The importance of accurate U-value calculations should not be underestimated. A correct outcome will ensure a building meets regulation levels of thermal performance. However, such is the prevalence of errant practice in relation to U-value calculations, it’s become something of an industry issue. If buildings are to meet required levels of energy efficiency, an accurate interpretation of heat transfer through building materials – which determines a U-value – is essential.
Achieving U-value requirements
U-values 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.
Accurate U-value calculations hold paramount importance, as they allow informed decisions to be made regarding material selection, building envelope design and overall energy efficiency strategies. A meticulous assessment of U-values ensures optimal thermal comfort for occupants, minimises heat loss and gain, and reinforces commitment to creating environmentally responsible structures that promote long-term sustainability and reduce carbon footprints.
Inaccuracy issues remain, however, with U-values used in Building Regulations submissions. This has a negative impact on energy compliance, puts homeowners in difficult positions and increases CO2 emissions. To combat this, it’s a requirement to discuss stipulated U-value outcomes for domestic new-build projects with an energy consultant. Advisors are able to offer professional guidance based on the Standard Assessment Procedure (SAP).
With SAP methodology, the thermal ratings of walls, floors, roofs, junction details and any renewable technologies are put into a metaphorical mix. The overall must correspond with or exceed the required regulations for the building. Elements can be changed, as long as the property achieves the required dwelling performance.
Working on the notional dwelling specification, which has to be followed as a minimum to achieve a pass, is an alternative to the SAP assessment. However, solely working to backstop values will result in failure.
SAP is a fairly complex assessment, as it allows for a compensatory approach to the elements involved. For example, if a roof’s height is preventing a U-value target from being achieved, the performance deficit can be made-up by installing additional insulation in wall and floor areas. 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 SBEM, 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 calculation also takes into account a property’s size, location, model, shape and construction.
Why Polyisocyanurate (PIR) panels are ideal 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. It means the construction industry is increasingly turning to Polyisocyanurate (PIR) panels, rather than mineral fibre-based insulation.
The benefits of PIR insulation are numerous. Its closed-cell structure ensures 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 when optimising 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. This negates costly repairs and maintains the material’s 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.
Engage with an expert for reliable U-value calculations
How can you ensure your home is as energy efficient as it is designed to be? Ensuring a property’s thermal envelope has been specified correctly is prerequisite to achieving a property’s as-designed performance.
The calculations involved in meeting this outcome should be performed by a professional using methodology outlined in BS EN ISO 6946:2017. In 1997, the guidance was expanded to include how tapered insulation should be calculated. This is now known as ‘Annex E’ calculations.
Achieving a U-value of 0.18 W/m²K requires specifying a tapered roof system that is based on the thermal resistance and thickness 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 in line with 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 a tapered insulation scheme to a 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.
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. Thus, purveyors of best practice in respect of building design should put a high price on true U-values.