TradeCalculator.co.uk
What Is Airtightness? — Building Envelope Testing
A plain-English guide to airtightness in UK construction: testing methods, Part L targets, common leakage paths, and sealing strategies.
Definition
Airtightness refers to the resistance of a building's envelope to uncontrolled air leakage. It is measured as air permeabilityin m³/(h·m²) at 50 Pascals pressure difference, representing the volume of air leaking through each square metre of the building envelope per hour. A lower number means a tighter building. Airtightness is a key factor in energy efficiency and is regulated under Part L of the Building Regulations.
When Is It Used?
Airtightness testing is required for all new dwellings under Part L 2021 and for new non-domestic buildings over 500 m². It is also relevant for retrofit projects targeting EPC improvements, Passivhaus certification, and any project seeking to minimise energy consumption. The test is typically carried out before the final Building Control inspection, after all services are installed but ideally before decoration.
Key Facts
- Part L 2021 backstop: 8 m³/(h·m²) at 50 Pa; practical target: 3-5 m³/(h·m²)
- Passivhaus target: 0.6 ACH at 50 Pa (approximately 1 m³/(h·m²))
- Testing must be done by a UKAS-accredited or ATTMA-registered tester
- A blower door test typically costs £200-£400 for a domestic property
- The air barrier line must be continuous and identifiable on drawings
- In masonry construction, the wet plaster coat acts as the main air barrier
- In timber frame construction, the vapour control layer (VCL) is the air barrier
- Smoke pencils or theatrical smoke machines help locate leaks during testing
Related Calculators
Use the U-Value Calculator for fabric performance, or the Heat Loss Calculator to see the impact of airtightness on heating demand. See also the MVHR glossary entry for ventilation in airtight buildings.
Next up: related pages
How We Calculate This
Frequently Asked Questions
Part L 2021 sets a reasonable upper limit of 8 m³/(h·m²) at 50 Pa for new dwellings, but the design air permeability used in SAP calculations is typically much lower. Most new-build developers target 3-5 m³/(h·m²) to pass the as-built SAP assessment. Passivhaus standard requires 0.6 ACH at 50 Pa, which equates to approximately 1 m³/(h·m²) depending on the building form.
A blower door test is performed by a UKAS-accredited tester. A calibrated fan is sealed into an external doorway and pressurises (or depressurises) the building to 50 Pascals. The airflow required to maintain this pressure is measured and expressed as air permeability in m³/(h·m²) of envelope area. The test takes about 2-4 hours for a typical house and costs £200-£400.
The biggest culprits are: the junction between walls and floors (especially at ground floor), service penetrations (pipes and cables through the envelope), around windows and doors, loft hatches, recessed downlights, electrical sockets on external walls, the junction between walls and roof, and poorly sealed cavity barriers. In timber frame buildings, the vapour control layer is the primary air barrier and must be continuous.
Yes, since Part L 2021 came into force (June 2022), all new dwellings must be pressure tested unless they are part of a development where a sample testing regime has been agreed with Building Control. Previously, developments of fewer than 2 units could avoid testing by using a default value of 15 m³/(h·m²) in SAP, but this is no longer permitted.
Not if ventilation is designed correctly. An airtight building must have controlled ventilation to maintain indoor air quality. Part F of the Building Regulations sets minimum ventilation rates. Options include MEV (Mechanical Extract Ventilation), PIV (Positive Input Ventilation), or MVHR (Mechanical Ventilation with Heat Recovery). The principle is "build tight, ventilate right."
Was this calculator helpful?
Last updated: March 2026
Verified against UK standards · estimates only, confirm with your supplier.