Glossary
Airtightness testing and what Part L expects in London
Resistance to air leakage. Tested at building-regs sign-off (in homes above a threshold size).
Resistance to air leakage. Tested at building-regs sign-off (in homes above a threshold size).
Airtightness is how much air leaks through the building fabric, measured as air permeability in m³/(h·m²) at 50 Pa. Approved Document L sets a maximum of 8 m³/(h·m²) for new dwellings, with 5 or better needed in practice to pass the SAP calculation. Extensions are not usually tested, but the same detailing decides whether the room is comfortable.
A blower door fan is sealed into an external doorway and the dwelling is pressurised to 50 pascals. The airflow needed to hold that pressure, divided by the envelope area, gives air permeability. A new dwelling in England must not exceed 8 m³/(h·m²), and the design air permeability used in the SAP calculation has to be met or bettered on test.
Extensions and most conversions are not pressure tested. Compliance is instead shown through the elemental U-value route, so the airtightness work is judged on the drawings and on site rather than by a number.
The dominant leakage paths in a London retrofit are the loft hatch, the junction between a suspended timber floor and the external wall, service penetrations under the kitchen units, unsealed downlights, and the gap behind dry-lined walls that connects the floor void to the loft.
A continuous air barrier line drawn on the section — one unbroken pencil line around the heated envelope — is the single most useful drawing for airtightness. Where the line has to break, the detail is drawn.
Making a house tighter without adding ventilation causes condensation and poor indoor air quality. Approved Document F sets the ventilation rates that go with the tighter fabric — trickle ventilators in habitable rooms, intermittent extract at 30 l/s in a kitchen or 15 l/s continuous, and 6 l/s intermittent in a bathroom.
Below roughly 3 m³/(h·m²), continuous mechanical ventilation with heat recovery becomes the sensible system, because natural background ventilation can no longer be relied on.
How airtightness plays out in Merton: London Borough of Merton (Merton Council) maintains 28 conservation areas, the Wimbledon Village Conservation Area and the John Innes Conservation Area among them, and there is no borough-wide Article 4 direction, so permitted development generally survives outside conservation areas. Anyone dealing with airtightness at a Merton address is therefore working to a £548 householder application decided in 8 weeks, or a £249 prior approval over 6 weeks where airtightness still qualifies.
| Regulatory maximum, new dwellings | 8 m³/(h·m²) at 50 Pa |
|---|---|
| Typical good practice | 3–5 m³/(h·m²) at 50 Pa |
| Passivhaus benchmark | 0.6 air changes per hour at 50 Pa |
| Test method | Blower door, pressurisation to 50 Pa |
| Governing document | Approved Document L, with Approved Document F |
| Applies to extensions? | Not usually tested; detailing still required |
No. Air pressure testing applies to new dwellings. An extension complies through U-values and detailing instead.
Only if ventilation is not designed with it. Tight fabric plus a properly sized MVHR or continuous extract system is comfortable and healthy.
An ATTMA-registered or UKAS-accredited tester, normally booked by the contractor near completion.
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