Question 1

What is the air permeability target for new dwellings?

Accepted Answer

The 2021 edition of AD-L sets a maximum design air permeability of 8 m³/(h.m²) at 50 Pa for new dwellings. The notional dwelling in SAP uses 5 m³/(h.m²) at 50 Pa, so in practice most new dwellings are designed to achieve 5 or better to meet the overall energy target. Very airtight dwellings (below 3 m³/(h.m²)) must have mechanical ventilation with heat recovery (MVHR) as natural ventilation cannot provide adequate air exchange.

Question 2

Is air pressure testing mandatory?

Accepted Answer

Yes. All new dwellings in England must be pressure tested to demonstrate compliance with the design air permeability value. Testing is carried out to BS EN ISO 9972 (Method 1 — building in use) and must be done by a UKAS-accredited body or a tester registered with an approved scheme (e.g. ATTMA). The test result is entered into the SAP calculation.

Question 3

What happens if a dwelling fails the air test?

Accepted Answer

If the tested air permeability exceeds the design value used in the SAP calculation, remedial sealing work must be carried out and the dwelling re-tested. Alternatively, the SAP calculation can be re-run with the actual tested value — but this may cause the dwelling to fail the overall CO₂ emission target, requiring compensatory measures elsewhere (e.g. better insulation, more efficient heating).

Question 4

Where are the most common air leakage paths?

Accepted Answer

The most common leakage paths in new-build dwellings are: around windows and door frames, at junctions between walls and floors/ceilings, through service penetrations (pipes, cables, ducts), at loft hatches, around recessed downlights, through the party wall zone (especially in cavity masonry), and at any joints in the vapour barrier or air barrier layer.

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

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

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

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

Part L Air Permeability & Airtightness

Air Permeability Targets (AD-L 2021)

Ventilation Strategy by Airtightness (AD-F & AD-L)

Common Air Leakage Paths and Sealing Methods

Testing Requirements

Scotland & Northern Ireland

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