Outdated EPC ratings based on estimates can mislead you about your building’s true energy performance. This isn’t a new problem for every building type, though – 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 score, 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 and undermining sustainability goals.
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.
The data allows building owners and professionals to:
- Target improvements: identify and address weaknesses in the building envelope, leading to reduced air leakage.
- Make informed Decisions: gain a clear understanding of the building’s actual energy performance for informed choices regarding energy-saving measures.
- Enhance value and marketability: demonstrate the effectiveness of improvements through better EPC ratings, potentially increasing the building’s value and marketability.
What Actually Happens During an EPC Test?
It’s worth clarifying what’s actually being measured when people talk about an “EPC test,” since the term covers two different things. A standard domestic EPC assessment involves an assessor visiting 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.
Air tightness testing – the focus of this guide – is a distinct, more specialised test: a blower door is used to pressurise the building and measure the actual rate of air leakage, producing a real figure rather than an estimate. Under RdSAP 10, that measured figure can now be fed directly into the EPC assessment, replacing the assumed default. So an EPC test and an air tightness test aren’t the same appointment – but as of RdSAP 10, the second can directly improve the accuracy of the first.
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. It introduced more detailed data collection, new property age bands and recognition for modern green technologies, including:
- New technologies: accurate accounting for solar PV battery storage, PV diverters, and small-scale hydro systems.
- Room-in-roof updates: roof rooms are now split into distinct types, each with individual window measurements and detailed insulation tracking.
- Granular data: all windows must be measured individually, specific light bulbs counted, and up to two alternative wall types can be noted per building part.
- Age Band M: a new construction age category for recently built modern homes.
Alongside these changes, measured air tightness test results can now be incorporated directly into EPCs – offering more accurate ratings, fairer recognition for buildings that have addressed air leakage, and a stronger incentive for owners and developers to invest in improving it.
The full breakdown of what changed under RdSAP 10 – from individually measured windows to the new evidence-first requirement for claimed upgrades:
It Won’t Always Improve Your EPC Score
It’s worth being upfront about this: measuring your air tightness and feeding it 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. If your building actually performs better than that 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 on the rating once the result comes in.
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) or, for more complex buildings, DSM (Dynamic Simulation Modelling), under Part L2 rather than the Part L1/RdSAP route used for existing homes.
The regulatory timeline runs well ahead of the domestic route:
- Part L 2002 first referenced air tightness testing in building regulations, though it was only recommended, not enforced, for large commercial buildings over 1,000 m².
- April 2006 (Part L2A 2006) made air testing mandatory for new commercial buildings over 500 m², and introduced SBEM itself as the National Calculation Methodology – requiring a verified ATTMA TSL2 test result to issue the final as-built EPC.
- 2008 saw BREEAM formally adopt ATTMA TSL1 and TSL2 standards into its rating criteria, recognising air tightness performance above the statutory minimum.
- Part L2A 2013 reinforced mandatory testing and tightened standard air permeability limits further, solidifying ATTMA Level 2 certification for commercial assessors.
Tying a measured air permeability result directly to a commercial EPC is effectively as old as SBEM itself – domestic EPCs are only now catching up to where commercial buildings have worked for the better part of two decades.
Commercial targets also vary far more by building type and scale than the domestic 5 m³/h·m² guideline below. Colsec’s refrigerated distribution centre in Leeds was tested to ATTMA TSL1 against a ≤3 m³/hr/m² Part L2 design target, and came in at 1.3. The Bristol Veterinary School’s Blagdon Building was tested to ATTMA TSL3 against a considerably tighter 2.00 m³/(h·m²) BREEAM-linked target, and achieved 1.85.
Two Bristol warehouses recently 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 Is a Good Air Tightness Score for a Domestic EPC?
New dwellings should aim for an air permeability of 5 m³/h.m² or lower to pass SAP calculations and produce a strong EPC. Below 3 m³/h.m², dwellings without mechanical ventilation receive no additional SAP benefit, since ventilation becomes the limiting factor rather than air tightness. For existing homes – which often sit around 15 m³/h/m²@50pa – bringing the score down toward 5 is a realistic target for most retrofit projects, and the improvement can now be reflected directly in an updated EPC under RdSAP 10. Commercial targets sit on a different scale entirely, driven by building type and use rather than a single guideline figure – see the examples above.
Improve Your EPC Rating with Air Tightness Testing
When you schedule an air tightness test with us, you get:
- Precise air leakage assessment: identifying weaknesses in your building envelope that lead to wasted energy and discomfort.
- Actionable insights: a clear understanding of your building’s airtightness, so you can target improvements effectively.
- A result that now counts: whether you’re working under domestic RdSAP or commercial SBEM, your measured air tightness score can feed directly into your EPC assessment.
What’s Next?
Commercial EPCs have relied on verified air tightness results for close to two decades; domestic ones are only just catching up under RdSAP 10. Either way, a genuine test can now replace an assumed default with a measured result – whether you’re a landlord working toward an EPC C target for MEES, or a commercial client needing ATTMA-certified proof for Part L2 or BREEAM. It’s not a guaranteed uplift in either case, 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.
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