The Americans are like us, but they are different: #3 R and U value units

Continuing the ‘Americans are like us but different posts’ and the previous post on Metric vs Imperial Units this post continues the units theme as there are big differences between how the UK/Ireland and the US handles Thermal Resistance.

The R-value therefore is a measure of thermal resistance:

Easy to read explanation:

A U value is a measure of heat loss in a building element such as a wall, floor or roof. It can also be referred to as an ‘overall heat transfer co-efficient’ and measures how well parts of a building transfer heat. This means that the higher the U value the worse the thermal performance of the building envelope. A low U value usually indicates high levels of insulation. – source

The physics/mathematical explanation:


Where, R= the temperature difference across an insulator divided by the heat flux (heat transfer per unit area per unit time).

The R-value is the reciprocal of the U-value (U=1/R)

All good so far, so where’s the difference? The units for R-values are square-metre kelvins per watt or m2·K/W (or equally, m2·°C/W). However, in the US the same calculation is given in units of ft2·°F·hr/Btu. It is therefore very easy to confuse UK/Ireland (SI) and US R-values, especially when R-values in the US are often cited without their units, e.g., R-3.5.

For more information, visit the Wikipedia page on R and U values.

I’m getting used to working on drawings with US R values and the next post will go a little deeper into the differences between UK/Ireland and US insulation thicknesses and resultant U-values.

So, if you would like me to help you with your drawings (whether you are a potential client or another designer (using Vectorworks) in either imperial or metric measurements, if you are in the UK or US, then why not CONTACT ME…

One thought on “The Americans are like us, but they are different: #3 R and U value units

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