What 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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