When you start looking at solid wall insulation as a potential solution for your home or commercial premises, you may come across the terms ‘thermal conductivity’ and ‘U-value’. In this article we are going to try to shed a little light on what they both mean!

Thermal conductivity

Thermal conductivity measures the ease with which heat can travel through a material by conduction, conduction being the main form of heat transfer through insulation. Thermal conductivity is often termed the λ (lambda) value.

The thermal conductivity is a constant for a particular material – it is not impacted by the thickness of the material. Although when comparing different materials, products with lower thermal conductivity values are better insulators (i.e. will be better at slowing heat loss).

At EWI Store we sell three different types of insulation (although there are more types available in the market). The table below shows the different thermal conductivity values for these materials:

Insulation MaterialThermal Conductivity
Expanded Polystyrene (EPS)0.031 W/mK
Extruded Polystyrene (XPS)0.034 W/mK
Rockwool Dual Density Boards0.036 W/mK

So based on the lowest thermal conductivity being the best insulator, from the table above, you can see EPS is the best insulator.

Now – thermal conductivity is useful, but we also need to incorporate the thickness of the insulation into the calculation. The thicker the layer of insulation, the slower the rate of heat loss and also the better at retaining heat a building will be. This is where the U-value comes in. The U-value signifies the heat lost through a given thickness of a particular material. This allows you to directly compare types and thicknesses of insulation.

The calculation is as follows:

U-value = Thermal conductivity / thickness (where the thickness is measured in metres).

When you are comparing U-values, a lower number is better – i.e. a lower U-value signifies less heat loss through a material; therefore it is a better insulator.

U-values and building regulations

Building regulations often provide a U-value number that needs to be achieved when building certain elements of a property. So for example when retrofitting external wall insulation on an old solid wall property, you need to achieve a U-value of 0.3W/m2K or lower. This can be achieved with any insulation material, although the thickness of the particular material will vary; for example when you are using EPS on a solid wall property, 90mm will suffice – but if you were going to use Rockwool, you would require 110mm.

Now, to get a true U-value figure you need to take into account all the different elements that make up the wall: the internal plasterboard, bricks or block work, insulation materials, and even the thin render top coats – they all very slightly change the u-value number.

So there you have it – hopefully that makes understanding U-values and thermal conductivity a little easier! If you would like us to calculate U-values to help you determine the thickness of insulation you require, then please let us know.