The risks of inaccurate U-value calculations revealed by RAVATHERM UK

Rob Firman, Technical and Specification Manager at RAVATHERM UK – manufacturer of POLYFOAM Extruded Polystyrene (XPS) insulation – highlights the risks of inaccurate U-value calculations and the need for a more rigorous approach during the specification process.



t’s an unfortunate truth that despite their importance, U-values and their calculation are not as well understood as they should be and that can lead to misleading calculations. As an insulation manufacturer, raising this issue can be tricky; any suggestion that calculations produced by others are unfair can sound like sour grapes.

However, accuracy and adherence to standards is an issue that cuts right to the heart of the built environment. U-values resulting from inaccurate calculations feed into energy assessment calculations (either SAP or SBEM, for domestic and non-domestic respectively) that are directly responsible for demonstrating compliance with Building Regulations.

In many cases, a miscalculation is the result of an honest mistake or lack of understanding, but alarmingly, there is also growing evidence that standards aren’t being adhered to.

Turning a blind eye

A scenario when this may occur is when the insulation specification is switched during construction and after project design stage compliance has been achieved. There is often an assumption that the new product will have the same declared thermal performance as the one it is replacing. The manufacturer will have to supply a calculation to prove this. If the result is not identical, difficult and costly, remedial measures will be required to make up the shortfall in performance. Sadly, in an effort to avoid that time and expense, it is not unknown for a revised calculation to be issued that says the same as the original.

Calculation issues and errors

There are several important factors to consider in a U-value calculation, particularly for inverted roofs such as the building’s location, the effects of rainwater cooling and ensuring the product’s ‘design’ thermal conductivity is used.

There are additional variables, any of which could cause a difference in result between two apparently ‘correct’ versions. Specific issues we have encountered recently include the following.

Rounding up R-values

The thermal resistance (R-value) of a construction material is its thickness divided by its thermal conductivity. An extruded polystyrene (XPS) product with a conductivity of 0.034W/mK, at 200mm thick, has an R-value of 5.88m2K/W or 5.85m2K/W if rounded down in accordance with BS EN 13164 to the nearest 0.05m2K/W. We’ve seen examples of calculations where it has been rounded up, yet the R-value cannot be any higher than that without breaking the laws of physics.

Correct thermal values for airspaces

U-value calculation software allows the dimensions of an airspace to be entered to ensure the correct thermal resistance is used.

Where one, or both surfaces, either side of the airspace has a low emissivity, ISO 6946 includes a formula to work out an increased airspace thermal resistance. We have seen examples of calculations where a greater airspace resistance has been included, despite the surfaces either side being standard high-emissivity surfaces. There has also been no justification as to why the higher resistance has been claimed in the airspace description.

Average UK rainfall figures

U-value calculations for inverted roofs account for the cooling effect of rainwater. The lower the rainfall, the lower the impact on the result. If the location of the building is known, rainfall data for that location, or one nearby, should be used (ISO 6946 refers to “data relevant for the location”). However, we have seen the UK’s average rainfall figure be used, even when the building’s location is known to have greater rainfall.

Raising standards

Misleading calculations call into question the construction industry’s ethics and values as well as its commitment to delivering a quality end product.

As a responsible manufacturer transparent about the calculations we produce and willing to talk through the results with any customer, it’s frustrating to know that inaccurate calculations make their way into the marketplace.

Unfortunately, there is insufficient knowledge in the industry, especially regarding relevant standards, which makes it very difficult for this issue to be adequately policed. This is exacerbated by the fact that training in calculations isn’t up to scratch.

Initiatives like the BBA’s competency scheme for the calculation of U-values are supported by a minority of insulation manufacturers and companies, but not enough to really put the spotlight on accuracy and competency.

A rigorous approach

Until this situation changes, we call on the specifiers and purchasers in the construction industry to join us in being more vocal about raising standards.

Test the manufacturers you work with. Obtain more than one calculation for the projects you are specifying and compare and question the results.

Seek clarity and be confident that a manufacturer does not simply sell you the thinnest solution with the biggest margin, but a product with an accurate U-value which will ensure a building – and our industry – performs to the highest standard.

Share this article

Login to post comments

About us

Future Constructor & Architect is a specification platform for architects and building contractors, which focuses on top-end domestic and commercial developments.

As well as timely industry comment and legislation updates, the magazine covers recent projects and reviews the latest sustainable building products on the market. Subscribe here.

Privacy policy

Latest updates


Sign up below to receive monthly construction, architecture and product updates from FC&A via email: