U-Value Calc

U-values are a vital indicator as to whether a building is compliant with current energy standards. Hitting the mark on sustainability, they help homes and businesses reduce their carbon footprint for the overall betterment of the environment. Technical Manager, Simon Blackham at Gradient highlights which considerations need taking into account to ensure the correct value is arrived at.


Gradient explains U-value calculations

Here Gradient explains how U-values measure the transfer of heat through a structure. In relation to an insulation productr example, the higher its thermal resistance value, the better its insulation performance.

When it comes to achieving required U-values to create domestic new-build projects with good insulation it’s a good idea to discuss requirements with an energy consultant who can give advice based on the Standard Assessment Procedure (SAP). With this methodology, the thermal ratings of walls, floors, junction details and any renewable technologies are put into a metaphorical mix, the performance of which will hopefully correspond with the required regulations for the building. These elements can be changed, as long as the property achieves the required dwelling performance and the insulation performs. 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 will produce a fail.

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.


Summing-up refurb calculations

In the majority of cases, flat roof and tapered insulation specialists Gradient will be asked to supply a U-value calculation based on building regulation Part L1B, which relates to the conservation of fuel and power in existing dwellings – the only building projects to which fixed U-values apply. Generally, this means a supplier might ask us to provide a system which meets a U-value of 0.18 W/m²K – the minimum thermal performance required for flat roofs. Gradient will then take into account the roof’s elements – the deck, air and vapour control layer (AVCL), insulation etc – and specify a solution that meets the required U-value performance based on the thermal resistance and thickness of each of the system’s components. As part of this process, condensation risk analysis is also provided.

Setting out a required U-value is one thing, but achieving it requires the highest levels of workmanship. Once installed, if there are gaps in the insulation or it slumps overtime, 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 outcome. This performance gap issue remains prevalent throughout the construction industry, although mooted changes in the assessment procedure could help address the situation. These include local authorities putting technology such as thermal imaging cameras to greater use in order to check that junction details and the like have been installed correctly and meet thermal performance requirements.


As Gradient has showcased previously, U-values are intrinsic to the way we measure the energy performance of buildings old and new in the effort to reduce our carbon footprint. Contacting an expert in the field of U-value calculation such as Gradient, which is able to create a unique insulation solution for a range of flat and tapered roof applications, would be time well spent for suppliers embarking on a new-build or renovation project. We have the technical insight and experience to ensure, whatever thermal performance is required, is met.