The Complete Guide to Airtightness Testing

The Complete Guide to Airtightness Testing

Everything you need to know about the blower door test — what it measures, how to prepare, what the results mean, and what to do if you fail.

Blower door test setup installed in a doorway to measure building airtightness and air leakage

What is an Airtightness Test?

An airtightness test — also called a blower door test or air pressure test — measures how much air leaks through the fabric of a building when it is pressurised or depressurised. A calibrated fan is fitted into an external opening (typically a front door), and the building is pressurised to 50 Pascals above atmospheric pressure. The airflow required to maintain that pressure differential is measured and used to calculate the building's air permeability.

Air permeability is expressed in m³/h·m² (cubic metres of air per hour per square metre of envelope area) at 50 Pascals. A lower number means a more airtight building. Passivhaus results are often expressed in ACH (air changes per hour), which relates to the building's internal volume rather than its envelope area.

Understanding Your Result

≤8 m³/h·m²UK Building Regulations minimum pass (Part L 2021)
≤5 m³/h·m²Part L 2021 design target — good performance
≤3 m³/h·m²Future Homes Standard / non-domestic target — excellent
≤0.6 ACHPassivhaus standard — outstanding

Why Does Air Leakage Matter?

🌡️ Heat Loss

Air leakage is responsible for 25–40% of heat loss in a typical UK home. Cold air entering through cracks displaces warm air and makes the building significantly more expensive to heat.

💧 Condensation & Mould

Warm moist air leaking through the structure reaches cold surfaces, condenses, and creates the conditions for mould growth and long-term damage to the building fabric.

😷 Indoor Air Quality

Uncontrolled air infiltration bypasses ventilation systems and HEPA filters, bringing in outdoor pollutants, allergens and particulates. Airtight buildings with MVHR provide far better air quality.

📋 Compliance

New buildings must pass an airtightness test under Part L of the Building Regulations. Failure delays sign-off and can require costly remediation works.

Diagram comparing an airtight building with controlled ventilation to a leaky building with uncontrolled air leakage through cracks and gaps

The 10 Most Common Leakage Points

  1. Wall–floor junctions — especially at the junction of the ground floor slab and external walls
  2. Wall–ceiling junctions — where plasterboard meets the perimeter wall
  3. Window and door reveals — around frames where they meet masonry or timber structure
  4. Electrical back boxes — sockets and switches recessed into external walls
  5. Pipe penetrations — where water supply, gas and drainage pipes enter the building fabric
  6. Cable penetrations — mains electricity, data, and telecommunications cables
  7. MVHR and ventilation duct penetrations — where ducts pass through the airtight layer
  8. Loft hatch and access panels — poorly fitted or without compression seals
  9. Eaves and ceiling–roof junctions — especially in warm roof or open-rafter constructions
  10. Oversite / ground floor perimeter — where the DPM or airtight layer meets the external wall below damp-proof course
💡 Tip: Passive Purple Brush is specifically designed for these critical junctions. Applied before the spray coat, it provides a fibre-reinforced, flexible seal at every one of these common failure points.

How to Prepare for Your Blower Door Test

  1. Complete the airtight layerEnsure the full airtight membrane application is complete and fully cured before booking the test. All junctions, penetrations and laps should be sealed.
  2. Close all intentional openingsClose and lock all windows, doors and roof lights. Seal trickle vents with temporary tape. Close all mechanical ventilation dampers.
  3. Seal or block service openingsTemporarily seal letter boxes, extractor fan outlets, open flues and any intentional ventilation openings that would otherwise allow air to escape during the test.
  4. Do your own smoke test firstBefore calling the tester, pressurise the building yourself with a portable fan and run a Wöhler smoke stick along all junctions and around all penetrations. Any smoke being drawn through indicates a leak — seal it with Passive Purple Brush before the official test.
  5. Document your airtight layerTake photographs of all sealed junctions, penetrations and membrane overlaps before they are covered up. This is valuable evidence if a dispute arises with building control and helps identify areas to investigate if the test result is unexpectedly poor.
  6. Book an accredited testerFor Building Regulations compliance, the test must be carried out by an ATTMA-registered tester (UKAS accredited for Part L). Your tester will bring the blower door equipment and issue a compliant test certificate.

Wöhler S 100 smoke stick used at a window to trace air leakage paths before a blower door test

If You Fail — Remediation with Passive Purple

If your test result exceeds the required limit, the tester can often help identify the main leakage locations while the equipment is on-site using smoke, thermography or by hand-feeling for air movement under pressure.

⚠️ Important: Don't panic if you fail. Most failed tests are remediated successfully, and the most common leakage sources are sealed quickly once identified.

Passive Purple Brush is ideal for remediation:

  • It can be applied to already-plastered or painted walls by cutting back the finish at the junction, applying Passive Purple Brush, and re-finishing over the top
  • Around service penetrations, Passive Purple Brush can often be applied without disturbing surrounding finishes
  • It cures quickly (4–6 hours), so a re-test can often be arranged the same or following day

Contact our technical team if you need advice on remediation — we can help you work out where to look and how to fix it.

Frequently Asked Questions

Who has to carry out an airtightness test?

All new dwellings in England must be tested unless they are part of a development where approved repeat testing procedures apply. All new non-domestic buildings must also be tested. The test must be carried out by an ATTMA-registered tester for the results to be accepted by building control.

When in the build programme does the test happen?

The test is carried out after the building is weathertight and all penetrations are sealed, but typically before the final decoration — so that remediation works are still accessible if needed. For new homes, it must be completed before final building control sign-off.

How long does the test take?

A typical domestic blower door test takes 2–3 hours on-site including set-up, testing (both pressurisation and depressurisation), and producing the report. For commercial buildings, testing takes longer and may need multiple fan set-ups.

How much does an airtightness test cost?

Typically £200–£500 for a single dwelling, depending on location and tester. Commercial tests are priced per visit and per zone. Re-tests are usually charged at a reduced rate.

Can I test my own building?

You can carry out a pre-test smoke check yourself (see above), but the official Building Regulations test must be carried out by an accredited tester. If you're a self-builder targeting Passivhaus, your Passivhaus certifier will specify the test protocol.

Related Resources

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