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Air leakage services - frequently asked questions

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Basic principles

What is air leakage?

Air leakage, air permeability and air tightness are all terms that refer to the uncontrolled loss of air from inside a building to the outside and the infiltration of air coming from outside to inside. This loss or gain of air through cracks, holes or gaps in the fabric of the building is often felt to us as draughts.

Achieving a good level of air tightness is important for the energy efficiency of the building. The benefits of improved insulation and more energy efficient heating systems are lost if warm air can leak out of the building and cold air can leak in. Poor air tightness can be responsible for up to 40% of heat loss from buildings.

What is air leakage testing in buildings?
A method of quantifying how much air leaks into or out of the building to check compliance with the requirements of Building Regulations.


Why carry out air leakage testing in buildings?

Building Regulations require that builders/developers prove the air tightness of a sample of new buildings on any development.

The regulations set a maximum allowable air permeability measure for all buildings but the specific performance required on any given unit will depend on the SAP or SBEM calculations which determine whether the building's Design Emission Rate (DER) is below the Target Emission Rate (TER).

What are the applicable Regulations?

Part L of the Building Regulations - The Conservation of Fuel and Power

Part L of the Building Regulations - The Conservation of Fuel and Power

Section 6 of the Scottish Building Standards

Northern Ireland
Part F of the Northern Ireland Building Regulations

When did air leakage testing become mandatory?

Since 2006, Building Regulations in England & Wales and Northern Ireland have required mandatory air leakage testing of new homes. Regulations in England & Wales were further revised in October 2010.

Since October 2010, for new building warrants, air leakage testing has been a mandatory requirement in Scotland.

What sort of properties need to be tested?

Approved Document L sets out the applicable regulations for England and Wales.

  • Approved Document L1A sets out the air tightness requirements for new dwellings.
  • Approved Document L1B states that reasonable provision should be made for continuity but no test requirements in existing dwellings.
  • Approved Document L2A deals with new buildings other than dwellings.
  • Approved Document L2B provides that extensions to buildings (other than dwellings) where the extension(s) is an area greater than 100m2 and more than a 25% increase in the floor area of the existing building, are covered by the requirements of ADL2A.
How much testing is needed?

Part L requires sample testing and sampling rates are applied to groups of dwellings that are defined as similar based on specific criteria described within the Approved Document.

Part L 2010 and 2013 increases the amount of testing required to 3 tests or 50% of each dwelling type, whichever is less. The criteria for defining similar dwelling types has also expanded such that there are now likely to be more tests required per development than was the case under Part L 2006.

Please note: Under Part L 2010 and 2013, plots that are not tested attract a penalty of +2.0 m3/(m2.h)@ 50 Pa in the as-built SAP, so if the target is 10 m3(h.m2)@ 50 Pa, a result of 8m3 /( h.m2) @ 50 Pa or better is needed from the tested plots to allow those units not tested to pass.

How are plots selected for testing?

For the 2006 Regulations the Building Control body usually makes the selection in consultation with the client to ensure testing is front loaded to identify any possible issues early.

For the 2010 and 2013 Regulations the test regime / similar dwelling type groups are determined by the testing organisation in collaboration with the Building Control body. Plots are chosen in line with the ADL requirement of 50% testing to be carried out within the first 25% of each dwelling type. What determines a similar dwelling type is defined in detail in the regulations.


How is an air leakage test carried out?

A temporary airtight screen is fitted into the entrance door of the dwelling with water traps filled (or temporarily blocked), trickle vents closed (sealed under Part L 2010) and extract vents sealed.

A fan is then mounted in the screen and operated to blow air into or out of the dwelling to create a pressure difference between inside and outside of approximately 50 Pa.

The air tightness of the dwelling is quantified by measuring the rate of airflow through the fan while a range of pressure differences between the inside and outside of the dwelling are maintained.

What result is required for a pass?

To pass an air leakage test in England and Wales, a dwelling must achieve ann air permeability result of 10 m3/(h.m2) (a minimum requirement in England and Wales; a recommended 'back stop' in Scotland).

However, the air permeability rate (APR) defined in the design stage SAP often imposes a more stringent target. If the tested APR is greater than that used in the design stage SAP, the as-built SAP may not comply.

The model specification used for Part L 2013 (England)* and Part L 2014 (Wales)** both assume an APR of 5 m3/(h.m2) so designers specifying the minimum required APR value will need to compensate by increasing performance in other areas of the specification in order to demonstrate compliance.

A test that does not achieve a Building Regulations minimum performance requirement would be classed as a fail. Should tests fail to achieve the necessary performance level, the property may need remedial work and re-testing. You may also be asked to test further examples of that dwelling type to restore confidence and show that remedial actions have been carried forward into the remainder of the build.

*Part L 2013 (England) comes into force on 6th April 2014 for buildings built in England only.

**Part L 2014 (Wales) comes into force on 30th July 2014 for buildings built in Wales only

Will air leakage testing disrupt work on site?
The time taken for testing varies with site conditions, including the complexity and size of the individual units, the number of units to be tested etc. The actual testing where readings are taken takes only around 15 minutes. We work with you to ensure minimal disruption on site.

Preparation, timing and testing

How do I know if I'm ready for a test?

Testing needs to take place in the final stages of construction when all dirty works have been completed and the site is clean. We will provide you with a checklist, in advance of testing, to let you know what needs to be in place and when.

Testing readiness checklist

What if I only have 110 volt on site?
We can still undertake testing but please make sure you let us know in advance so that we can ensure the correct equipment is brought to site. The fans used for this type of testing require a significant amount of stable power and portable generators are often not sufficient.
Can I get some peace of mind that I'll pass, prior to testing?
We can carry out pre-testing site visits and early testing to see that the units are on track to meet their targets. Talk to us about your requirements.

Results and failures

Will I get the results on the day?

Yes, we will give you preliminary test results on site to allow you to immediately demonstrate the measured performance to other parties and we will follow this up with a full report and test certificates within 5 - 7 working days of your test.

When using NHBC Building Control and/or our Energy Rating services we can share this information with them to save you time and speed up clearing of conditions and production of EPCs (unless you choose to opt out of this information sharing at order stage).

Can you email the report to me?
Yes, the completed and signed report will be emailed to you. Printed copies are available if required.
Can you send the report straight to Building Control?
Yes, we do by default where you use NHBC Building Control, to save you time (unless you choose to opt out of this at order stage). Also, where we are providing your Energy Rating services we can send your report to colleagues to speed up the process for you receiving EPCs.
What happens if a test fails?

In line with the regulations you are required to carry out remedial sealing to ensure the development meets the relevant performance requirements and re-test. Our test engineer will often be able to provide remedial advice and identify the air leakage paths on the day of the test.

You may also be asked by Building Control to test further examples of the construction elsewhere on the development in order to restore confidence and/or highlight the extent of the issue. Depending on the nature and extent of the leakage paths, we may be able to re-test on the same day or if not, come back to site at a later date.

What are the common problem areas?

Commonly air leakage is observed in the following areas:

  • around service penetrations (pipe work etc) including those enclosed / boxed-in
  • behind fitted units or behind bath and shower tray panels
  • at the junctions of external walls and floors
  • around windows and doors, generally at the junction of the frame and the recess or threshold
  • at socket points and around electricity consumer units.

Common air leakage points diagram

Tester credentials

Can anyone with the right equipment do the tests?
Approved Document L states that the testing body should be registered with BINDT. NHBC is UKAS accredited to undertake air leakage testing, we are members of ATTMA (Air Tightness Testing and Measurement Association) and associate members of BINDT. We are also UKAS accredited to undertake sound insulation testing, and many of our engineers undertake testing for air and sound on the same visit.
Should I use a non accredited company?
Absolutely not. Your reports will not be accepted by the necessary approval bodies and you may have to pay for testing again by an accredited company.

Discuss your project

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Downloads and links

Part L compliance solution


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Technical guidance on Building Regulations

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