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Part L Air Leakage Testing Of Buildings

Air pressure testing is a crucial process that evaluates a building's air tightness, referred to as 'air permeability' or 'leakage' rate. The assessment helps identify uncontrolled air leakage through gaps, holes, or cracks in the building's fabric, impacting energy efficiency and increasing operational costs.

Why Do I Need Air Pressure Testing?

We've covered 'What is Air Pressure Testing?' but why do you need it?
Regulations in England, Wales, Scotland, and Northern Ireland mandate air pressure testing for new residential and commercial constructions since 2006 (England, Wales, and Northern Ireland) and 2010 (Scotland). The goal is to ensure buildings meet or surpass the air permeability rating specified in design stage SAP or SBEM calculations, contributing to high-quality energy efficiency. Failure to maintain air tightness can lead to significant heat loss, up to 40%.
Extended commercial properties may require testing, while residential extensions or conversions, though not legally obligated, often undergo the test for peace of mind.

How Is It Done?

You or your architects should provide us with the complete drawings (floor plans, sections, elevations and site plan). Once we have all the information, we’ll use the air tightness target found in your design stage SAP or SBEM calculations as a benchmark. Our technicians calculate air tightness via the following process:

  • We'll install a large fan temporarily, usually in a doorway.
  • Next, we will temporarily seal openings to the property and switch the fan on.
  • Air pressure is gradually increased and decreased, recording the pressure differential at each step.
  • The total air flow required to achieve a pressure differential of 50 Pa is calculated and divided by the total building envelope area. This provides the leakage rate in m³/h.m² @ 50 Pa.
  • This calculation will show the building's air tightness or air permeability.
  • Contractors can continue work within the property but all windows and doors have to remain closed for the duration of the test.

When and How Long

Testing should be conducted towards the project's end, and the duration depends on property size and preparation, generally lasting no more than two hours.

Addressing Failure

Common factors contributing to failure include insufficient sealing around windows, spaces behind fitted units, cracks around sockets, gaps at the wall-floor junction, and spaces around pipework. UKBC offers consultation, on-site training, and additional testing services if a building fails, including smoke testing, depressurization, and thermography to pinpoint problem areas.

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Smoke Testing

LaDuring a smoke test, our technicians will fill the building with smoke and pressurise the space. This will provide a visible indication of the problem areas, as the smoke will flow through the openings. In less extreme situations, we can use devices known as ‘smoke pencils’ to complete the task.


Depressurisation is conducted by pressurising the whole building, forcing air out through any gaps or cracks. When our technicians reduce the pressure, there will be draughts as the air rushes back into the room, indicating where the air is escaping.


Thermography uses infrared cameras to detect where the hottest and coldest spots are in the house.


Common questions asked by our clients

Air permeability testing is a mandatory requirement under Building Regulations Part L for new houses and large extensions (25%+ of existing buildings). The test evaluates a building's air tightness, ensuring it meets specified standards.

All new houses and large extensions exceeding 25% of existing buildings must undergo air permeability testing as per Building Regulations Part L.

The specific air permeability rating required may vary, but the aim is to meet or exceed the standards set in Building Regulations Part L.

The number of units to be tested can depend on various factors, and it's advisable to refer to specific regulations or authorities for accurate information.

According to Part L Building Regulations 2000 in the UK, air tightness testing is mandatory for new buildings to ensure energy efficiency.

Air leakage is the uncontrolled flow of air through gaps, holes, or cracks in the building's fabric.

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Energy Performance Certificates

Energy Performance Certificate (EPC) is required by law when a building is built, sold or put up for rent. If you are a landlord or homeowner and need to provide an Energy Performance Certificate (EPC) this will need to be completed by an accredited domestic energy assessor. They will carry out the assessment and produce the certificate. Energy Performance Certificates (EPCs) give information on how to make your home more energy efficient and reduce carbon dioxide emissions.

What does the EPC say about the property?

An Energy Performance Certificate gives clear information about the energy performance of a property. An EPC will also make recommendations about how the property could improve it’s energy performance.

How long does it take to perform an EPC assessment?

An assessment takes anywhere from 25 minutes to 1 hour for a domestic EPC and a couple of hours to a couple of days for a Commercial EPC depending upon the size of the commercial property. Factors such as the size of the property, it’s age and general access to it, will have an influence on the time. Generally, an energy assessment won’t cause much inconvenience but, it’s important that the energy assessor can get access to any loft space and heating appliances (e.g. a boiler). An internal and external inspection of the property is also required for a certificate.

What is an energy assessor looking for?

  • Noting the construction of the walls and checking wall insulation.
  • Confirming how many windows are double glazed.
  • Counting the number of open fireplaces.
  • Checking the make and model of the boiler under EU SEDBUK regulations.
  • Looking for green energy devices such as solar panels.
  • Counting the number of low energy light bulbs.
  • Confirming the type of heating system used e.g. gas, electric, underfloor etc.
  • Noting down the type of material used to insulate any hot water cylinders.
  • Assessing the heat loss through roof, check loft insulation.
  • Checking for excessive window area in larger houses.
  • Taking precise measurements of conservatories and extensions.
  • Counting how many habitable rooms are heated.
  • What is the procedure once the assessor has visited?

    Once the assessor has visited property, the information is uploaded onto the Government authorized software. A certificate is produced and emailed to the customer within 24 hours after the payment is received.