Air Permeability Test

When you construct or renovate a building, you invest heavily in insulation, high-performance windows, and efficient heating systems. But if the building envelope is riddled with gaps and cracks, much of that investment leaks away—literally. An air permeability test measures exactly how much uncontrolled air escapes through your building’s fabric, giving you the data you need to achieve compliance, reduce energy bills, and create comfortable indoor spaces.

In this comprehensive guide, you’ll learn what an air permeability test actually measures, how it fits within UK building regulations, the testing methods used on site, and practical steps to ensure you pass first time.

What Is an Air Permeability Test?

An air permeability test measures the rate of uncontrolled air leakage through a building’s fabric when subjected to a specified pressure difference—typically 50 Pascals (Pa). In simple terms, it quantifies how “leaky” or “airtight” a building is by measuring the amount of air that escapes through unintentional openings in the building envelope.

You may hear this test referred to as an air pressure test, air leakage testing, or air tightness testing. These terms all describe the same fundamental process: measuring how much air passes through gaps, cracks, and poorly sealed junctions rather than through designed ventilation routes.

The test specifically targets unintentional air paths—think gaps around window frames, cracks at floor-wall junctions, holes around service penetrations, and poorly sealed skirting boards. Controlled ventilation systems such as MVHR units, extract fans, and trickle vents are temporarily sealed during testing because they represent deliberate, designed airflow rather than unwanted leakage.

In the UK, air permeability is reported in cubic metres per hour per square metre of envelope area at 50 Pa (m³/(h·m²) @ 50 Pa). This value feeds directly into SAP and SBEM energy calculations, affecting the building’s predicted energy consumption and, ultimately, its EPC rating.

Key Fact	Detail
Unit of measurement	m³/(h·m²) @ 50 Pa
Standard test pressure	50 Pascals
Primary standards	BS EN ISO 9972:2015, CIBSE TM23:2022
Main UK regulations	Approved Document L (England & Wales), Section 6 (Scotland)
Mandatory for	All new dwellings (since June 2022 updates)
Typical compliant range	3–5 m³/(h·m²) @ 50 Pa

Since the June 2022 updates to Approved Document L in England and Wales, air pressure testing is now a mandatory requirement for all new dwellings. Achieving a good air permeability score is essential not just for regulatory compliance, but for handover, warranty sign-off, and long-term occupier comfort.

Air Permeability, Building Design & Regulations

Air permeability sits at the intersection of several critical building performance factors: energy efficiency, moisture control, and indoor air quality. As buildings become more highly insulated to meet low-carbon targets, controlling air movement through the envelope becomes increasingly important.

Understanding the distinction between air permeability and ventilation is fundamental. Air permeability refers to uncontrolled leakage—warm air escaping and fresh air infiltrating through other unintentional openings in an unplanned manner. Background ventilation and whole-house ventilation, by contrast, are designed, controlled airflow routes that ensure healthy indoor environments while minimising unnecessary heat loss.

The regulatory framework in England addresses both aspects:

• Approved Document L (Conservation of fuel and power) sets requirements for envelope air tightness to limit heat loss
• Approved Document F (Ventilation) establishes minimum ventilation rates to maintain indoor air quality and control water vapour
• Similar requirements exist in Scotland’s Section 6 and Wales’ Part L of the building regulations

A critical change arrived in June 2022 (effective from June 2023 for most projects): sample testing of new dwellings was effectively removed. Every new dwelling now requires an individual air pressure test to demonstrate compliance—no more relying on representative samples from larger developments.

For designers, this means targeting an air permeability significantly better than the bare regulatory maximum. SAP 10.2 calculations require realistic air leakage inputs to demonstrate that the Dwelling Emission Rate meets or beats the Target Emission Rate. With the Future Homes Standard on the horizon (expected from 2025), these targets will only tighten further.

The Impact of Air Permeability on Energy Efficiency

High air permeability directly increases heat loss in winter and unwanted heat gain in summer. When air infiltration is excessive, heating and cooling systems must work harder—and run longer—to maintain comfortable temperatures, driving up energy consumption and costs.

Consider a simple comparison: a dwelling achieving 10 m³/(h·m²) at 50 Pa versus one achieving 3 m³/(h·m²). The leakier building might experience 20-30% higher space heating demand over a year, translating to substantially higher energy bills and carbon emissions. That difference compounds over the building’s lifetime.

Perhaps more frustrating is when air leakage undermines otherwise excellent thermal performance. You can specify the best insulation and achieve impressive U-values, but if the air tightness layer is compromised, warm air simply bypasses your insulation through gaps and cracks. The result: cold draughts, surface condensation on cold spots, and mould growth around junctions—the very problems good design should prevent.

Well-controlled air permeability enables right-sized heating systems. A heat pump, for example, can be specified at lower capacity (and lower cost) when air leakage is minimised, because peak heating loads are reduced. This improves running costs, reduces carbon emissions, and enhances thermal comfort year-round.

However, there’s an essential caveat: buildings must not be made “too tight” without adequate mechanical ventilation or carefully designed natural ventilation strategies. An airtight envelope without proper controlled ventilation systems leads to stale air inside, elevated CO₂ levels, excessive water vapour accumulation, and serious moisture problems. The goal is controlled air tightness paired with deliberate, effective ventilation—not simply sealing everything shut.

Air Permeability vs Ventilation & Background Ventilation

Understanding the difference between air permeability and ventilation is critical for both compliance and building performance.

Air permeability quantifies unintentional air flow through cracks, gaps, and poorly sealed junctions in the building envelope. This air movement is uncontrolled, unpredictable, and driven by wind pressure and temperature differences—not design intent.

Designed ventilation encompasses trickle vents, mechanical extract fans, MVHR systems, and other deliberate airflow routes. These are sized and positioned to deliver specific air flow rates that maintain indoor air quality while minimising unnecessary energy loss.

Part F of the building regulations sets minimum background and whole-building ventilation rates to manage moisture, odours, and indoor pollutants. These requirements apply regardless of how airtight the building is—in fact, they become more important as air tightness improves because natural air infiltration no longer provides incidental ventilation.

For retrofit projects, a newer concept has emerged: Background Ventilation Testing (BVT). Used primarily under PAS 2035:2023, BVT measures the air change rate (expressed as air changes per hour, or ACH) rather than envelope leakage at 50 Pa. This helps assess whether retrofit measures—such as new insulation or window replacements—have compromised the ventilation strategy of an existing building.

Aspect	Air Pressure Testing (APT)	Background Ventilation Testing (BVT)
Primary purpose	Measure envelope air loss at 50 Pa	Measure background air change rate
Typical unit	m³/(h·m²) @ 50 Pa	Air changes per hour (ACH)
Test pressure	50 Pa (or 4 Pa for LPP)	Low pressure, closer to natural conditions
Main application	New builds, Part L compliance	Retrofit projects, PAS 2035
Established since	~2002 in UK regulations	More recent, mainly post-2019

Air pressure testing is the long-established method for new builds, mandatory since the early 2000s. Background ventilation testing is more recent and primarily supports retrofit assessors in ensuring that energy improvements don’t create unhealthy, under-ventilated existing dwellings.

The Air Permeability Testing Process

The air permeability test uses the blower door method as the primary technique for demonstrating compliance in new dwellings. This approach is codified in BS EN ISO 9972:2015 and further detailed in CIBSE TM23:2022.

From a practical standpoint, here’s what happens when a tester arrives on site:

1. Pre-test inspection: The tester walks through the building to check completion status and identify any obvious gaps or unsealed penetrations
2. Preparation: Controlled ventilation openings (trickle vents, extract fan terminals) are temporarily sealed; internal doors are opened for whole-building testing
3. Equipment setup: A calibrated fan unit is installed in an external door frame, connected to pressure sensors and data logging equipment
4. Testing: The fan creates a controlled pressure difference between inside and outside while measuring the air flow rate required to maintain that pressure
5. Data collection: Measurements are taken across a range of pressures (typically 10-60 Pa) to calculate the leakage rate at 50 Pa
6. Reporting: Results are calculated, compared against design targets, and a formal certificate is issued

Tests can be performed in pressurisation mode (blowing air in), depressurisation mode (drawing air out), or both. Calm weather conditions—wind speeds below 6 m/s and stable temperatures—improve accuracy significantly.

Fan Pressurisation (Blower Door) Method

The blower door fan method remains the industry standard for air permeability testing in residential and commercial buildings alike.

Setup: A calibrated fan is mounted in a temporary frame that seals into an external door or window opening. The fan connects to digital pressure sensors and a laptop or data logger that records airflow and pressure readings in real time.

Preparation: Before testing begins, all controlled ventilation openings must be temporarily sealed. This includes trickle vents, mechanical extract fan terminals, and any other intentional openings. Windows and external doors are closed, while internal doors are left open to allow the test to measure the whole building envelope as a single zone.

Testing procedure: The fan creates a pressure difference of 50 Pa between the air inside the building and the ambient conditions outside. While maintaining this reference pressure, the equipment measures the air flow rate required to sustain the pressure difference. Measurements are typically taken across a range from 10 Pa to 60 Pa, allowing the software to calculate an accurate leakage rate at the standard 50 Pa.

This method is directly referenced in CIBSE TM23 and ISO 9972, making it the standard approach for demonstrating Part L compliance across England, Wales, and most of Europe.

Accuracy: In calm conditions with wind speeds below 3 m/s, typical uncertainty is around ±10%. As wind speeds increase toward 6 m/s, uncertainty rises to approximately ±20%. This is why test scheduling should consider weather forecasts—testing on a blustery day introduces additional variability that could affect borderline results.

Low-Pressure Pulse (LPP) Method

The Low-Pressure Pulse method offers an alternative to traditional blower door testing, using a fundamentally different approach to measure building air tightness.

Instead of a large fan creating sustained pressure, LPP uses a compressed air tank placed inside the building to deliver short pulses of air. The equipment measures how internal air pressure rises during the pulse and then decays as air escapes through the envelope.

Operating conditions: LPP works at much lower pressures—typically 2-15 Pa rather than 50 Pa. Results are reported at a reference pressure of 4 Pa, which is closer to natural pressure differences caused by wind and temperature.

This method has gained traction for testing existing buildings and retrofit projects. It’s recognised under CIBSE TM23 and aligns with practices in France, Switzerland, and parts of the USA where 4 Pa reference values are standard.

Accuracy and limitations: LPP offers an estimated uncertainty of around ±3-4%, potentially more precise than blower door testing in some conditions. However, current LPP equipment can only pressurise—there’s no depressurisation mode available. The method is accepted for some compliance routes under Part L where specifically agreed with building control.

Factor	Blower Door	Low-Pressure Pulse
Test pressure	50 Pa	2-15 Pa (reported at 4 Pa)
Equipment	Fan in door frame	Compressed air tank
Typical uncertainty	±10% (calm conditions)	±3-4%
Pressurisation/depressurisation	Both available	Pressurisation only
Best suited for	New builds, compliance testing	Existing buildings, retrofit

Relating Results at 4 Pa and 50 Pa

Because blower door tests report at 50 Pa while LPP reports at 4 Pa, converting between the two values introduces additional uncertainty that assessors and designers must understand.

The relationship between air leakage at different pressures is not linear. A building with a given air leakage rate at 4 Pa will have a higher (but not simply proportional) rate at 50 Pa due to the physics of airflow through small openings.

For situations where conversion is necessary, SAP 10 guidance provides a BRE-derived formula:

AP50e(4t) = 5.26 × AP4^0.924

Where AP50e(4t) is the estimated 50 Pa air permeability derived from a 4 Pa test result (AP4).

Example calculation: If LPP testing returns a result of 1.5 m³/(h·m²) at 4 Pa, the estimated 50 Pa equivalent would be: 5.26 × 1.5^0.924 = approximately 7.5 m³/(h·m²) @ 50 Pa

Conversions should only be used where absolutely necessary—for example, when an LPP test is the only available data but a 50 Pa figure is required for SAP input. Any converted value must be clearly labelled as estimated, with the formula source and assumptions documented.

What Is a Good Air Permeability Score?

Air permeability scores indicate how much air escapes through each square metre of building envelope area per hour when subjected to a 50 Pa pressure difference. Lower numbers mean better performance—less uncontrolled air loss, better heat loss prevention, and improved thermal comfort.

For new dwellings in England and Wales, designs typically target around 5 m³/(h·m²) at 50 Pa or better to meet Part L 2021 requirements with comfortable margins. As the Future Homes Standard approaches, expect these targets to tighten further.

Here’s how to interpret different performance levels:

Air Permeability Score	Performance Level	Typical Characteristics
Above 10 m³/(h·m²)	Poor / Leaky	Noticeable draughts, high heating costs, likely Part L failure
7-10 m³/(h·m²)	Mediocre	May pass older regulations, struggles with current standards
5-7 m³/(h·m²)	Compliant	Meets minimum requirements, typical new-build standard
3-5 m³/(h·m²)	Good Practice	Comfortable margins, improved energy performance
Below 3 m³/(h·m²)	Very Airtight	Passivhaus-approaching, requires mechanical ventilation with heat recovery

There’s an important nuance for very airtight buildings: Part L does not reward extremely low air permeability without appropriate ventilation. A dwelling achieving 2 m³/(h·m²) but relying only on natural ventilation gains no additional SAP benefit over one at 3 m³/(h·m²) with MVHR. The regulations recognise that ultra-tight envelopes need controlled ventilation systems to function properly.

For commercial buildings, appropriate targets vary significantly by use. An office building might target 3-5 m³/(h·m²), while a warehouse with large doors and loading bays might realistically achieve only 8-10 m³/(h·m²). These targets should be agreed during design stages with the energy modeller and building control.

Preparing for an Air Permeability Test

Good preparation on site is the most reliable way to achieve a first-time pass, avoiding re-test costs (typically £200-400 per visit) and programme delays that can hold up handover.

Completion Requirements

Before the tester arrives, ensure the following items are complete:

• Building envelope finished: All external walls, roofs, and floors forming the thermal envelope must be complete with final finishes
• Windows and external doors installed: Fully glazed and operational, with all seals in place
• Penetrations sealed: Cable entries, pipe runs, duct penetrations, and meter box installations must be properly sealed with appropriate materials
• Loft hatches fitted: With compression seals if specified
• Skirting boards and architraves installed: These often mask the junction between walls and floors where air leakage paths commonly occur
• Service voids accessible: Bath panels may need removal to check sealing behind

Temporary Sealing for the Test

The tester will require certain openings to be sealed before testing:

• Trickle vents closed and taped
• Extract fan terminals covered or sealed
• Flues and chimneys temporarily sealed (as directed by the tester)
• Open drainage pipes capped or ensured that traps are filled with water
• Letterboxes taped shut

Pre-Test Walk-Through

A thorough snag check before the tester arrives can identify obvious issues that would cause failure. Pay particular attention to:

• Service penetrations through external walls and ceilings
• Gaps around window and door frames
• Junctions between floor, wall, and roof elements
• Meter boxes and consumer unit locations
• Soil pipe penetrations through the envelope
• Areas behind bath panels and kitchen units

Finally, ensure the site team knows the test is happening. Coordinate with all trades so that external doors aren’t opened mid-test, and confirm safe access, working power sockets, and adequate lighting.

Practical Design & Construction Tips

The most effective air tightness strategies begin at design stage, not during snagging.

Define a clear airtightness line: Drawings should show a continuous airtight layer throughout the building, clearly indicating how this layer is maintained at junctions, corners, and penetrations. This line should be unambiguous—every trade needs to understand which elements form the barrier.

Specify materials early: Include appropriate tapes, membranes, grommets, and sealants in employer’s requirements and trade packages. Generic descriptions like “seal all penetrations” lead to inconsistent application. Instead, specify products and methods explicitly.

Train the workforce: Toolbox talks and on-site demonstrations ensure that all trades understand the airtightness strategy. Electricians, plumbers, and mechanical installers need to know they cannot simply punch holes through membranes without proper sealing afterwards.

Consider interim testing: On larger developments, testing a representative plot during construction stages—before plasterboard goes up—allows identification of systematic issues while they’re still easy to fix. A proactive approach to early testing can prevent costly remedial works across multiple plots.

Collaborate across disciplines: Air tightness is not solely an architectural issue. M&E designers specify penetrations; structural engineers detail junctions; contractors execute the work. Regular coordination ensures the airtightness strategy survives from drawing board to completion.

Air Permeability Testing in Existing Dwellings & Retrofit

Air testing is no longer just for new construction. It’s increasingly valuable in existing buildings undergoing retrofit, supporting both energy improvement programmes and solutions to damp and mould issues.

Under PAS 2035:2023, retrofit projects often require assessment of how proposed measures will affect building ventilation. Installing insulation, replacing windows, or sealing draughts can significantly reduce air infiltration—which is good for energy efficiency but potentially problematic for indoor air quality if not managed properly.

RdSAP 10 improvements: The latest version of RdSAP (used for existing dwelling EPCs) now allows Domestic Energy Assessors to enter measured air permeability values rather than relying on default assumptions. This means a retrofit project that achieves measurable air tightness improvements can demonstrate better EPC outcomes, supporting funding applications and property valuations.

Occupant considerations: Testing in occupied homes requires sensitivity. Scheduling should work around residents’ routines; noise from equipment needs explanation; temporary sealing of vents requires clear communication about what’s happening and why. The goal is to gather accurate test results while causing minimal disruption.

Link to background ventilation testing: After insulation or draught-proofing works, BVT can verify that minimum air change rates are still achieved. This is particularly important in existing dwellings where the original ventilation strategy may have relied partly on controlled (or uncontrolled) air infiltration that has now been reduced.

Training and Qualifications for Testers

Air permeability testing is typically carried out by certified professionals operating under recognised competent person schemes such as ATTMA (Air Tightness Testing and Measurement Association) or the Elmhurst Airtightness Scheme.

Typical training content includes:

• Principles of air leakage and building physics
• Test methods (blower door and pulse techniques)
• Equipment setup, calibration, and maintenance
• Data interpretation and quality assurance
• Reporting requirements and regulatory context

Demand for qualified testers has risen significantly since the 2021-2022 updates to Part L in England, Wales, and Scotland made testing mandatory for all new dwellings. This demand will grow further as the Future Homes and Future Buildings Standards take effect.

For contractors and energy assessors interested in expanding their services, Level 1 air pressure testing and background ventilation testing qualifications are available through approved training providers. The investment in certification opens access to a growing market as the construction industry increasingly recognises air tightness as a core quality indicator.

Conclusion & Next Steps

Controlling air permeability is central to delivering buildings that are energy-efficient, comfortable, and healthy. As UK regulations tighten toward net-zero carbon targets under the Future Homes and Future Buildings Standards, understanding and achieving good air tightness will only become more important.

The key points to remember:

• Air permeability measures uncontrolled air leakage through the building envelope at 50 Pa
• The blower door method is standard for new builds; low pressure pulse offers an alternative for existing buildings
• Target values around 3-5 m³/(h·m²) represent good practice for new dwellings
• Airtight buildings must have adequate controlled ventilation to ensure compliance with Part F and maintain indoor air quality

Early planning is essential. Defining the airtightness line at design stage, specifying appropriate materials, and training site teams all contribute to first-time test passes. Reactive snagging after a failed test is far more expensive than proactive attention during construction stages.

For architects and designers: Define your airtightness strategy clearly on drawings and ensure it’s buildable at every junction and penetration.

For contractors and site managers: Use the preparation checklist before every test, and consider interim testing on early plots to catch systematic issues.

For homeowners and developers: Engage qualified testers and energy assessors early in your project to agree target values, testing strategy, and the ventilation approach that will ensure your building performs as intended.

Whether you’re constructing new buildings or undertaking retrofit projects on an existing building, the necessary tests and the expertise to interpret test results are fundamental to achieving buildings that deliver on their design promises—comfortable, efficient, and compliant.