while RSI is Metric, and both have a U-value equivalent.

"Can you give some examples?"

Sure! Here's an example of how to convert an R-value of an exterior wall to a U-value:

R21 Wall Insulation (1/21) is 0.048 U-Value in Imperial terms (USA). If I multiply this value by a factor of 5.678 you get a metric U-value of 0.27.

...Now let’s look at it from a window perspective.

1.4 Metric U-value, when divided by 5.678, is an imperial U-value of 0.25, which is an R4 window.

The challenge with U-value numbers is we tend not to apply the units of measure with the number and mix up metric and imperial U-values. The units for metric are W/(m2K) and the units for imperial are BTU/(hrFf2).  Note the use of meters in the metric unit and feet in the imperial unit.

A common comment I get is “your window has a U-value of 1.47 while these other windows have a U-value of 0.30. This is a classic metric/imperial mistake. A U 0.30 is actually a U 1.7 in metric. 

So here is all the background info.  The funny part is, that now with technology, we can just google it - but now you know. Stay tuned for Part 2 to find out how windows achieve specific U-values.

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Centra Windows is not only the manufacturer but also the installer of its product, which enables us to offer a full warranty, eliminating any confusion between product performance and installation. We view the house as a system and understand how each of the elements must work correctly to maximize the comfort and energy efficiency of your home. Contact our local Employee Owners at 1-888-534-3333 or send us an email at for more information about this topic and many more. You can also sign up for our biweekly newsletter, providing all the tips and tricks about the window industry.

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