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Creating an airtight building is one of the most effective ways to improve energy efficiency, reduce heat loss, and meet current building regulations such as Part L. Whether you’re working on a new build, a commercial building, or upgrading an existing building, controlling air leakage is key to achieving good performance.
Uncontrolled air leaks in the building envelope can lead to higher energy bills, reduced comfort, and issues such as moisture intrusion. By focusing on air permeability and preventing air leakage early in the build process, you can create a more efficient and reliable building.
In this guide, we’ll explain how to achieve airtightness, the key principles to follow, and when to use airtightness testing services to ensure your building meets the required standards.
What is air permeability?
To achieve good airtightness, it’s important to understand how air permeability is measured and why it matters.
Air permeability is the amount of air leakage through a building’s envelope, including walls, floors, and roofs, as well as around openings such as windows and doors. It is tested under a set reference pressure of 50 Pa and expressed as m³/h·m² (hr m2).
In simple terms, the higher the air permeability, the more air leaks a building has.
Excessive or uncontrolled air leakage can lead to:
- Increased heat loss and higher space heating demand
- Reduced energy efficiency and higher energy bills
- Poor indoor air quality due to unfiltered air flow
- Greater risk of moisture intrusion and condensation
By controlling air leakage, you create a more consistent and energy-efficient building. This allows ventilation to be properly managed using systems such as mechanical or natural ventilation, rather than losing heat through unintended gaps in the building.
Key principles for achieving airtightness
To achieve airtightness, it’s important to take a planned, consistent approach with your target air permeability in mind. Whether you’re working on a new build, low-energy home, or upgrading existing buildings, these key principles will help reduce air leakage, improve energy performance, and support compliance with Part L.
Define a continuous airtight layer
A continuous airtight layer (or airtight barrier) is the foundation of good building airtightness. Without it, air leaks can occur throughout the building envelope.
- Identify the airtightness layer early in the design phase
- Make sure it runs continuously across walls, floors, roofs, and all junctions
- Avoid gaps caused by changes in building materials or poor detailing
A clearly defined airtight layer makes it much easier to control air leakage and achieve your target air permeability rate.
Design robust junction details
Junctions are one of the most common areas for uncontrolled air leakage, so getting these details right is essential.
- Pay close attention to windows and doors, wall-to-floor connections, and roof interfaces
- Make sure all junctions are sealed and properly connected to the airtight barrier
- Reduce weak points where external wind can push air into the building
Well-designed junctions play a key role in preventing air leakage and maintaining overall thermal performance.
Specify the right materials
Choosing the right materials is key to controlling air leakage and creating a reliable airtight barrier.
- Use airtight membranes, tapes, and sealants designed for airtightness
- Install vapour control layers where needed
- Make sure materials are compatible with each other
- Use suitable products for sealing porous surfaces
All components need to work together to form a strong, continuous airtight layer.
Prioritise quality workmanship
Even the best design can fall short if it isn’t installed correctly. Good building airtightness depends heavily on how well the work is carried out on site.
- Make sure all trades understand the importance of airtightness
- Check that air barriers and sealing products are installed properly
- Carry out regular quality checks during construction
Consistent workmanship helps reduce air leaks and avoid problems during the final test.
Manage service openings and building services
Openings for pipes, ducts, and cables are a major source of air leakage in both new dwellings and existing buildings.
- Seal around all pipes, ducts, and cables passing through the building envelope
- Use airtight grommets, collars, or similar sealing solutions
- Coordinate with mechanical ventilation systems and electrical installations
Careful attention to these areas helps prevent air leakage and ensures your building meets its air permeability target during airtightness testing.
Taking a proactive approach to airtightness early in your project can help avoid costly issues and improve overall building performance. If you need support, Falcon Energy offers expert advice and professional testing services to help you confidently meet your air-permeability targets.
How to improve airtightness and reduce air leakage on site
Improving building airtightness requires a planned approach during construction to reduce air leakage and achieve your target air permeability.
- Carry out pre-completion checks: Inspect the airtight layer before finishes are applied. Identify and fix air leaks early to avoid costly rework.
- Use interim air testing: Undertake air testing before the final test. Support accurately determining air leakage while areas are still accessible.
- Coordinate between trades: Ensure all trades understand the airtightness strategy. Prevent damage to the airtightness layer during installation.
- Document the airtightness strategy: Provide clear drawings and specifications. Ensure consistent implementation across the build.
- Engage specialists early: Work with experts in airtightness testing. Reduce the risk of failure and project delays.
Taking these steps during construction will improve overall energy performance, reduce the risk of failure at the final test, and ensure your building meets its required air-permeability target.
The role of airtightness testing
Airtightness testing, often referred to as a blower door test, is used to measure the level of air leakage in a building and to confirm whether it meets the required air permeability targets under Part L.
The test involves installing a calibrated fan in an external doorway and pressure testing the building using the fan pressurisation method. This creates a controlled pressure difference, allowing air leakage to be measured at a standard reference pressure of 50 Pa. The results show how much air is escaping through the building envelope and help identify areas of uncontrolled air leakage.
Timing is key to achieving a good result. It’s recommended to undertake air testing at two stages:
- Interim test (recommended): Carried out once the airtight layer is complete but before finishes are applied, making it easier to identify and fix air leaks.
- Final test (required): Completed at the end of the project to demonstrate compliance with Approved Document L and meet testing requirements.
Leaving testing too late can increase the risk of failure, particularly if issues are hidden behind completed finishes.
Ultimately, airtightness testing plays a vital role in verifying a building’s airtightness, improving energy performance, and ensuring it meets its target air permeability rate.
Airtightness requirements under Part L
Building regulations in the UK are set out in Part L, which defines minimum standards for airtightness, air permeability, and overall energy efficiency.
To comply, buildings must:
- Meet a maximum air permeability target
- Undergo airtightness testing for most new-build projects
- Provide verified results to demonstrate compliance
While meeting the minimum standard is required, many projects aim for lower air permeability rates to reduce the risk of failure at the final test and improve overall energy performance.
For existing buildings, improving airtightness can also lead to noticeable energy savings, particularly where air leakage is high.
Summary
Achieving good building airtightness is essential for improving energy efficiency, reducing heat loss, and meeting the requirements of Part L. By understanding air permeability and addressing air leakage early, through a well-designed airtight layer, careful detailing, and the right materials, you can significantly improve overall building performance in both new and existing buildings.
A consistent approach during construction, combined with timely airtightness testing, helps ensure your building meets its target air permeability rate and avoids issues at the final stage. By preventing uncontrolled air leakage and managing airflow effectively, you can reduce energy bills, improve comfort, and achieve long-term energy savings.
Book airtightness testing with Falcon Energy
Ensure your building meets Part L requirements with professional airtightness testing from Falcon Energy. We provide reliable testing, clear results, and practical advice to help improve performance and reduce air leakage.
Book your airtightness test today, or speak with our team for a free, no-obligation quote. Call us on 01403 253439 or email info@falconenergy.co.uk.
Frequently Asked Questions
You can reduce air leakage by installing a continuous airtight barrier, sealing service openings, using vapour control layers, and ensuring high-quality workmanship. Early design and coordination are key to controlling air leakage.
While not always mandatory, airtightness testing in existing buildings can identify air leaks, improve energy performance, and help reduce energy bills, making it a valuable step in retrofit projects.
If a building fails to meet its air permeability target, remedial work is required to reduce air leakage, followed by a retest to demonstrate compliance with Part L before the project can be signed off.
This article provides general regulatory guidance and does not constitute legal advice. EPC obligations and exemptions should always be assessed on a case-by-case basis.