What are U values?

When it comes to specifying building materials, it's easy to get caught up in technical terms and industry jargon - and making sense of all those numbers can be pretty complicated.

For example, what's a U-value and why is it important?

Simply put, U-values measure how effective a material is as an insulator. The lower a U-value is, the better it is at providing insulation.

If you want to get more technical, U-values describe the thermal transmittance of a building material. That is, how easily heat can travel through a structure made from certain materials. A single material can have a U-value, and so can composites.

To calculate a U-value, the rate of heat transfer through the structure is divided by the difference in temperature across the structure. This results in U-values being measured in watts per metres squared kelvin, or W/m²K.

U-values are important because they describe how energy efficient a property is. A building with good thermal wall insulation will have a lower U-value, and therefore be more efficient and cheaper to keep warm in the winter.

As well as the materials used, the workmanship and installation standards of a property can also affect thermal transmittance. If insulation is not installed properly, there could be gaps and cold bridges, which will cause thermal transmittance to be higher than desired. U-values take all types of heat loss into consideration, including conduction, convection and radiation.

Prior to construction, you can estimate a structure's U-values by considering each layer of the building envelope and doing some simple maths.

  • Start by establishing the thermal conductivity (K-value) of each product used (for example, surface coatings, bricks, insulation, blocks and plaster). These numbers should be available from the manufacturers.
  • Measure the thickness of each element.
  • Calculate the thermal resistance (R-value) of each element by multiplying the K-value by the thickness.
  • Add the R-values together. Then divide one by the R-value.

It's important, however, to remember that this calculation is not the actual U-value of the finished structure. That's because air gaps, cold bridges and other issues will also affect the real figure.

For the most accurate measurement once the building is completed, U-values can be measured using a heat flux metre. Several measurements will need to be taken over a continuous period of about two weeks (or over the course of a year for ground floor slabs).