What is U-Value and why is it important?

Have you ever wanted to know what makes a window U value what it is? 

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

Let’s first begin with understanding what a window U value actually is. 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. 

Maybe it is best to give you 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.

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. 

Want to learn more? We would like to extend the invite to you and your company to sign up for a Lunch & Learn with us!  


Anton Van Dyk
Vice President of Sales & Business Development



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