Outdated EPC ratings based on estimates can mislead you about your building’s true energy performance. That’s a longstanding problem for domestic ratings in particular – commercial EPCs have tied verified air tightness results directly into the calculation for close to two decades, while for domestic ratings, it’s only just arrived. As part of the RdSAP 10 methodology, residential air tightness data can now be input directly, bringing homes into line with an approach commercial buildings have worked to since 2006.
In this blog you’ll learn:
- Why traditional EPCs have relied on unreliable estimates
- What actually happens during an EPC test, and how that differs from air tightness testing
- Why commercial EPCs have used verified air tightness results for close to two decades, and why it’s new for domestic ratings
- What’s changed under RdSAP 10 for existing homes
- What counts as a good air tightness test result, for both domestic and commercial buildings
The Challenge: Unreliable Traditional EPCs
EPC assessments have traditionally relied on estimated air leakage values, which often underestimate the actual leakage in existing buildings, resulting in inaccurate ratings. This can have real consequences: imagine a building owner who installs a less efficient heating system based on an EPC that suggests good energy performance, when in reality, high air leakage means the building loses significant heat – negating the benefits of the new system.
What Actually Happens During an EPC Test?
An “EPC test” refers to a standard energy performance certificate assessment: an assessor visits the property to carry out a full on-site energy evaluation – fabric, heating system, glazing, insulation and more – before producing the certificate alongside clear, prioritised advice on the improvements that would move the rating up a band. On its own, that visit doesn’t measure air leakage directly; it relies on an assumed default figure for a building of that age and type, unless a separate air tightness test has been carried out.
Is Air Tightness Testing the Solution for Accurate EPCs?
Air tightness testing is a non-destructive method for precisely measuring a building’s air leakage rate, done by pressurising the building’s interior slightly and measuring the rate at which air escapes. That single, real figure replaces guesswork with something you can actually act on: where the building is losing heat, and whether the fix is worth the spend. It’s also a distinct, more specialised test from the standard EPC assessment described above – the two aren’t the same appointment.
It Won’t Always Improve Your EPC Rating
It’s worth being upfront about this: measuring your air tightness and feeding the result into an EPC doesn’t automatically make the rating better, whether you’re working under domestic RdSAP or commercial SBEM. Without a test, the assessment assumes a default air permeability figure for a building of that type and age – and that default varies significantly depending on which methodology applies:
- Existing Domestic – 15 m³/h/m²@50Pa
- Existing Non-Domestic – 25 m³/h/m²@50Pa
SBEM’s non-domestic default isn’t fixed at 25, either – it can drop to 15 or even 10 m³/h/m²@50Pa for buildings constructed under more recent Building Regulations, or rise as high as 35 m³/h/m²@50Pa for particularly leaky, uninsulated or metal-clad structures.
If your building actually performs better than the relevant default assumption, testing and inputting the real figure will likely improve your score. But if it performs worse – which is common in older buildings that haven’t been specifically draught-sealed – the measured result can bring the rating down instead.
This is exactly why it’s worth talking to an assessor before testing purely for EPC purposes. They can help you judge whether your building is likely to test better or worse than the default assumption for its type, so there are no surprises once the result comes in.
Domestic EPCs: Air Tightness Testing Arrives Under RdSAP 10
RdSAP 10 launched on 15 June 2025, replacing RdSAP 2012 (v9.94) as the official UK methodology for assessing existing home energy performance and generating EPCs, with more detailed data collection and recognition for modern green technologies. Alongside these changes, measured air tightness test results can now be incorporated directly into EPCs, giving buildings that have already addressed air leakage a more accurate rating.
Commercial EPCs: Air Tightness Testing Since 2006
For commercial and non-domestic buildings, none of this is new. Air tightness testing has been tied directly into EPC calculations for two decades, not two years. Non-domestic EPCs are generated via SBEM (the Simplified Building Energy Model; for existing buildings, a certified air test result can replace the assumed SBEM default, provided it’s still representative of the building’s current fabric. Assessors can’t simply estimate a lower figure without that certificate on file, which is exactly why this matters most for older buildings trying to prove they’re genuinely tighter than the SBEM default assumes, often to lift a failing MEES rating
Case Study: Air Tightness Testing for EPCs in Practice
Two Bristol warehouses found themselves rated on default air permeability assumptions that didn’t reflect how the buildings actually performed – one facing a failing MEES rating, the other a handful of points short of an EPC A. A single air test on each replaced assumption with measurement:
What’s Next?
Every EPC starts from an assumption until you test it. If you’re a landlord chasing an EPC C to meet MEES, a genuine air tightness test replaces that assumption with the real number your building achieves. It won’t always work in your favour – which is exactly why it’s worth talking through your building’s likely result before booking a test.
We carry out air tightness testing that feeds directly into your EPC assessment – domestic under RdSAP 10, commercial via SBEM and ATTMA-certified testing – replacing assumed defaults with a measured result.