What is the Part L airtightness requirement?

Part L 2021 (Conservation of Fuel and Power) requires a maximum air permeability of 8 m³/h.m² at 50 Pa for new dwellings. The Future Homes Standard (expected 2025) targets 5 m³/h.m². Passivhaus standard requires 0.6 air changes per hour at 50 Pa, which is roughly 1-2 m³/h.m². Most new builds achieve 3-5 m³/h.m² with good practice.

What are the main leakage paths in a building?

The biggest leakage paths are: service penetrations (pipes, cables entering the building), window and door junctions, loft hatches, letterboxes, and recessed spotlights. In older buildings, suspended timber floors and party walls are also major sources. Sealing these penetrations with appropriate tapes, membranes, and grommets is the most cost-effective way to improve airtightness.

How is airtightness tested?

A blower door test (BS EN ISO 9972) uses a calibrated fan installed in an external doorway to pressurise or depressurise the building to 50 Pa. The fan speed needed to maintain this pressure difference tells you the total air leakage rate. The result is expressed as m³/h.m² of envelope area. Testing costs £300-600 for a typical dwelling and is mandatory for new builds.

Can a building be too airtight?

Yes — without adequate ventilation, an airtight building can suffer from poor indoor air quality, condensation, and mould. Buildings achieving below 5 m³/h.m² should have mechanical ventilation with heat recovery (MVHR). Below 3 m³/h.m², MVHR is essential. The principle is 'build tight, ventilate right' — control air movement rather than relying on unplanned draughts.

What is the difference between air permeability and air changes per hour?

Air permeability (m³/h.m²) measures leakage per unit of building envelope area — used in UK Building Regulations. Air changes per hour (ACH or n50) measures how many times the building's internal volume is replaced per hour — used in Passivhaus. To convert: ACH = (Air permeability × Envelope area) ÷ Building volume. A typical house has a ratio of about 3-4.

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Airtightness Estimator — Air Permeability at 50 Pa

Estimate building airtightness from leakage paths. Calculate air permeability in m³/h.m² at 50Pa against Part L and Future Homes Standard targets.

Envelope Area

Total external surface area

Build TypeSets target air permeability

Windows

1.5 m³/h each

External Doors

3.0 m³/h each

Loft Hatches

4.0 m³/h each

Service Entries

Gas, water, electric entries

Recessed Spotlights

0.5 m³/h each

Extract Fans

2.5 m³/h each

How We Calculate This

This estimator calculates approximate airtightness by summing leakage contributions from individual building penetrations and the general fabric leakage rate.

The formula

Penetration leakage = Σ(Item count × Leakage rate per item)

Fabric leakage = Envelope area × Base leakage rate

Air permeability = Total leakage ÷ Envelope area

For example, a house with 250 m² envelope, 10 windows (15 m³/h), 3 doors (9 m³/h), 20 spotlights (10 m³/h), and 3.0 base rate: Fabric = 250 × 3.0 = 750. Total = 750 + 34 = 784 m³/h. Permeability = 784 ÷ 250 = 3.14 m³/h.m².