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Incorrect U-Value Calculations Jeopardise Our Future

U-values are vital. Not only do they determine the anticipated thermal efficiency of a building’s fabric, they are a must for new-build and refurbishment projects.


U-values reveal whether a property has met regulation energy standards. But why are the calculations executed so carelessly? Calculations should and must be done in strict accordance and conformance to standards and regulations. Frustratingly, at present, the calculations are frequently carried out with little regard of the importance of compliance. This disregard is deeply worrying, placing homeowners at risk and causing the increase of CO2 emissions – quite the opposite of what is intended.

Incorrect U-value calculation is a big problem for the industry. To provide insight as to what can be done, James Wilkinson, Design Team Manager at flat and tapered roof insulation specialists Gradient, argues why accurate and compliant U-value calculations matter, highlighting to specifiers the need to ensure they have not been mis-sold and are providing their clients with the thermal performance they require.

Meeting U-value requirements

U-values measure the transfer of heat through a structure. They feed into a building’s overall performance alongside Psi-values and renewable technologies such as heat pumps and solar panels. The lower a U-value is, the more thermally efficient the build-up is – meaning the more cost-effective homes are to heat.

There are, however, high levels of inaccuracy that exist in U-values used in Building Regulations submissions. This hefty issue is having a negative impact on compliance and the energy efficiency of the building, putting homeowners in difficult positions and increasing CO2 emissions.

When it comes to achieving required U-values for domestic new-build projects it’s a requirement to discuss stipulated outcome with an energy consultant who can give advice based on the Standard Assessment Procedure (SAP). With this methodology, the thermal ratings of walls, floors, roofs, junction details and any renewable technologies are put into a metaphorical mix, the performance of which will hopefully correspond with or exceed the required regulations for the building. These elements can be changed, as long as the property achieves the required dwelling performance. The alternative to carrying-out the SAP assessment is to work on the notional dwelling specification, which has to be followed as a minimum to achieve a pass. Working to backstop values only, however, will result in failure.

The SAP assessment is quite complex, as it allows for a compensatory approach to the elements involved. For example, if a designer is struggling to hit a U-value target due to height issues with a roof, the performance deficit can be made-up by installing additional insulation in areas such as walls and floors. With new-builds, therefore, adjustments to U-value outcomes can tip the balance in favour of achieving the required performance targets. For commercial properties, Simplified Building Energy Modelling (SBEM) is the approved national calculation methodology used to highlight their energy efficiency. With the SBEM process, a property’s overall U-value compliance is determined by the thermal outcomes of individual elements such as walls, floors, pitched and flat roofs. A property’s size, location, model, shape and construction are also taken into account as part of the calculation.

The value of selecting Polyisocyanurate (PIR) panels for a building’s fabric

Poorly-insulated building fabric is a major contributor to domestic energy wastage and buildings falling short of U-value requirements. To help combat this, the construction industry is increasingly turning to Polyisocyanurate (PIR) panels, rather than mineral fibre-based insulation.

There are numerous benefits associated with PIR insulation. Its closed-cell structure means it doesn’t absorb water, allowing the thermal performance and reliability of the product to be retained over time.

With lambda values as low as 0.022 W/mK, PIR provides excellent performance. This, coupled with its slim composition, means it requires less space to achieve the same U-value as other insulation materials. This is of particular benefit to housebuilders looking to maximise interior living space in multi-property developments with limited plot size.

Unlike fibrous insulation, which deteriorates over time when damp sets in, PIR insulation’s structural strength enables a consistent performance that will last, negating costly repairs and maintaining its thermal qualities. PIR insulation is also renowned for its adaptability. It is the ideal solution for a range of applications such as floors, walls, pitched and flat roofing.

Expert and trusted U-value calculations

Efficiency is at the forefront of people’s minds when buying anything from a fridge or TV to a car. This was born out perfectly in the “diesel-gate” scandal, where thousands of car owners discovered their vehicles were not giving the level of MPG efficiency they were promised. Why should the built environment be any different? How can you be sure that your home is as energy efficient as it was designed to be? You may keep a car for 10 years but a house will stand for far longer. One of the best ways to start is to ensure your thermal envelope has been designed correctly and the calculations involved are performed by a competent person in the correct manner.

The methodology which should be used is outlined in BS EN ISO 6946:2017. In 1997, this guidance was expanded to include how tapered insulation should be calculated. This is now known as ‘Annex E’ calculations. To achieve a U-value of 0.18 W/m²K requires specifying a tapered roof system that is based on the thermal resistance and thickness each of each of its components i.e: the deck, air and vapour control layer (AVCL), insulation and waterproofing. A condensation risk analysis may also be provided. Only calculations done to Annex E methodology for tapered roofs are accurate and compliant. U-value calculations, using the average thickness of insulation only, will give wildly inaccurate and non-compliant results.

Designing the tapered insulation scheme to the target U-value is one thing, but achieving it requires the highest levels of workmanship. Once installed, if there are gaps in the insulation due to poor installation or an element such as an AVCL is omitted, the desired thermal performance or vapour resistance will not be met and the building will fall short of its as-designed proposal.

When it comes to engaging with a specialist to carry out U-value calculations, it’s important to employ one that uses the Annex E method. However, regardless of the expert you choose, it’s imperative that the information given is accurate. If not, a rebuild could well be required, and roof refurbishment can be extremely costly. Furthermore, incorrect U-value calculations will compromise a roof, and the overall building’s, thermal performance. Ultimately, this will have a negative impact on the environment, with the increase in CO2 potentially thwarting the UK government’s ambition to achieve net-zero emissions by 2050.

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