U-Values, R-Values, and Thermal Conductivity made easy!

Eco RollYou’ll soon find when shopping for insulating materials that the process is brimming with technical terms and confusing figures that seem to exist only to confuse. These are, however, actually pretty useful and will provide you with the powerful knowledge necessary to navigating your way to the right product.


A U-value tells you how much heat a given thickness of a particular material loses through conduction, convection, and radiation. This measure of heat loss is always used within Building Regulations.

Basically, the lesser the U-value, the better the material works an insulator.

Do the maths

The U-value is calculated by taking the reciprocal of the R-value and adding convection and radiation heat losses. In practise, this is a complicated calculation and would be best figured out using calculation software. The result is expressed in Watts per metre squared Kelvin (W/m2K). Generally, an uninsulated cavity wall has a U-value of approximately 1.6 W/m2K, whilst a solid wall has a U-value of approximately 2 W/m2K.

The U-value equation looks like this:

U-Value Equation


An R-value shows how well a given thickness of material stops heat from moving through it. Unlike a U-value, the greater the R-value, the better the material works as an insulator.

Do the maths

The R-value equation looks like this:

R-Value Equation

  • l is the thickness of the material in metres
  • λ is the thermal conductivity in W/mK
  • The R-value is measured in metres squared Kelvin per Watt (m2K/W)

R values of different materials, such as a cavity wall and the insulation used inside it, can simply be added to each other to find the total thermal resistance.  In this way, an R-value is a relatively simple way to compare insulating materials where you have the thermal conductivity of each. It also allows you to compare different thicknesses of the same insulating material.

What makes R-values slightly less valuable than U-values is that they only take loss of heat through conduction into consideration, whilst heat is actually lost by several different methods. Both convection and radiation are left out when working out an R-value.

So, although they are more complicated, U-values can be more useful.

metalThermal Conductivity

Thermal conductivity, measured in λ or Lambda, tells you how easily a material lets heat flow through it independently of the material’s thickness.

A lower thermal conductivity is better, because this means that heat moves across the material at a slower rate.

Thermal conductivity is measured in Watts per Metre Kelvin (W/mK).

Insulating materials usually have a thermal conductivity from 0.008 W/mK (vacuum insulated panels – very effective, very expensive), to 0.061 W/mK (some wood fibres).

Thermafleece sheep wool insulation ranges from 0.035 W/mK (Ultrawool) to 0.040 W/mK (Natrahemp).

Using U-Values, R-Values, and Thermal Conductivity

When you’re looking for the right insulating material, bear in mind that:

  • Higher numbers are better when comparing R-Values and Thermal Conductivity
  • Lower numbers are better when comparing U-Values
  • The U-Value gives the most accurate measure of insulating ability, but it can be more difficult to work out.

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