FANS & VENTILATION SYSTEMS T


here are a range of tools that can aid understanding about a home’s measured energy performance. Airtightness results


from tests, such as Pulse or blower door, are very effective for assessing air changes and the associated heat loss in the form of air leakage pathways and draughts. On the flip side, such tools can also determine the appropriateness of installed ventilation, helping to ensure systems are fit for purpose and avoid damaging effects, such as damp and mould.


The new EPC


At the time of writing, details about the new Energy Performance Certificates (EPCs) are imminent, but the Government has already made it clear the metrics it intends to use to improve understanding about building energy performance. While the current EPC offers merely a cost estimate of a property’s energy bills, the ‘new’ EPC will take a more forensic approach. Its four headline metrics are expected to be:


¡ Energy cost: a fuel bill estimate, to help people make decisions about which energy efficiency home improvements to make. ¡ Fabric performance: the thermal performance of a building’s walls, roof, floor and constructed elements, to emphasise how a well-insulated, energy-efficient space is more comfortable to live in. ¡ Heating system: the building heating source, with information on its efficiency and environmental impact, to speed up adoption of lower carbon heating solutions. ¡ Smart readiness: how readily a building can have smart technologies integrated, to optimise resident energy consumption and enable them to benefit from cheaper smart tariffs. There will also be a raft of secondary metrics, starting with two that could potentially grow to include indicators about a property’s health impacts. ¡ Carbon: the estimated carbon emissions from a building from the energy use of its occupants. ¡ Energy use: overall energy consumption, as well as potential energy efficiency improvements By using fabric performance as a metric, this will


make evidence-based insights, such as airtightness testing, more important to accurately assess thermal performance. This new EPC information will support decision making around building fabric retrofit. At the same time, any measured energy performance information used to determine EPC ratings can also be used to specify and size heat pumps and other heating systems more accurately, while also guiding the design and specification of other HVAC installations.


Measured data replaces assumptions


The shift to measured data has already begun. From June 2025, domestic energy assessors have been able to input measured airtightness results into the RdSAP10 methodology, which is used to calculate EPC ratings for existing homes. This new input can


14 December 2025


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Moving from ‘best guess’ to evidence-based will support HVAC system design


This clear shift towards understanding real-world building energy performance means airtightness testing is becoming a vital tool in HVAC design and retrofit strategy, moving us away from ‘best guess’ solutions, towards evidence-based installations. That’s according to Luke Smith, managing director of Build Test Solutions (BTS)


Left: Luke Smith, managing director of Build Test Solutions (BTS)


bump up the point score by between one and three, potentially increasing the EPC rating. EPC reform brings an even greater need for


insight, moving the dial further to reflect real-world property energy performance, rather than relying on assumptions and default values. And while the new EPC will initially be for new homes, this will be rolled out across all buildings to base ratings on live fuel prices, actual home occupancy and energy use, and information about how the specific building is constructed and heated.


The use of airtightness testing also has practical design benefits for HVAC specification and design. Measured data feeds directly into heat loss calculations, with the MCS heat loss calculator now also accepting measured airtightness results. This enables engineers to right-size HVAC systems and avoid over- or under-specification. The result is reduced installation and running costs for residents


– one of the most tangible outcomes of the move to data-led design. More widely, measurement can not only provide the


evidence base for retrofit strategy but also provide the data to confirm effectiveness upon completion. Homeowners will be recognised for this work by a more accurate EPC system that reflects the work done.


A tool for compliance and confidence


Under the latest PAS 2035:2023 framework, which delivers guidance for whole-house energy-efficient retrofit, airtightness testing is no longer an optional step – it is embedded into the entire compliance process. Any project involving fabric upgrades – from insulation to window replacements – must include a defined airtightness and air leakage testing


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