What it is

Since 2006 all new domestic and commercial buildings have been required to be tested for Air Permeability, in order to comply with Part L1 and Part L2 of the Building Regulations.

This means that for a dwelling to be deemed as habitable and meet Part L1A of the Building Regulations it needs to achieve the standard of air tightness set and be energy efficient. (The target Air Test value is set within the Design Stage SAP) In subsequent amendments to Building Regulations the levels of air tightness have become more stringent as dwelling emission rates have been lowered. This has put a greater emphasis on building more air tight houses.

Air Tightness Testing is a method of calculating this by quantifying how much air leaks in or out of the building.
For this we use the term ‘Air Leakage’ which is defined as ‘the uncontrolled flow of air through gaps and cracks in the building fabric’

Areas for potential leakage which should be given particular attention during the build process are:

  • Behind Kitchen/Utility units or fitted bedroom or bathroom furniture.
  • Where all walls meet a floor.
  • Around windows and external doors.
  • Around any pipework leading into or out of the building.
  • Around sockets and other electrical points.

Testing Regime

October 2010, L1A Building Regulations required that Air Pressure Testing is carried out on 50% of a particular ‘dwelling type’ or a minimum of three units of each ‘dwelling type’, whichever is the least number.

There are a number of different ‘Dwelling Types’

  • Detached Houses
  • Semi-Detached House
  • End Terrace House
  • Mid Terrace House
  • Top Floor Flat
  • Mid Floor Flat
  • Ground Floor Flat

In addition to be classed as the same ‘dwelling type’

  • they must employ the same construction methods
  • the difference in floor area must be less than 15%
  • they must have a difference of no more than one significant opening

For Example


1 OR 2

3 OR 4






It is advisable at an early stage to contact Northern Air Tightness Testing Services so that we can advise on the number of tests required. This can then be agreed with your local Building Control Officer.

Where there are multiples of the same dwelling type, it is normal for the first ones completed to be tested, this will help to ensure that any week areas identified can be addressed on the remainder of that dwelling type.

Where the sample house has been air pressure tested it must achieve a score of two below the ‘design’ air test for this result to be applied to other dwellings of the same ‘type’

For example; Seven houses built of the same ‘dwelling type’, designed to an air test score of 7m3(m2/hr). Three would need to be tested and achieve a score of 5m3(m2/hr) or better for the result to be applied to the remaining four properties.

Following a successful test we can provide all necessary reports and certificate of compliance whilst on site if necessary.

How To Do An Air Test

Testing needs to take place in the final stages of construction when all work to the internal envelope has been completed and ideally when power and water are connected.

On the day of the Air Tightness Test one of our highly specialised NATTS engineers will complete a pre-test survey of the building. This is to ensure that all of the property is in a fit state to be tested and to identify any areas that can be addressed prior to the test taking place to help achieve the desired outcome. These potential problems will be minimised if the advice contained within the checklist which we will have previously provided, is followed.

The accuracy of the air tightness test will also be affected by environmental conditions such as temperature and wind speed and external barometric pressure. These measurements will need to be taken prior to the test and again on completion.

Once the engineer is satisfied that the needs have been met for the test conditions, the air tightness test equipment can be set up and the test process outlined below completed.

  • Ensure all windows are shut and trickle vents closed or sealed
  • All internal doors should be open
  • Apply temporary seals to all room extractors and externally vented cooker hoods
  • Temporary seal to chimneys


  • Ensure central heating boiler is switched off
  • All traps should be filled/sealed
  • Assemble cloth door and aluminium frame to an external door aperture
  • Internal and external pressure monitoring tubes located and fixed appropriately


  • Fan mounted on aluminium frame and sealed within cloth door
  • Using fan control device to pressurise or depressurise the property
  • Readings taken across a range of pressure differentials
  • Result calculated
  • Pass certificate issued



Part L1A of the Building Regulations calls for sample testing of dwellings and at NATTS Ltd we carry out Air Tightness Testing to establish the Air Leakage on a completed structure and ultimately supply you with the required certification to obtain Local Authority Building Control Approval.

The Air tightness of a building plays a significant part in how energy efficient it is. Air leakage in a building will result in poor energy efficiency and up to 40% heat loss. If you’re investing in measures such as a highly efficient boiler, extra insulation, or innovations in energy conservation your efforts could be in vain if the building’s air tightness isn’t as high as it could be. We all want our homes to be energy efficient cost effective comfortable or have as little impact on the environment as possible

The benefits in making a building as airtight as possible include:

  • a reduction in CO2 emissions
  • increased comfort levels for building users
  • substantial savings in heating bills for the consumer
  • reduction of harmful irritants entering a building, such as pollen, dust, insulation fibres, that are generally drawn from under floor boards, the attic or directly from the outside.