Airtightness Testing

Air Tightness Testing for Passivhaus dwellings

Properties that are built with their ecological footprint in mind are also known as ‘passive houses’. To qualify for passive house accreditation, low levels of air leakage need to be achieved.

We advise carrying out tests throughout the build of your property. Testing throughout the first and second phase will help ensure the lowest score possible. A final air test at the end of the build will be required for compliance and accreditation purposes.

Airtightness describes how well the building envelope resists unintended air movement through gaps and cracks when designed openings are closed. The envelope includes external walls, roofs, exposed floors, windows, external doors and the junctions and penetrations that connect them. A more airtight envelope has less uncontrolled leakage, although it still needs a deliberate ventilation system designed under Part F.

Air permeability is the quantified Part L metric. It is the measured air-leakage flow per hour divided by the building’s internal envelope area, at a stated pressure difference between indoors and outdoors. Its complete unit is cubic metres per hour per square metre of envelope area, written m³/(h·m²), and the pressure must always be stated.

The common fan-pressurisation result is AP50: air permeability measured or derived at a 50-pascal pressure difference. The recognised low-pressure-pulse result for dwellings is AP4, measured at 4 Pa. These are not the same direct measurement. SAP applies different calculation treatments to them; a fan-derived AP50 is divided by 20 for its infiltration estimate, while direct pulse AP4 uses the formula 0.263 × AP4^0.924. A converted estimate should not be presented as though it were a direct result at the other pressure.

Air permeability is also not simply “air changes per hour”. Two buildings can have the same leakage airflow but different permeability or air-change results because their envelope areas and volumes differ. A useful test certificate must identify the method, reference pressure, envelope area, result and units.

The statutory basis is Regulation 43 of the Building Regulations 2010. For erection of a building to which the relevant Part L fabric requirement applies, it requires pressure testing in circumstances and by a procedure approved by the Secretary of State. Unless qualifying certificate data provides the required information, results and supporting data must be notified to building control no later than seven days after the final test. Regulation 43 allows the authority to accept an appropriate certificate from a person registered by Elmhurst Energy Systems Limited or ATTMA.8

Approved Document L is statutory guidance rather than the legislation itself, but it sets out the recognised compliance route and names the approved test procedure. Under Volume 1, 2021 edition incorporating 2023 amendments:

• every new dwelling should receive an air-pressure test;
• the limiting result is 8.0 m³/(h·m²) at 50 Pa or 1.57 m³/(h·m²) at 4 Pa;
• DPER, DER and DFEE must be recalculated using the measured result and must meet TPER, TER and TFEE respectively;
• a failure of either the limiting value or calculated criteria requires improvement and further testing; and
• all results, including failures, should be reported to building control.1

The design air permeability is the project target used in the design assessment. The assessed or as-built value is the completed dwelling’s measured result. They are not interchangeable. The as-built submission should retain the final test certificate, underlying report and SAP evidence, as well as earlier failed or interim test records where applicable.

England’s present approach is stricter than the former lighter-touch treatment. In 2002 guidance, pressure testing was presented as an alternative quantified demonstration of infiltration control, with a 10 m³/(h·m²) at 50 Pa benchmark. Statutory pressure-testing duties were introduced in 2006, but the accompanying domestic guidance used type-based sample testing rather than testing every home.

By the 2010 guidance, the general sample was three dwellings of each dwelling type or 50% of that type, whichever was less. Small developments could, in defined cases, rely on recent same-type/same-builder evidence or use an assumed 15 m³/(h·m²) at 50 Pa in the energy calculation. The current 2021 Volume 1 ended that domestic sampling approach: every new dwelling should be tested.

The important lesson is not simply that the required sample grew. The direction of travel has shifted evidence from representative construction towards the measured performance of each completed home. Published 2026 guidance retains every-dwelling testing. Airtightness strategy, inspection records and test readiness are therefore increasingly central to repeatable delivery across a development.

Neither Regulation 43 nor current Approved Document L sets a universal “test at first fix” date. The final compliance test must represent the completed envelope in the permitted configuration, and its result must feed the as-built calculation. The best programme often includes two distinct activities:

• An early diagnostic test or inspection after the intended air barrier has become continuous but before key junctions and service routes are concealed. This is a professional risk-control measure, not a universal regulatory requirement.
• The final compliance test when external doors, windows, hatches, seals and service penetrations are complete and the building can be prepared under TM23. Time should remain for permanent repairs, retesting, calculation updates and reporting.

Before booking the final test, confirm the address and plot reference, tested envelope, internal envelope-area calculation, design target, ventilation strategy, building completeness, access and power, calibration evidence and tester scope. The report file should identify the method and pressure, equipment serials and calibration, weather, test preparation, raw or derived data, result and units. Retain photographs and all failed, interim and final reports. Regulation 43 requires results and supporting data; TM23 governs detailed reporting practice.

New-Build Dwellings

Under current England Approved Document L Volume 1, every new dwelling should be tested and its measured result used in the as-built SAP calculation. Flats and houses are not exempt merely because they share a type. Attached dwellings require a properly defined test envelope, and test strategy should be agreed early where party interfaces or common areas complicate the boundary.

Conversions and Material Changes of Use

Converting a building into a dwelling, adding a flat or changing the number of dwellings can be a material change of use under Approved Document L. Bringing an unheated loft or garage into the heated dwelling can change its energy status. Current existing-dwelling guidance sets fabric, fittings, opening and services provisions, but it does not create the new-dwelling Regulation 43 whole-envelope test duty merely because existing fabric is improved.1 Determine whether the scheme also involves erection or other work that engages a different route.

An air test may still be professionally valuable. It can establish a pre-works baseline, identify paths before insulation or linings are installed, check post-works quality, support a ventilation assessment or satisfy a funder or specification. Those are diagnostic or contractual reasons, not proof of a blanket statutory requirement.

Domestic Retrofit

Ordinary insulation, window replacement or airtightness improvement does not automatically require a Part L blower-door test. The higher priority is to define the actual work and understand how reduced infiltration affects ventilation, moisture and combustion safety. Where energy-efficiency work affects ventilation, current Approved Document F says provision should meet its relevant standard or be no less satisfactory than before; it warns that reducing infiltration can make indoor air quality inadequate.

Baseline and follow-up testing can make deep retrofit outcomes more measurable, but interpreting an existing building requires care. Heritage fabric, moisture pathways, chimneys, party leakage and incomplete compartmentation can make indiscriminate sealing harmful or the test boundary uncertain.

Non-Domestic Buildings

New non-dwellings under current Approved Document L Volume 2 normally require pressure testing and have an 8.0 m³/(h·m²) at 50 Pa limiting value. The measured result must also allow as-built BPER and BER to meet TPER and TER.6 Read more about the calculation route at Part L SBEM Calculations.

A building below 500 m² total useful floor area can avoid testing only if the calculation uses 15 m³/(h·m²) at 50 Pa. That is a deliberately conservative model input, not evidence that the building achieves 15 or a general permissible test limit. Defined alternatives also exist for qualifying modular buildings, genuinely impractical large or complex tests and self-contained compartmentalised units with no internal connections. The representative-area route is narrow; there is no generic non-domestic percentage-sampling rule.

An ordinary renovation or material change of use does not automatically carry a whole-building pressure-test duty. A non-domestic extension is treated as a new building for this purpose where its useful floor area is both over 100 m² and over 25% of the existing building’s total useful floor area. Scope and testability should be agreed with building control. Completed operational evidence may also support Non-domestic EPCs, while planning-stage policy commitments may be addressed through Energy Statements.

Part O is a separate summertime-overheating requirement, not another SAP airtightness test. Current Approved Document O applies to new residential buildings; it does not apply merely because an existing residential building is extended or a building undergoes a change of use. Proportionate overheating analysis can nevertheless be prudent in a conversion or deep retrofit, especially where insulation and airtightness increase while secure window opening, noise or urban pollution constrain heat removal.

For new residential work within Part O, a more airtight envelope can reduce winter heat loss but also reduce incidental summertime air movement. Part O can require more heat-removal provision than the Part F purge minimum, and usable openings may be constrained by security, safety, noise, pollution and fall risk. Modelled infiltration, ventilation flows, shading and actual opening assumptions should therefore be coordinated. Part O’s dynamic route uses CIBSE TM59 subject to the Approved Document’s limits, while cooling used for the Part O solution may also need representation in SAP. See Part O Overheating for a dedicated scope and assessment guide