R value v U value – what’s the difference?

When you’re talking about thermal performance, you end up referring to everything in terms of it’s “value’, R value, U value, K value, lambda value – but what does this mean?  This article focuses on the two most commonly discussed values – R value and the U value.

R value is typically used to tell us how thermally effective insulation is e.g. R2 wall insulation batts







U value is typically used to tell us how thermally effective a window is e.g. High performing glazing has a U value of 1.0.  Low performing glazing has a U value of 6.5.

R value is the inverse of U value – SO

U value = 1/R (W/m2K)  said as watts per square meter kelvin

R value = 1/U (m2k/W) said as square meter kelvin per watt



To demonstrate these conversions:

An R2 wall batt has a U value of 0.5

U value = 1/2 = U value 0.5

A seemingly high performance double glazed window with a U value of 3 has an R value of 0.33

R value = 1/3 = R value 0.33

So although a window U value of 3 might seem good, the R value is only R0.33 which is not very good at all in terms of insulation capacity.  By doing these calculations it is easy to see how windows are the weakest point of any building envelope!

To improve your U value of your window, you can do the following:

  • use double or triple glazing
  • use a noble gas in between the glass (e.g. argon)
  • use low e coatings
  • specify thermally broken frames

Get in touch with us using the “Chat now” button to find out more about improving the thermal performance of your project!

Jessica Allen 

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