Projects

"AES Sustainability recommended air testing for several properties that were close to, but just below, an EPC C rating. The testing confirmed their real-world performance and the success of the retrofit works, enabling them to achieve a SAP C rating. This allowed us to secure funding and avoid further capital expenditure on additional technologies. Given the cost-effectiveness, we are now applying this approach to other retrofit properties.”

Bryan Padley, Asset Manager, CHS Group

Project Details

As part of a wider retrofit programme aligned with PAS 2035, AES Compliance & Testing supported Cambridge Housing Society (CHS) in improving the energy performance of a selection of existing dwellings.

These properties - typical of those managed by social landlords - received comprehensive fabric and services upgrades, aiming to raise their EPC scores to band C or above. This was essential for meeting funding eligibility under the Warm Homes: Social Housing Fund (WH:SHF) and supporting broader decarbonisation targets.

Historically, many retrofit projects have relied on assumed values for airtightness due to the lack of measured data. However, by carrying out air tightness testing on these properties, AES was able to provide real, project specific data. With the introduction of RdSAP10 (effective from June 2025), these actual test values can be used in EPC calculations – and in this case, they made a measurable difference, directly lifting the properties into a higher EPC band.

How We Met the Requirements

AESCT recommended and conducted air permeability testing following the completion of fabric and services measures. These included insulation improvements, window upgrades, and heating system replacements.

The air tightness testing process was designed to:

• Measure how much heat is lost due to air leaks in the building

• Confirm that insulation and sealing work was effective

• Support the design of ventilation systems to maintain good indoor air quality

• Provide post-insulation performance evidence aligned with PAS 2035’s approach

Using measured airtightness values, the properties’ energy models were updated to reflect real airtightness performance. This was a crucial step in demonstrating the effectiveness of the retrofit - and in these cases, it was the measured air test results that lifted the properties into EPC band C, making them eligible for WH:SHF funding and aligning with CHS’s energy targets.

Outcome

The results clearly demonstrate that measured airtightness can significantly improve EPC outcomes as seen in the table below.

Key outcomes included:

• EPC ratings increased from Band D to Band C in all Cornish Close properties, achieving eligibility for WH:SHF

• Accurate reflection of retrofit quality, avoiding undervaluation from default assumptions

• Improved design confidence in associated ventilation strategies, mitigating condensation risks

• Verified performance supporting funding bids, compliance reporting, and tenant communication

While all properties had already received significant fabric and services upgrades, it was the measured air tightness value that ultimately pushed the EPC scores over the Band C threshold. In similar cases, the only remaining intervention required to guarantee WH:SHF compliance would be the addition of solar PV, where not already installed. Measured air tightness avoided the need for more costly interventions, making it a cost-effective way to unlock Band C EPC ratings.

Associated Services 

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