How to calculate U-Values

U-value Calculator

This week we are celebrating the launch of our new U-Value Calculator so for the next five days we will be talking about U-values; what they are, how they are calculated and what our online U-value calculator can provide.

To find out when they are posted keep an eye on our Twitter feed or look at the bottom this post where we will be adding links to all the blogs when they are posted.

We are starting by looking at what a U-value is and why you would want to calculate one.

What is a U-value?

The U-value assesses the rate of heat loss through all the thicknesses of the combined elements that make up a building component such as a wall, floor or roof. It is measured in units of W/m2.K (Watts per metre squared Kelvin). It is a way of measuring the insulating properties of the building element. The lower the U-Value, the better insulated the building element is. So a wall with a low U-Value should prevent heat loss better than a wall with a high U-Value.

Why do we use U-Values?

The use of U-values allows for the comparison of different build ups for applications, so the insulating properties of a solid wall could be compared to that of a cavity wall for example, or allows the comparison of two different types of insulation at different thicknesses.

It is the most accurate way of measuring the insulating properties of the application or material and Building Regulations/Standards specifies U-Values which have to be achieved when building or refurbishing a building. For example when building in England, a pitched roof has a recommended best starting point of a U-value of 0.11W/m2.K or lower if it is a new build; for a refurbishment or extension the U-Value should be 0.18 W/m2.K or lower.

Without knowing the U-value of a wall or a floor or roof, you won’t know how energy efficient the whole building will be. The U-values are used as part of SAP assessments and EPC’s for houses.

Before you start any building work you should calculate the U-value as part of the design process, to make sure that meets or is lower than that specified by Building Regulations/Standards.

So how do we calculate a U-Value

To calculate a U-Value you have to start with the R Value or resistivity of the elements that make up the building element you are trying to calculate the U-Value for. The R-Value refers to a material’s ability to resist heat transfer at a certain thickness – the higher the better when looking for an insulation material. It is measured in m2.K/W. The R-Value is calculated as R = l/λ where l = the thickness of the material in metres and λ (lambda) is the thermal conductivity of the material in W/m.K.

So for example to work out the R-Value of 60mm of Kingspan Kooltherm K8 Cavity Board, which has a thermal conductivity or λ value of 0.020W/m.K:

R-Value = l/λ = 0.06/0.02 = 3.00 m2.K/W

U-Value formula

For a particular application, such as a cavity wall, which is built up of a brick outer leaf, 50mm clear cavity, Kooltherm K8 Cavity Board, dense block and skim coated plaster board, then the R-Value for each of the elements would be calculated in order to work out the U-Value.

U Value is the reciprocal of all resistances of the materials found in the building element. To calculate the U-Value of the building element the R-Value of all the different components that make up that element will be considered.

U-Value = 1/(Sum of all R-Value)

U-Value (of building element) = 1 / (Rso + Rsi + R1 + R2 …)

where Rso is the fixed external resistance, Rsi is the fixed internal resistance and R1, R2 etc are resistivity of all elements within the application including that of cavities within the construction.

This is the basic formula and by using this you could work out the U-Value for a particular application, however there are also other factors which need to be taken into account including thermal bridging factors of fixings or stud work.

There is a simpler solution.

Using an online u-value calculator

Calculating the U-value by hand can get quite complicated, time consuming and is unnecessary these days. Our online u-value calculator has over 6000 U-values pre-calculated for a range of common applications and the U-value calculations produced have been approved under the ‘BBA/TIMSA Scheme for Calculation Competency Part 1 – U value and condensation risk’.

By selecting the elements for the particular wall, floor or roof build up you wish to know the U-Value for, a pre-calculated U-Value can be supplied to you without the stress.

Take a look at our online u-value calculator and let us know what you think either in the comments here, on Twitter or LinkedIn or by emailing us on To find out more about the u-value calculator and how it works check out the rest of the posts in this series which are linked below.

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Kingspan Insulation is a market leading manufacturer of optimum, premium and high performance rigid insulation products and insulated systems for building fabric and building services applications.

0 comments on “How to calculate U-Values
  1. How can the overall U value of a house be measured to demonstrate whether its component parts work as well as their calculated U value

    • Hi Sally

      Area-weighted U-value calculations could combine roof, wall and floor elements. Unfortunately we only provide free U-value calculations for the individual elements, so we are unable to help you any further with this. However, have a Google – you’re likely able to find somebody else who can!

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