Airtightness entered mainstream UK regulation through the energy-efficiency requirements of Part L of the Building Regulations. The framework has tightened over successive editions, but it remains a minimum-standards regime — and the gap between a regulatory pass and genuinely good performance is large.
Approved Document L — the legal requirement
In England, Part L (conservation of fuel and power) sets the airtightness requirement for new dwellings, expressed as air permeability (m³/h·m² at 50 Pa). The key points:
- New dwellings must demonstrate airtightness, normally by physical testing, and meet a maximum air permeability limit (historically a limiting value around 8 m³/h·m², with most designs targeting lower to meet the overall energy calculation).
- Since the 2021 edition (in force 2022), the regime tightened: more dwellings must be tested, the previous option to assume a default poor value without testing was effectively removed, and untested dwellings can no longer simply assume a penalty figure.
- The as-built airtightness result feeds into the SAP energy calculation, so a worse-than-designed test result can push the whole dwelling out of compliance.
- Testing must be carried out by a competent, registered tester (see ATTMA below), and the result is documented for Building Control.
ATTMA — who is allowed to test
The Air Tightness Testing & Measurement Association (ATTMA) is the UK competent-persons scheme for airtightness testing. Tests submitted for Building Regulations compliance must generally be carried out by an ATTMA-registered tester (or equivalent UKAS-accredited scheme), following the relevant Technical Standard:
| Standard | Applies to |
|---|---|
| ATTMA TSL1 | Dwellings (domestic) |
| ATTMA TSL2 | Non-domestic buildings |
| BS EN ISO 9972 | Underlying international test method |
Registration matters because the result is only as trustworthy as the tester's competence and equipment calibration. An unregistered or careless test — wrong preparation, mis-measured geometry, single-point shortcuts — can produce a number that looks compliant but is meaningless.
The Part F link — you cannot tighten without ventilating
Crucially, the airtightness requirement is tied to ventilation. Approved Document F (means of ventilation) sets minimum ventilation provision, and the tighter a dwelling is built, the more important compliant, designed ventilation becomes. The 2021 editions strengthened this link, recognising that increasing airtightness without adequate ventilation creates condensation, mould and indoor-air-quality risks. Regulation now expects 'build tight, ventilate right' to be applied together — not airtightness in isolation.
How Building Regs compare to Passive House
| Standard | Requirement | Verification |
|---|---|---|
| Part L (new dwelling) | ≈ 8 m³/h·m² limit; ~5 commonly designed | Mandatory test (ATTMA registered) |
| Good airtight practice | 1–3 m³/h·m² | Tested, with an air-barrier strategy |
| EnerPHit (retrofit) | ≤ 1.0 ACH₅₀ | Mandatory blower door, both directions |
| Passive House (new build) | ≤ 0.6 ACH₅₀ | Mandatory blower door, both directions |
What this means for retrofit
Most airtightness regulation targets new build; existing homes are largely unregulated for airtightness unless a notifiable change triggers it. That is both an opportunity and a risk. The opportunity: air-leakage reduction is one of the most cost-effective retrofit measures available. The risk: improving airtightness without simultaneously upgrading ventilation — exactly the mistake the regulations now guard against in new build — is alarmingly common in unregulated DIY and contractor retrofits, and it causes condensation and mould. Apply the Passive House discipline whether or not regulation forces you to.