Getting ready for a building air tightness test is a key step in ensuring your project meets modern energy efficiency standards and complies with Part L of the Building Regulations. Whether you’re managing a small residential build or a large commercial development, a smooth testing process starts with good preparation.
This guide will walk you through what needs to be done on-site before the test takes place — helping you avoid unnecessary delays, retesting fees, or compliance headaches.
What Is a Building Air Tightness Test?
A building air tightness test (also called an air leakage test or air permeability test) measures how much air escapes through gaps and cracks in the building’s fabric. It’s a way of checking how “leaky” the building is and whether it meets the standards set out in Approved Document Part L of the Building Regulations.
The test is typically carried out using a blower door system, which pressurises or depressurises the building to measure airflow and identify leakage rates. Results are expressed in cubic metres per hour per square metre (m³/h·m²) at a pressure of 50 Pascals.
You can read more about how the process works in our guide: What is Air Tightness Testing?
Why Does Preparation Matter?
Preparing your site properly can make the difference between a pass and a fail. According to the NHBC Foundation, around 35% of new homes fail their first air tightness test due to issues that could have been avoided with better on-site checks.
Uncontrolled air leakage doesn’t just affect compliance — it impacts energy bills and carbon emissions too. The UK Committee on Climate Change reports that over 25% of heat loss in poorly sealed buildings happens through unintended air gaps.
With regulations getting stricter, especially in the lead-up to the Future Homes Standard, it’s more important than ever to be test-ready.
How to Prepare for a Building Air Tightness Test
1. Choose the Right Time for Testing
Your building should be externally complete before testing. That means:
- All windows and doors are fully installed and operational
- Any external cladding or sealing is finished
- No large temporary openings are left exposed
If the test is carried out too early, there’s a high risk of failure — and a need for costly retesting.
2. Seal Up All Unused Openings
Temporary holes for services, ducts, or cables should be sealed up using:
- Expanding foam
- Mastic
- Fire collars or grommets where required
Look out for common problem areas such as pipe penetrations, boiler flues, behind kitchen units, or at the junction of floors and walls.
3. Finish Wet Trades First
Plastering, screeding, and painting should be done before the test. These finishing stages often help seal gaps and joints, and damp materials can interfere with accurate pressure readings.
4. Switch Off Mechanical Systems
Mechanical ventilation systems (e.g. MVHR or extractor fans) should be:
- Switched off before testing
- Temporarily sealed or taped up
This ensures the test measures only uncontrolled air leakage, not intended airflow through vents.
5. Have the Right Info Ready
On test day, make sure your testing engineer has:
- Floor plans and elevations (preferably digital)
- The building’s internal volume or envelope area if known
- Clear access to the building and a main external door
It also helps to have someone on-site who knows the building layout and can assist if quick fixes are needed.
6. Do a Walk-Through Inspection
Before the testing team arrives, do a final walkthrough and check:
- Window and door seals are fully installed
- Loft hatches and riser cupboards are closed and sealed
- Services that penetrate walls or floors are fully sealed
Need help getting ready? Our Air Tightness Testing service includes optional diagnostic testing with smoke pencils and thermal cameras to identify leaks before the test begins.
7. Keep the Test Area Clear
The blower door rig is usually set up at a main external door. Ensure:
- The area is clean and free of tools or debris
- There’s good access inside and outside
- No trades are working in the building during the test
What Happens on Test Day?
A qualified ATTMA-accredited engineer will arrive on-site and:
- Set up calibrated test equipment at the main doorway
- Carry out pressurisation and depressurisation tests
- Record pressure differentials, temperature, and air flow
- Provide immediate feedback on the result
If the building passes, we’ll issue your air test certificate within 48 hours. If it fails, we’ll highlight the problem areas and advise on what to do next.
To see exactly what’s included in our testing packages, visit our air tightness testing page.
Be Prepared, Stay Compliant
A well-prepared site is key to passing your building air tightness test the first time. By sealing common leakage points, finalising finishes, and providing your engineer with clear access and information, you can avoid project delays and meet regulatory requirements with confidence.
We also offer full building compliance services to help you meet all relevant standards in one go.
If you’re ready to book a test or have questions about your site’s readiness, contact our team — we’re here to help.