Failing an air tightness test can be frustrating, especially when deadlines and handovers are looming. A fail air tightness test result is more common than many expect, but it’s usually avoidable with the right preparation and diagnostic support. At Air Tightness Solutions, we work closely with every client to help ensure they pass first time. Our engineers provide clear, practical advice before testing — and we highlight common problem areas well in advance to avoid surprises on the day.
According to UK Building Regulations guidance, airtightness is a mandatory part of demonstrating overall energy performance in new buildings. Let’s break down the most frequent causes of failure, how they affect compliance, and — most importantly — how to fix them quickly and efficiently.
Understanding What “Fail” Means in an Air Tightness Test
A building fails an air tightness test when the measured air permeability exceeds the design air leakage target — usually 5.0 m³/h·m² @ 50 Pa for dwellings under current UK standards.
These targets are in place to ensure buildings meet energy performance expectations. A failed test indicates the building allows more uncontrolled airflow than permitted, meaning it doesn’t comply with regulations until resolved.
Refer to Approved Document Part L for technical guidance and threshold values.
Common Causes of Air Tightness Test Failure
1. Unsealed Service Penetrations
A common oversight is failing to seal around pipes, cables, and ducts that pass through building fabric. Problem areas include boiler cupboards, floor voids, and behind kitchen units.
Fix: Use mastic, expanding foam, or grommets to seal all visible penetrations. Ensure any fire-stopping requirements are also met where services pass between compartments.
2. Poorly Installed Doors and Windows
If window frames or external doors are not sealed properly, they may allow air to leak even when closed. Frame-to-wall junctions are often weak points.
Fix: Inspect for gaps, damaged compression seals, or faulty latching. Replace, adjust, or reseal as necessary, especially around the edges of openings.
3. Loft Hatches and Access Panels
Ceiling hatches, riser doors, and access panels are often unsealed or loosely fitted, allowing significant leakage under pressure.
Fix: Add foam or rubber seals and ensure secure closures. Tape over temporarily if needed for testing purposes.
4. Unsealed Ventilation Terminals
Extractor fans, trickle vents, and MVHR outlets can skew test results if left open during testing.
Fix: Temporarily cover or seal all ventilation outlets. Don’t forget air bricks or passive vents.
5. Skirting and Floor Junction Gaps
Air leakage at floor-to-wall junctions, behind skirting boards, or within floor voids can dramatically affect results.
Fix: Apply mastic to skirting edges and seal floor void penetrations. Ensure continuity of air barrier materials across levels.
6. Poor Site Readiness
Tests carried out too early — before full envelope sealing or with trades still on site — frequently fail. Movement, open access points, or temporary works often introduce avoidable leakage paths.
Fix: Follow a structured checklist and ensure all necessary finishing work is complete before scheduling your test. Use our preparation guide: Prepare for a Building Air Tightness Test
What to Do After a Failed Test
If your building fails, stay calm. It’s relatively common, and with the right approach, it can usually be resolved without significant disruption.
Request Diagnostic Testing: We offer targeted smoke testing and thermographic surveys to help identify specific leakage areas. This removes guesswork and supports focused remediation.
Book a Retest Promptly: After addressing issues, we can often offer same-day or next-day retesting. This helps keep your project on schedule and ensures updated certification without long delays. Learn more on our Air Tightness Testing service page
How to Prevent Failure in Future Projects
Avoid future failures by integrating airtightness into the design and build process early. Use continuous air barrier strategies, train trades on sealing standards, and schedule internal inspections before the test. Our Building Compliance Services can support this from design through to completion.
Need Help After a Failed Air Tightness Test?
If you’ve received a failed result, or want to ensure you avoid one, our experienced team is here to help. We offer professional advice, diagnostics, and retesting — all designed to get your building compliant as efficiently as possible.
Contact us to speak with an expert and get your project back on track.