If you’re approaching practical completion on a new build and someone mentions air tightness testing, don’t panic — but don’t ignore it either. It’s a legal requirement for most new residential and commercial buildings in the UK, and failing to organise it at the right time can cause delays that push back your completion certificate.
Here’s a clear, practical guide to what air tightness testing involves, when you need it, what to expect on the day, and how to give your project the best chance of passing first time.
What Is Air Tightness Testing?
Air tightness testing — sometimes called a pressure test or air permeability test — measures how much air leaks in and out of a building through unintentional gaps and cracks in the building fabric.
The test uses a large fan (a blower door) fitted into an external door or window frame. The fan depressurises the building and measures the airflow required to maintain a stable pressure difference. The result is expressed in m³/(h.m²) at 50 Pascals — in plain English, how many cubic metres of air leak through each square metre of the building envelope per hour.
The lower the number, the more airtight the building — and the better it performs for energy efficiency.
Why Is It a Legal Requirement?
Part L of the Building Regulations — which covers the conservation of fuel and power — requires that new buildings meet minimum energy efficiency standards. Air tightness is a key part of that, because even the most well-insulated building performs poorly if warm air is constantly escaping through gaps.
The Part L 2021 update introduced tighter air permeability targets for new homes and commercial buildings. The design target (set in your SAP calculations or SBEM calculations) must be confirmed by an on-site test before Building Control will issue your Completion Certificate.
In other words: no test, no sign-off. It really is that simple.
Who Needs Air Tightness Testing?
Generally speaking, you’ll need a test if you’re building:
- A new residential property (house, flat, or apartment building)
- A new commercial building (offices, retail, industrial)
- A conversion where the building is being treated as a new dwelling
- A Passivhaus-certified project (which has much stricter targets)
If you’re carrying out a refurbishment that doesn’t add a new dwelling, you may not need a formal test — though diagnostic air leakage testing can still be a useful tool for identifying where heat is escaping. Speak to an assessor if you’re unsure.
What Are the Pass/Fail Criteria?
There isn’t a single blanket pass/fail figure — the target depends on the type of building and the standards it was designed to.
For most new homes under Part L 2021, the design air permeability target is typically 8 m³/(h.m²) or better, with a maximum permitted of 10 m³/(h.m²). The as-built test must achieve the design target set in the SAP calculations.
For commercial buildings, SBEM calculations set out the equivalent targets under Part L non-domestic.
Passivhaus projects have a far stricter requirement of 0.6 air changes per hour at 50 Pa — which requires very careful detailing throughout the build.
If your design target is ambitious, it’s worth discussing this early with your assessor. Getting involved at the design stage can help identify any potential weak points before they become expensive problems.
How Is the Test Actually Carried Out?
On the day, here’s what typically happens:
- Preparation: All intentional ventilation openings (trickle vents, extract fans, chimneys) are sealed before the test. Doors and windows should be shut.
- Fan installation: The blower door fan is fitted into an external door opening.
- Pressurisation/depressurisation: The fan runs through a series of pressure differentials, usually in both pressurisation and depressurisation modes.
- Readings: Airflow measurements are taken at each pressure point and the results are extrapolated to give a figure at 50 Pa.
- Report: Results are calculated on-site and a formal report is issued, usually on the same day or within 24 hours.
The test itself typically takes around 1–2 hours for a single dwelling, though this can vary depending on the size and complexity of the building.
How to Give Your Project the Best Chance of Passing
Here’s the thing — air tightness is built into your building, not bolted on at the end. The result depends on how carefully the building has been constructed. That said, there are a few practical steps that help:
- Use a good membrane and seal laps and penetrations properly. The majority of air leakage in new builds comes from poorly taped joints or unsealed service penetrations (pipes, cables, ducts).
- Get a pre-completion check. Some developers bring in a tester for an informal air pressure check before the formal test. This gives you a chance to find and fix any major leakage paths before it counts.
- Communicate with your trades. Plasterers, electricians, and plumbers all have the potential to compromise the airtightness layer. Make sure they understand what they’re working with.
- Don’t rush the test. Book the test when the building is genuinely ready — all finishes complete and services installed. Testing too early creates problems.
What Happens If the Building Fails?
It’s not the end of the world. If a building fails its air tightness test, you have two main options:
- Identify and fix the air leakage paths and re-test. An experienced tester can often help you locate the main leakage areas during the test using smoke pencils or other diagnostics.
- Compensate through other means (limited and very project-specific — always check with your assessor).
Remediation usually involves sealing around service penetrations, re-taping membrane joints, or addressing specific areas identified during testing. Most failures can be rectified and the building re-tested.
What About Other On-Site Tests?
Air tightness is usually just one of the tests required for Building Control sign-off. Depending on your project, you may also need:
- Sound insulation testing (between dwellings, or between dwellings and commercial spaces)
- Ventilation flow rate balancing (confirming mechanical ventilation systems deliver the specified airflow rates)
- Smoke shaft testing (for certain residential developments)
- Passivhaus testing (for certified schemes)
If you’re juggling multiple tests, it makes sense to use a single provider who can coordinate everything. Our as-built testing services cover all the above — one contact, one programme, no coordination headaches.
Frequently Asked Questions
When should I book the air tightness test? Book it once the building is fully finished internally — all finishes complete, services installed, and intentional openings identifiable. It should happen before Building Control inspection, not after. Aim to have it in your programme at least 2–3 weeks before your target completion.
Can I test individual flats within a larger development? Yes. For apartment blocks, individual units are typically tested to a representative sampling strategy agreed with Building Control. Not every flat necessarily needs to be tested, but a minimum number will.
How long does an air tightness test take? For a typical single-storey or two-storey house, approximately 1–2 hours on-site. For larger or more complex buildings, allow longer.
Do I need to be on-site for the test? Someone with site access and knowledge of the building should be present — usually the site manager. The tester will need access to all habitable rooms and may need to identify service penetrations.
Does the air tightness result affect my EPC? Yes. The as-built air permeability result feeds directly into the final SAP calculation, which generates the new build EPC. A significantly worse result than the design target can affect the EPC rating.
How much does air tightness testing cost in the UK? For a single dwelling, you can typically expect to pay in the region of £200–£400, depending on size and location. Larger developments benefit from economies of scale. Contact us for a specific quote for your project.
Ready to Book Your Air Tightness Test?
ARMEEC provides air tightness testing across the UK, with nationwide coverage and programme-friendly scheduling. We’re ECMK registered and Building Control accepted — and we understand that construction programmes don’t wait around.
Visit our air tightness testing service page for more information, or request a free quote and we’ll respond within 2 hours.