The required thermal transmission – aka U-value – for a construction element (floor, wall or roof) determines the thickness of thermal insulation that has to be incorporated within that element’s build-up.

For architects and specifiers, accurate U-value calculations are therefore essential – whether seeking to ‘only’ achieve the minimum building regulations compliance, or meeting higher levels of performance through voluntary standards and assessment methods. How much do you know about how U-values are calculated, and how much do you need to know? Read on to find out!

What is BS EN ISO 6946:2017?

From a building physics perspective, the movement of heat energy through building materials is complex. This complexity increases in construction elements that are themselves built up in a complex way, and thermal transmittance can only be calculated through a method known as numerical modelling.

Many elements, however, essentially comprise a series of layers. Most of these layers are a uniform thickness of a single material, while some feature regular ‘interruptions’ such as a timber structure or metal fixings.

For these build-ups, a simplified form of calculation called the combined method can be used to calculate U-values. It is this simplified ‘combined method’ that is set out in BS EN ISO 6946, the current version of which is the 2017 edition.

The ‘BS EN ISO’ part of the title indicates that it is a British and European Standard adopted from an internationally agreed standard.

Who uses U-value calculations produced by the combined method?

The simple answer is: a lot of people!

If you or your practice produce your own U-value calculations then you will probably use a software program that is based on the methodology set out in BS EN ISO 6946.

If you request U-value calculations from external consultants or product manufacturers, then most of the calculations you receive will follow the combined method. Somewhere on the calculation, it will probably state that the result has been calculated to BS EN ISO 6946 (it may also refer to a document called BR 443, which you can read more about here).

Because of its simplified nature, using the combined method is much faster than relying on numerical modelling for all calculations. The details of the combined method set out in BS EN ISO 6946 have been validated by numerical modelling, so it can be used with confidence – as long as it is applied to the correct types of constructions.

Do I need to know BS EN ISO 6946 in detail?

The answer to this question depends on how closely you engage with U-value calculations.

For anybody carrying out calculations and working out insulation thicknesses, it is helpful to know some of the underlying ‘workings’ of the methodology. Rather than just accepting the output of the software, it can help to explain why the software is producing the result that it is – and can help to steer design decisions long before formal calculations even need to be undertaken.

In addition, if you go down the route of gaining training or accreditation in calculating U-values, it might be necessary to produce some U-value calculations by hand. That is not possible without understanding the mathematical processes involved.

For construction professionals who obtain calculations from consultants and product manufacturers, it is generally the case that you are relying on the knowledge of the person or organisation producing the calculation. While it may not be necessary to understand the mathematics of the calculation method, it is important to engage with the party doing the calculation to ensure that your proposed build-up is being properly represented.

We said that the method needs to be applied to the correct types of constructions, and this is essential. Unfortunately, there are too many occasions where people try to obtain U-values for complex constructions that go beyond the limit of the combined method. Software programs depend on the ability of the user, and cannot identify when the user is trying to model a scenario that cannot be accurately represented.

Equally unfortunately, we have also experienced occasions where somebody has asked us to carry out a U-value calculation that is unsuitable for the combined method.

When we explain that what they’re asking for needs to be calculated by numerical modelling instead, they become frustrated at the need to source an alternative type of calculation – rather than appreciating that we have saved them from obtaining a calculation that could have fundamentally misrepresented the performance of their design.

About Polyfoam XPS

When you request a U-value calculation from Polyfoam XPS, you can do so with the confidence that you’re benefiting from our team’s many years of experience in the construction industry and insulation manufacturing.

We work closely with architects to ensure that the combined method calculations we produce are appropriate to the project and an accurate reflection of the proposed design.

To find out more about our technical support offering, contact us about your project, read more about standards and current construction trends on our blog, view our online CPD session, or subscribe to receive our monthly newsletter.