Airtightness Estimator — Air Permeability at 50 Pa

Part L 2021 (Conservation of Fuel and Power, Volume 1: Dwellings) sets a limiting (backstop) air permeability of 8 m³/h.m² at 50 Pa for new dwellings — the absolute maximum permitted. The notional dwelling used for the energy-performance check is specified at 5 m³/h.m², so designing to 5 is the good-practice target that eases overall compliance, not a separate legal limit. The Future Homes Standard was published on 24 March 2026 and comes into force on 24 March 2027 (with a 12-month transition). It keeps the 8 m³/h.m² legal backstop but tightens the notional dwelling to 3 m³/h.m² at 50 Pa, with the notional specifying mechanical extract ventilation (dMEV; many builders fit MVHR instead). Passivhaus requires 0.6 air changes per hour at 50 Pa, roughly 1-2 m³/h.m². Most new builds achieve 3-5 m³/h.m² with good practice.

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.

A blower door test (BS EN ISO 9972, with UK testing to ATTMA TSL1) 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, expressed as m³/h.m² of envelope area. Costs are market-variable: a single new-build dwelling test typically starts from around £95 plus VAT, with more for larger or complex dwellings and volume discounts on multi-plot developments — treat any figure as indicative and get a quote. Since June 2022 every new-build plot in England needs its own air test.

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.

Air permeability (q50, m³/h.m²) measures leakage per unit of building envelope area — used in UK Building Regulations. Air changes per hour at 50 Pa (n50) measures how many times the building's internal volume is replaced per hour — used in Passivhaus. To convert: n50 = (Air permeability × Envelope area) ÷ Building volume. For a typical dwelling the envelope area and internal volume are numerically similar, so q50 and n50 come out roughly EQUAL (ratio ≈ 1) — the difference grows only for very small or very large/unusual building forms. Do not confuse this with the everyday natural air-change rate: SAP uses the 'divide by 20' rule of thumb (background infiltration ≈ q50 ÷ 20) to estimate the air-change rate under normal pressures rather than the 50 Pa test pressure.