What is a Good Air Tightness Test Result?
What Is a Good Air Tightness Test Result?
A good air tightness test result for a new build dwelling is typically around 5 m3/(h.m2) @ 50Pa. This figure is widely considered an achievable, industry-average score that most well-built properties can reach without specialist construction techniques. However, the specific target for your building will be determined by your SAP calculations, and achieving a better result can make compliance with Part L significantly easier.
Air tightness testing — also known as air pressure testing, air leakage testing, or air permeability testing — measures how much air escapes through the building envelope. It is a mandatory requirement under Part L of the Building Regulations for all new dwellings and many non-domestic buildings.
Understanding Air Tightness Scores
Air permeability is measured in m3/(h.m2) @ 50Pa, which represents the volume of air (in cubic metres) that leaks through each square metre of the building’s external envelope per hour when a pressure difference of 50 Pascals is applied. The lower the number, the more airtight the building.
Here is how different air tightness scores compare:
| Score (m3/(h.m2) @ 50Pa) | Rating | Notes |
|---|---|---|
| 8.0 | Maximum permitted | Part L 2021 backstop value |
| 5.0 - 7.0 | Average | Typical target for standard new builds |
| 3.0 - 5.0 | Good | Above average, achievable with attention to detail |
| 1.0 - 3.0 | Excellent | High-performance buildings, MVHR required |
| Below 1.0 | Exceptional | Approaching Passivhaus standards |
The maximum permitted air permeability under Approved Document Part L 2021 is 8 m3/(h.m2) @ 50Pa. Any building that exceeds this value will fail its air tightness test outright, regardless of what the SAP calculations specify.
How Your Air Tightness Target Is Set
Your specific air tightness target is not a fixed number — it is determined by your building’s SAP calculations. The SAP assessor models the entire building and calculates what air permeability score is needed for the dwelling to achieve an overall pass.
A building with a strong specification elsewhere — for example, excellent insulation, minimal thermal bridging, and renewable technologies — will have more flexibility on the air tightness target. Conversely, a building with marginal performance in other areas will need a tighter air permeability score to compensate.
Key factors that influence the target include:
- Insulation performance — better U-values in walls, floors, and roofs reduce the required air tightness score.
- Heating system — heat pumps perform better in SAP than gas boilers, giving more flexibility on air tightness.
- Renewable technologies — solar PV panels or waste water heat recovery reduce the carbon calculation, easing the air tightness requirement.
- Thermal bridging — lower psi values from good junction details improve the overall fabric performance.
Most dwellings end up with a target of around 5 m3/(h.m2) @ 50Pa, though this can range from 3 to 7 depending on the specification.
What Happens at Different Score Levels?
5 to 8 m3/(h.m2) @ 50Pa — Standard Performance
Buildings in this range are adequately sealed but still allow reasonable natural air movement through the fabric. Natural ventilation with trickle vents and intermittent extract fans is typically sufficient for indoor air quality.
3 to 5 m3/(h.m2) @ 50Pa — Good Performance
At this level, the building is noticeably tighter and heat loss from air leakage is significantly reduced. Natural ventilation is usually still appropriate, though continuous mechanical extract ventilation (MEV) may be recommended in some cases.
Below 3 m3/(h.m2) @ 50Pa — High Performance
Buildings achieving below 3 m3/(h.m2) @ 50Pa are considered very airtight. At this level, whole-house mechanical ventilation with heat recovery (MVHR) becomes necessary. The building is too tight for natural ventilation to provide adequate fresh air, and without mechanical ventilation, there is a significant risk of:
- Condensation and mould growth
- Poor indoor air quality
- Excess humidity
MVHR systems recover up to 90% of the heat from extracted air, making them energy efficient. However, they add cost to the build and require ongoing maintenance.
Below 0.6 m3/(h.m2) @ 50Pa — Passivhaus Level
The Passivhaus standard requires an air change rate of 0.6 air changes per hour or less at 50Pa. This is an exceptionally high level of airtightness that demands specialist construction techniques, meticulous attention to detail, and typically multiple air tests during the construction phase.
Factors That Affect Your Test Result
Several construction factors directly influence the air tightness test result:
- Quality of sealing — how well gaps around service penetrations, junctions, and openings have been sealed with appropriate tapes, membranes, and sealants.
- Window and door installation — poorly fitted frames or missing seals are common sources of air leakage.
- Service penetrations — pipes, cables, and ducts passing through the external envelope must be sealed properly.
- Loft hatches — a common weak point if not fitted with effective draught excluders.
- Skirting boards — gaps behind skirting boards where plasterboard meets the floor allow air to escape into the wall cavity.
- Construction quality — consistency of workmanship across the entire building is crucial.
How to Achieve a Better Score
For practical guidance on improving your air tightness result, read our detailed guide on how to pass an air tightness test. Key principles include:
- Plan for air tightness from the design stage
- Brief all site trades on the airtightness strategy
- Seal all service penetrations through the building envelope
- Use appropriate tapes and membranes at junctions
- Fit draught excluders to loft hatches and integral garage doors
- Seal behind skirting boards and at plasterboard-to-floor junctions
- Ensure windows and doors are properly fitted and sealed
Air Tightness Testing With UKBC
UK Building Compliance provides professional air tightness testing services for dwellings and commercial buildings across the UK. Accredited by ATTMA, our technicians deliver accurate results with on-site diagnostics as a standard part of the service.
If your building does not meet its target on the first test, we will help you identify the specific sources of air leakage and advise on the most effective remedial measures before retesting.
For the best value, book air tightness testing alongside ventilation testing and sound insulation testing — we offer discounted rates for combined packages.
Call 01455 242297 or request a free quote today.
Frequently Asked Questions
What is a good air tightness test result for a new build?
Most new build dwellings target around 5 m3/(h.m2) @ 50Pa, which is widely considered an achievable average score. Results below 3 are excellent but require mechanical ventilation (MVHR) to manage indoor air quality.
What is the maximum air permeability allowed under Part L?
Approved Document Part L 2021 sets a maximum air permeability of 8 m3/(h.m2) @ 50Pa for new dwellings. However, most buildings will need to achieve a lower target as determined by their SAP calculations.
What does m3/(h.m2) @ 50Pa mean?
This is the unit of air permeability. It measures the volume of air (in cubic metres) leaking through each square metre of the building envelope per hour, when a pressure difference of 50 Pascals is applied. Lower values mean less air leakage.
Does a better air tightness score always mean a better SAP result?
Generally yes, as less air leakage means less heat loss. However, very airtight buildings (below 3 m3/(h.m2) @ 50Pa) will require MVHR systems, which adds cost. Your SAP assessor will determine the optimal balance for your project.
Can I improve my air tightness result if I fail?
Yes. UKBC provides diagnostics as standard during testing. Common remedial measures include sealing around service penetrations, improving seals at junctions, and addressing gaps around windows and doors. A retest can then be carried out.
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