Part L Air Permeability & Airtightness
Air permeability targets, testing requirements, common leakage paths, and implications for ventilation strategy, referencing Approved Document L: Conservation of Fuel and Power, Volume 1 - Dwellings (2021 edition, effective 15 June 2022).
Air Permeability Targets (AD-L 2021)
| Parameter | Value | Notes |
|---|---|---|
| Maximum design air permeability | 8 m³/(h.m²) at 50 Pa | Backstop — must not exceed this |
| Notional dwelling value | 5 m³/(h.m²) at 50 Pa | Used in SAP notional dwelling |
| Good practice target | 3-5 m³/(h.m²) at 50 Pa | Achievable with careful detailing |
| Passivhaus standard | ≤ 0.6 ACH at 50 Pa | Approximately 1-2 m³/(h.m²) |
| MVHR threshold | ≤ 3 m³/(h.m²) at 50 Pa | Below this, MVHR is typically required |
Ventilation Strategy by Airtightness (AD-F & AD-L)
| Air Permeability Range | Suitable Ventilation Systems |
|---|---|
| > 5 m³/(h.m²) | System 1 (intermittent + trickle vents) or System 2 (PSV) |
| 3-5 m³/(h.m²) | System 3 (continuous MEV) or System 4 (MVHR) |
| < 3 m³/(h.m²) | System 4 (MVHR) required |
Common Air Leakage Paths and Sealing Methods
| Leakage Path | Typical Contribution | Sealing Method |
|---|---|---|
| Window/door frames | 15-25% | Expanding foam, tape, or sealant between frame and wall |
| Wall/floor junctions | 10-20% | Flexible sealant or membrane at junction |
| Wall/ceiling junctions | 10-15% | Continuous plaster bead or sealant |
| Service penetrations (pipes, cables) | 10-20% | Intumescent sealant, grommets, or fire-rated foam |
| Loft hatch | 5-10% | Draught-sealed, insulated hatch with compression seal |
| Recessed downlights | 5-15% | Airtight rated fittings or airtight covers above |
| Party wall zone | 5-15% | Sealed cavity at eaves level, continuous plaster |
Testing Requirements
| Requirement | Detail |
|---|---|
| Test standard | BS EN ISO 9972 (Method 1 — building in use) |
| Mandatory for | All new dwellings |
| Tester accreditation | UKAS or ATTMA registered |
| Test pressure | 50 Pa (both pressurisation and depressurisation) |
| Result unit | m³/(h.m²) of envelope area at 50 Pa |
Scotland & Northern Ireland
Scotland has its own airtightness requirements under Technical Handbook Section 6 (Energy), with a maximum air permeability of 10 m³/(h.m²) at 50 Pa for dwellings. Northern Ireland follows Technical Booklet F2. Wales follows its own edition of AD-L with similar targets to England. Testing requirements vary by jurisdiction.
Related Calculators
Use the U-Value Calculator for thermal performance, or the Part F Ventilation Rates for ventilation strategy guidance.
Always check the latest edition of Approved Documents on the Planning Portal (planningportal.co.uk) as regulations are updated periodically.
How We Calculate This
Frequently Asked Questions
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Last updated: April 2026
All calculations are estimates. Verify with your supplier.