Understanding U-Values


As we know, winter in the UK can bring some harsh weather; freezing temperatures, snow, wind and rain. No one likes a cold house, but how can you be sure that your home is retaining as much heat as possible?

Understanding U-values of different wall materials will stand you in good stead to making your self-build or renovation thermally efficient.

U-values are an indication of how much heat loss happens through a given thickness of a material. For insulating purposes, if the U-value is closer to 0 the better.

It’s important to use the U-value to determine what materials you want to use when creating your next home. To give an example:

  • a brick and block cavity wall with mineral wool insulation giving a wall thickness of 312mm has a u-value of 0.3 W/m­­2K
  • a timber frame stud wall including 140mm mineral wool insulation has a u-value of 0.29 W/m­­2K
  • structural insulated panels of 140 mm thickness bonded to polyurethane creating an overall thickness of 220mm will give a u-value of 0.19 W/m­­2K

U-values are measured in watts per square metre per degree Kelvin. So for a brick and block cavity wall with a U-value of 0.3 W/m­­2K, would mean that for every 1 ⁰C degree difference between inside and outside the wall, 0.3 watts will be transferred every square metre.

Cavity Fill

Different wall types will have unique U-values based on each element of the wall and its thermal resistance. Each element has an R value, the measure of thermal resistance – the higher the value the better thermal performance and heat retention.

The thickness of a wall and the number of elements used has a large impact on its u-value, the thicker and more insulated the lower the u-value. However the airtightness of a property needs to be considered as well, a good u-value will not compensate for a leaky, poor-quality build.

To calculate the U-value: https://www.uvalue-calculator.co.uk/


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