Air tightness testing is a crucial part of delivering energy-efficient, regulation-compliant new buildings. If you’re a developer, builder or self-builder, you’ll likely need to carry out this test to meet the requirements of Part L of the UK Building Regulations.

In this guide, you’ll learn what an air tightness test is, why it matters, what it involves, and how it affects compliance and energy performance.

Key Takeaways

• An air tightness test measures how much uncontrolled air escapes from a building.
• It is a legal requirement under Part L of the UK Building Regulations for most new builds.
• The test result affects your SAP calculation and EPC rating.
• Common failure points include window junctions, service penetrations, and poorly sealed loft hatches.
• You can improve your chances of passing with proper preparation and construction practices.

What Is Air Tightness Testing?

An air tightness test – also known as an air permeability test, air pressure test or air test – measures how much uncontrolled air escapes through gaps and cracks in a building’s envelope. It helps assess how airtight a property is and whether it meets modern energy efficiency standards. The result is recorded as an air permeability test score, which quantifies airflow per square metre of surface area.

If you’re managing a self-build or new development, meeting building air tightness test requirements is key to ensuring compliance and achieving a satisfactory EPC rating.

Why Air Tightness Testing Matters

If your building has excessive air leakage, it will be harder to heat, less energy-efficient, and more expensive to run. Draughts and inconsistent temperatures can also affect comfort for future occupants.

Good airtightness contributes to a higher EPC rating and supports your SAP result. That’s why it’s so important to understand why air tightness is important for both building performance and regulatory success.

Is Air Tightness Testing Legally Required?

Yes, air tightness testing is legally required for most new residential and commercial buildings in the UK. You must demonstrate that your project meets air permeability targets set out in Part L of the building regulations before it can be signed off. 

To stay ahead of any surprises, it’s worth reviewing how recent changes to Part L affect air tightness testing and what that means for your project.

When Should Air Testing Be Done?

You should schedule the air tightness test once the building fabric is complete but before internal finishes and mechanical systems are commissioned. This allows time to address any issues without holding up completion.

If it’s your first time managing a test, it’s worth understanding when air tightness testing should be carried out during the build to avoid costly retesting or hold-ups later on.

How the Air Tightness Test Works

To carry out the test, all vents, windows, doors and penetrations are temporarily sealed, and a fan is used to pressurise or depressurise the building, known as a blower door test. The amount of air needed to maintain a fixed pressure reveals how much uncontrolled leakage is present through gaps in the envelope.

To avoid delays or unexpected failures, it’s important that your team is clear on what the air tightness test process involves, from sealing procedures to how the building should be prepared on the day.

What Happens If a Building Fails an Air Tightness Test?

If your air tightness test result doesn’t meet the required standard, you’ll need to identify and seal the leak points before retesting. This can add costs and delay sign-off.

Knowing what happens if you fail an air test and how to fix it can help you react quickly and keep things moving.

How to Improve Your Chances of Passing an Air Tightness Test

Attention to detail during construction is the single biggest factor in test success. Sealing around service penetrations, using membranes properly, and keeping trades accountable are all key.

If you’re aiming for a first-time pass, make sure you know how to prepare for an air tightness test effectively and review some practical tips for passing.

What Is a Good Air Permeability Test Result?

While the legal maximum for dwellings is 8 m³/h·m² at 50 Pa, most new builds aim for 5 or below. More ambitious designs – like low-energy or Passivhaus-standard homes – may target even tighter levels.

Having a clear sense of what qualifies as a good air tightness result can help you set realistic targets from the outset of your project.

Where Does Air Leakage Typically Occur?

Common areas of air leakage include:

• Pipework penetrations
• Loft hatches
• Window and door frames
• Junctions between materials

Identifying and being mindful of the most common sources of air leakage in buildings will help your team reduce test-day surprises.

Can Air Tightness Be Improved?

Yes — and it’s often easier to improve than you might think. Small adjustments during or after the build can dramatically improve test results. 

For example, resealing service penetrations, adding tape to poorly joined membranes, or addressing gaps around windows can significantly reduce air leakage. 

Understanding how to improve air tightness in a building at this stage could save you from costly delays or retesting.

Do All New Builds Need an Air Test?

Most new homes must be tested. In some cases – like developments with repeated house types – sample testing may be permitted, but increasingly, one test per unit is the norm.

​​If you’re managing a new build, you can read more about air test requirements for new builds and how the rules apply across different development types.

How Does Air Tightness Affect Your EPC?

Your air test result is a required input for your SAP assessment, which in turn determines your EPC. 

Without a valid result, the EPC can’t be completed – so an air test is essential for producing an EPC where SAP calculations are required.

Helping Site Teams Stay on Track

Getting airtightness right often comes down to what happens on site. Everyone involved – from subcontractors to site managers – needs to understand the build stage actions that affect air test outcomes. That’s why it helps to break the process down into clear, practical steps, like those covered in our air tightness testing checklist for builders, which outlines key areas to keep in focus throughout the build.

At a broader level, developers and contractors benefit from understanding how air leakage can affect compliance and sign-off, including where it typically occurs and how small decisions early on can affect the project at the test stage.

Final Thoughts

Air tightness testing isn’t just a regulatory requirement – it’s a valuable quality control tool that contributes to comfort, efficiency and long-term performance.

With proper preparation, good site practices and an understanding of the regulations, most builds can pass first time and move forward without delay.

On 15th December 2021, the government published their updated Building Regulations for England, which included amendments to Approved Document F (Ventilation) and Approved Document L (Conservation of fuel and power), as well as the release of a new approved document for Overheating (Part O).

Here, we’ll take a look at how the changes to Part L, in particular, affect one of our services – Air Tightness Testing.

Air tightness is an important element of building regulations in the UK. Testing air tightness in new properties, as per Part L of UK Building Regulations can make a big difference to the energy efficiency of properties as well as how comfortable and well-heated they are for occupants. Better standards of air tightness will mean that there is less air leakage occurring, which in turn will make for a more robust building envelope and better heat retention and energy efficiency.