Airtightness Testing:
The Complete Guide for New Builds

What is an Airtightness Test?

An airtightness test (blower door test) measures how much air leaks through your building envelope — the walls, roof, floor, windows, doors, and all the junctions between them. A calibrated fan is mounted in an external doorway and depressurises the building to 50 Pascals. The airflow required to maintain that pressure difference is measured and expressed as air permeability in m³/hr/m² at 50 Pa.

Under Part L 2019, all new dwellings in Ireland must be airtightness tested. The result directly affects your BER calculation — a leaky building will score significantly worse than a tight one, even if every other specification is identical.

Airtightness Values: What's Good?

Understanding the scale helps set realistic targets for your build:

• Part L default (no test): 7.0 m³/hr/m² — this is assumed if you don't test. It heavily penalises your BER.
• Part L maximum: 5.0 m³/hr/m² — the worst result you can achieve while still meeting regulations (though most new builds perform better)
• Typical new build: 3.0–5.0 m³/hr/m² — what most well-built new houses achieve
• Good new build: 1.5–3.0 m³/hr/m² — achievable with careful attention to airtightness detailing
• Excellent: Below 1.0 m³/hr/m² — approaching passive house territory
• Passive house standard: 0.6 ACH at 50 Pa (approximately 0.6 m³/hr/m²)

When to Test

Timing is critical. The ideal point for airtightness testing is when the building envelope is complete but before final finishes cover potential air paths. Specifically:

• Plastering complete (wet plaster acts as an air barrier on internal walls)
• Windows and external doors installed and sealed
• First fix electrical and plumbing complete (penetrations sealed)
• Attic insulation not yet installed (allows access to seal any identified leaks)
• Before second fix and before skirting boards, architraves, and floor finishes

Testing at this stage gives you the chance to identify and fix leaks before they're permanently hidden behind finishes.

Most Common Failure Points

After testing thousands of new dwellings, we consistently see the same air leakage paths:

• Service penetrations: Where pipes, cables, and ducts pass through the building envelope — particularly at eaves level and through ground floors
• Window and door reveals: Gaps between the frame and the masonry, especially at the head and sill
• Eaves junction: Where the wall meets the roof structure — this is the single most common leakage path in Irish construction
• Utility entry points: Gas, water, electricity, and broadband entry points through external walls or floors
• Attic hatches: Poorly sealed or lightweight hatches without compression seals
• Recessed downlights: Each unprotected recessed light is a direct air path through the ceiling
• Socket boxes on external walls: Back boxes that penetrate the air barrier

How to Prepare for Your Test

Good preparation typically improves results by 1-2 m³/hr/m². Before we arrive to test:

• Seal all temporary openings and unfinished penetrations with tape
• Close all windows and external doors
• Open all internal doors
• Ensure all mechanical ventilation is off
• Block any open chimneys or flues (we can do this on arrival)
• Run a smoke pencil around suspected leak areas to identify issues

What Happens if You Fail?

If the result exceeds Part L requirements, don't panic. The test identifies where air is leaking, which means you know exactly what to fix. Common remedial measures include additional sealant around window frames, taping of service penetrations, sealing of eaves junctions, and installing airtight membranes around attic hatches. A retest after remedial work typically shows significant improvement.