In this blog, you’ll learn:

The Air Tightness Testing & Measurement Association (ATTMA) defines ‘air leakage’ as the “uncontrolled flow of air through gaps and cracks in the fabric of a building. It is sometimes known as infiltration or draughts”. This means that the measurement of air permeability is how ‘leaky’ or ‘tight’ a building is and this characteristic has critical implications for both energy efficiency and indoor air quality.

Air Permeability & Building Design

Understanding and managing air leakage is not just about energy conservation; it’s a fundamental aspect of sustainable building practices. Effective control of air permeability is essential for meeting the demands of stringent building codes and for reducing the environmental impact of construction.

Air permeability is not to be confused with ventilation but it’s important to understand ventilation in relation to building regulations as the two are interconnected.

Ventilation in buildings is regulated by Part F of the Building Regulations, which outlines the requirements for adequate and effective ventilation systems to ensure good indoor air quality and prevent excess moisture within the building fabric. This part of the regulations is crucial for maintaining the health and comfort of occupants by specifying minimum ventilation rates for different types of spaces, from residential to commercial and everything in between.

In the UK, all new developments generally require an Air Tightness Test as part of Building Regulations. The test is conducted before occupancy, influencing energy ratings. 

For those in the fields of property development, home building, and architecture, understanding air permeability and its effects on design is crucial for delivering projects that are not only high in quality and efficiency but also compliant with regulatory standards.

The Impact of Air Permeability on Energy Efficiency

A building with high air permeability, meaning more leaks, can lead to significant energy loss. In winter, warm air escapes easily, forcing heating systems to work overtime, while in summer, hot air can infiltrate, making cooling systems less efficient. This not only increases energy consumption and costs but also puts additional strain on HVAC systems, potentially reducing their lifespan.

On the other hand, a building with controlled air permeability minimises unwanted air flow, maintaining a stable internal environment and reducing the need for heating and cooling. This balance is crucial, as too little air exchange can lead to stagnant air and associated health issues, while too much can result in unnecessary energy use and discomfort.

Indoor Air Quality

The rate of air exchange directly influences a building’s indoor air quality. Properly managed air permeability ensures that there is enough fresh air coming in to dilute and remove pollutants, such as volatile organic compounds (VOCs), allergens, and moisture, which can lead to mould growth. This is particularly important in modern, energy-efficient designs where buildings are made as airtight as possible to conserve energy.

Thermal Insulation 

Air permeability and thermal insulation are two sides of the same coin in building performance. While thermal insulation focuses on resisting the conductive flow of heat through the building’s fabric, air permeability deals with the movement of air. Together, they determine the building’s overall thermal efficiency. Effective insulation can be undermined by poor air tightness, as even the smallest gaps can lead to significant heat loss, demonstrating that both must be optimised to achieve the best performance.

The Air Permeability Testing Process

Air tightness testing, also known as a blower door test, is a diagnostic procedure used to quantify the air permeability of a building’s envelope. This test involves creating a pressure differential between the inside and outside of the building to force air through any unsealed cracks or openings, thereby measuring the air leakage rate.

There are 3 different levels of air testing that have been established from the size and complexity of a building:

The Stages of Air Permeability Testing


Before the test, the building is prepared by sealing off designed openings such as extraction vents and air bricks. Internal doors are left open, and heating or cooling systems are turned off to ensure accurate measurements.

Equipment Setup 

A blower door, which is a powerful, calibrated fan mounted into an exterior door frame, is the primary tool used. The fan is capable of both depressurizing and pressurising the building to test air infiltration and exfiltration rates, respectively.

Conducting the Air Permeability Test 

The blower door fan is activated to either pull air out of the building (depressurize) or push air into it (pressurise), gradually increasing the pressure difference. This process is closely monitored using specialised gauges that measure the pressure differential and the amount of air needed to maintain it.

Data Collection & Analysis 

The air flow rate required to maintain a range of building pressure differentials (between 10-90 Pascals) is recorded. This data is used to calculate the building’s air permeability, typically expressed in cubic metres per hour per square metre of envelope area at 50 Pascals (m³/h·m²@50Pa).

What Score Determines Good Air Permeability?

When it comes to air tightness testing, a score near 10 suggests the building isn’t quite efficient enough. Ideally, you’d want a score just under 5, which typically indicates the building is sufficiently airtight to pass the test and earn a certification. Falling short of this benchmark can mean failing the test, which can delay meeting building regulations. 

The more airtight the building, the better it is for maintaining a comfortable environment and saving on heating costs. However, if a building scores below 3, it’s so airtight that mechanical ventilation might be necessary to prevent issues like dampness and poor air quality, which could affect health.
The Part L Future Homes Standard requires air testing for all new dwellings, eliminating sample testing. Dwellings without mechanical ventilation will receive no benefit in SAP for scores below 3 m³/(h.m²).

Practical Tips For Getting Air Permeability Right

Improving air permeability in new buildings involves a series of strategic steps from the design phase through to construction, focusing on creating a tight building envelope while ensuring proper ventilation.

If you’re a housebuilder, architect or property developer, here’s how you can achieve this.

Stakeholder Education

Ensure that everyone involved in the project, from designers to contractors, understands the importance of air tightness and their role in achieving it. This collective awareness will help in maintaining a high standard of work throughout the project.

Design Consideration 

From the outset, incorporate air tightness objectives into the architectural design. Opt for building shapes and layouts that minimise complex junctions where leaks are likely to occur.

Material Selection 

Choose high-quality, air-sealing materials for the building envelope, including walls, roofs, windows, and doors. Ensure that these materials are compatible and can be effectively sealed together.

Construction Quality 

Implement strict quality control measures during construction to ensure that the workmanship meets the air tightness criteria set out in the design. This includes proper installation of insulation, membranes, and sealants.

Ventilation Strategy 

While striving for air tightness, also plan for controlled ventilation to maintain indoor air quality. This may involve incorporating mechanical ventilation with heat recovery (MVHR) systems to provide fresh air and prevent moisture buildup without compromising the building’s thermal efficiency.

Air Permeability Testing & Verification 

Conduct blower door tests at various construction stages to identify and rectify leaks early on. This proactive approach allows for adjustments before the building is completed, ensuring that the final structure meets or exceeds air tightness standards.


Controlling air permeability is fundamental to designing buildings that are comfortable, healthy, and energy-efficient. It requires a careful balance, taking into account the need for ventilation and fresh air while minimising unnecessary energy loss. 

By considering air permeability alongside thermal insulation, architects and builders can ensure that a building performs optimally, providing a comfortable and sustainable environment for its occupants.

Working with Building Energy Experts

Our Air Tightness Testing service tells you exactly how airtight your building is. 

This can help you plan a decarbonisation strategy, identify ways to save energy and meet regulatory requirements. Our experienced local air tightness engineers work across new build, residential, commercial, and retrofit environments, so can provide whatever support you need to get the job done. Contact us for a free consultation to discuss your needs.

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