Air Tightness Testing

APT Sound Testing provides air tightness testing throughout the United Kingdom. We undertake air testing on all types of projects from single dwelling to the largest commercial buildings. We are one of the few companies that are UKAS accredited to undertake both Air & Sound testing in compliance to Building Regulations and Standards. This means we can undertake Air, Sound and Vent Testing Packages during the same visit which provides our clients with cost savings and improved on site co-ordination. Unlike many companies we do not subcontract our works.

Air testing to achieve satisfactory levels of air tightness became a legal requirement in 2006 in England and Wales, under Building Regulations Part L and is becoming increasingly important to help produce greener buildings, via reducing carbon emissions due to the fact that energy is not being needlessly lost from residences via large air leakage paths etc. Building Regulation Part L defines ‘air leakage’ as the uncontrolled flow of air through gaps and cracks in the fabric of a building. The general public recognise it as draughts.

When you lose energy through the building fabric this is known as the building emission rate. New projects/developments must meet specific requirements for air testing and emission rates. The usual target for a standard dwelling is 5.0m3/hr/m2; however for commercial buildings the target rate is often much lower – around 3.0m3/hr/m2

Wherever air infiltration occurs, there is usually exfiltration somewhere else in the building. During the summer, infiltration can bring humid, outdoor air into buildings. In winter, exfiltration can result in moist indoor air moving into cold wall cavities and may result in condensation and mould and/or rot which could result in serious damage to the property.

We undertake air tightness consultancy and design reviews to help support our clients from the design stage of the project through to the precompletion air testing to achieve building control signoff. If you need to undertake your testing on Saturdays or Sundays, we offer an ‘out of hours testing’ service to help reduce the impact of testing during working hours.

We also offer substantial cost savings for combined air and sound testing packages. This is due to our multi skilled engineers being UKAS accredited to carry out the Air & Sound Testing, in fact most of our clients now use our ‘3 step’ precompletion package as it negates the need to employ multiple companies.

On site co-ordination is also improved, with both tests being undertaken on the same day in a planned methodical manner. We can also provide out of hours testing (if required) to help reduce the impact of testing during working hours.

We provide a friendly expert advice for helping properties achieve required emission rates for air testing. Simply the use our contact form on this page, or call our offices, to chat about your specific air testing requirements with our knowledgeable team of air tightness consultants.

Air tightness testing is carried to check the air leakage that occurs through a buildings envelope. The air test checks amount of conditioned air that escapes or gets wasted through gaps or air leakage paths through the building fabric. If the building is more air tight, less energy will be being needlessly lost, thus lowering the carbon emissions of the building. Air tightness testing also shows how efficient a building is at retaining conditioned air which will also put less straight on your mechanical and ventilation system. Failing to maintain adequate air tightness can lead to up to 50% of heat loss from within a building to the outside atmosphere.

Air testing is mandatory in the United Kingdom and is governed by Part L of the Building Regulations, this split into two parts; for domestic properties Part L1A should be used and for commercial properties Part L2A should be referenced.

Most air tests can be completed in 1-2 hours although some can take much longer, if low air leakage rates are required to comply with Passivehaus standards etc. Also other factors such as the size of the property, how many apertures need to be taped over; however even on very large dwellings the duration of the actual air test should take no longer than 2 hours if the building has been adequately prepared.

We also offer a smoke testing service to locate the air leakage paths within a building envelope which can also take extra time; however this often allows our clients to seal and pass their building at the first attempt even if their building initially fails the air test.

It depends on the location and the number of tests that are required on each site; it can be as low as £75 plus VAT per unit if we are undertaking multiple tests. Please contact us at to obtain a no obligation quote along with some friendly advice for you project.

As a general rule, the nature of the current building regulations requires that all new dwellings achieve an air leakage of 5m3/hm2 or less. That is the air leakage rate per hour, per square metre of envelope area. An air test results between 3 – 5m3/hm2 would usually be acceptable by building control and your SAP assessor as a good and acceptable result, although you need to check each your designed SAP Assessment as there may be items within your buildings design that that may affect this figure as the dwelling may be struggling to meet its emissions targets and a low air leakage may compensate for other areas, either arising from poor design, or factors beyond the developer’s control.

The air tightness test is undertaken in-line with Part L and the ATTMA standard. The pressure differential is measured across the envelope of the building by means of the temporarily installation of a large fan inside a door panel. Thereafter, a range of static pressures and environmental readings are taken. The fan is switched on and the air pressure in the property is gradually increased or decreased and the differential pressure is recorded 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 to provide the leakage rate in m3/h.m2@50Pa.

In all cases the more complete a development, the more likely it is to meet air testing requirements. When we send out our quotation we also forward a specific air leakage checklist to help our client prepare for the testing.

Basically if you meet the following criteria will generally have a much better chance of passing the testing at the first attempt.

a. The building envelope should be fully complete; this includes walls, floors and ceilings.
b. All doors and windows must be fully fitted and able to shut tightly against their seals.
c. All electric fittings must be installed and functional.
d. All mechanical fittings must be installed and functional.
e. Gaps within walls and floors must be sealed.
f. All service penetrations must be fully sealed through the building envelope.
g. Bathrooms and kitchens must be fully fitted and all service penetrations sealed.
h. All mechanical ventilation turned off with grilles sealed.
i. All trickle vents to windows and doors must be sealed.
j. All fireplaces must be sealed.
k. Ensure water is present in soil pipes.
l. 240v power must be available on site.

Airtightness Testing for New Buildings

An airtightness test is a whole building test that measures how easy it is for air to leak through a building envelope (walls, floor, and roof). In residential developments, construction air tightness testing is often referred to as blower door tests, this is because the main part of the test equipment is a blower door – basically a large fan. For residential air test, a single blower door is used to pressurise (supply air) into a dwelling. For larger non-dwellings, such as commercial buildings with a floor area of over approx. 500m2 commercial fan testing equipment must be used, this usually comprises of a hard door panel that houses 3 fans.

In all cases, both dwellings and commercial buildings must be fully prepared prior to the air test visit, to offer the best chance of the building passing at the first attempt. The main areas to temporarily seal are AC/HVAC intake and exhaust grills, kitchen, utility and bathroom extract fans, relief dampers, etc.  Here is a more detailed air tightness checklist to help clients prepare for the air test visit.


Why is Air Tightness Testing useful?

The blower door test equipment measures the airflow (how much air is moved into or out of the building) and the corresponding pressure difference acting across the building envelope/enclosure.  There are many reasons the air leakage rate can be useful, such as

  1. Any air leaking out of the building envelope causes energy to be consumed when replacement air is conditioned, either via cooling or heating. With information from an air leakage test you can estimate how much air will leak in both cold and warm weather, or how much more windy weather will cause a building to leak relative to calm weather.  The resulting energy impact can then be considered and used for equipment sizing calculations and predictions of operating energy consumption.
  2. You can identify air leakage paths that you didn’t know about.  Via blower door and smoke testing, we can accurately record where your leaks are, sometimes it can be a few large holes, however, more often than not its lots of smaller air leakage paths that account for the main air leakage issues. Our experienced air tightness engineers can quickly identify and record the leakage paths during an airtightness test.
  3. Draughts, through the building can badly affect the occupiers of the dwellings and commercial buildings such as offices. If cold damp air is blowing into the building it can be uncomfortable in both hot and cold weather. In the past we have undertaken pressurised smoke testing to buildings because the clients’ workers refused to work in the building. When we tested the building, we found it had an air leakage rate of 25m3/hr/m2. Once we had identified the air leakage paths and the clients’ contractors had sealed the areas, the building achieved an air leakage figure below 5m3/hr/m2. The result was further complaints from the occupiers.
  4. Excess air leakage can lead to condensation issues, as the air can blow through the floors, walls, and ceilings and usually contains water vapour, these uncontrolled air leaks can cause condensation, mould, and rot issues, which can be unhealthy for building occupiers. So, quantifying how much air passes through the building envelope are extremely important to understand. Only with a good air barrier can we know where the air is coming from and have a chance that air quality (and quantity) can be controlled and condensation kept to a minimum.
  5. Large amount of air leakage can bring polluted outdoor air into a building from, for example, buy main road, fuel garage, bins, restaurants, or external ventilation. Knowing how much air infiltrates through your building enclosure/envelope will help identify how much risk there is of ingress of outdoor pollutants. Any uncontrolled air that enters a building is often loaded with pollutants, especially if you are close to a main road etc. so trying to provide a robust and good air barrier is very important. Only with a good air barrier can we control pollutants within the building enclosure.


Air Tightness Testing for Retrofit Projects

Airtightness targets are useful within the design process for both new buildings as well as retrofit projects. Air tests are the only way you can accurately quantify the building enclosure performance and provide key input into the mechanical designer’s load and energy, as well as helping to lower the air leakage rate of the building via pressurised smoke testing and remedial sealing works. It has been proven that Airtightness tests are one of the most important parts of the construction process, as they provide confirmation that airtightness targets are met and, if timed properly e.g., undertaken before final decorations, give the building contractors an opportunity to address problems via remedial sealing, prior to the final precompletion test and building handover.


What results do you get from an air tightness test?
The results of an airtightness test in basic form are the pressure difference across the building envelope/enclosure, the total airflow, and the airflow direction (pressurise or depressurise).  During the testing 10 airflow measurements are collected using a range of pressure differences and flow directions, the usual pressures are between 25-70Pa in 5Pa increments.  This air test data is plotted to quantify the relationship between the fan airflow and internal and external pressure difference.   Although the test takes into account multiple pressure levels, the result is shown at a single pressure (50Pa) from the curve and reported as such – as shown below.

For dwelling and commercial buildings, the single test pressure used for reporting is almost always 50 Pascals. The airflow is reported in cubic meters per hour per metre square of building envelope (m3/hour/m2).

In most cases, as the size of the building under test increases, the airflow measured will also increase and the types of test equipment will also change from single to multiple blower door units. To allow for easy comparisons between different buildings, two calculation methods are used to calculate the airflow with respect to building size:

  1. Divide the airflow by the area of the building enclosure being tested (m3/hr/m2)


  1. Divide the airflow by the volume of the building (ACH@50)

in many cases its more common to use the airflow by the area of the building enclosure being tested or m3/hr/m2. This is mainly due to the differences between surface-to-volume ratios. It is more common, and usually more accurate to provide the leakage rate at a specific pressure (usually 50 Pa) in terms of flow per unit area, in other words as cubic metre per house per square metre of envelope area (m3/hr/m2).

The envelope area means the total six sides of the building, which includes the four walls, floor, and ceiling. It’s worth noting that all (areas) that are exposed to the outdoor air are part of this equation, this can mean awkward ‘one off; envelope protrusions such as large canopy overhangs etc, that are not compartmented from the rest of the building.  This reporting method accounts for the size of the building and therefore allows you to make better comparisons between varying types of buildings.  This envelope area type of testing is increasingly finding favour in the residential community as well that’s why Building Regulations Part L1/L2 stipulates this type of testing.

In some rare cases, clients need the air leakage worked out to account for air changes per hour (ACH). Air changes per hour is simply the volume of air leaked per hour, divided by the volume of the building. The test data is then reported as a number in “ACH@50”.  Air changes per hour is sometimes used to report air leakage for commercial and more unusual buildings and/or enclosures such as laboratories, data centres etc.

Our Air Tightness Testing procedure

Our fan system mounts into a standard size door-sized housing (900mm x 2000mm) and is adjusted to suit the size of the door frame.

We then set up the blower door equipment as follows.

  1. Firstly, we find a suitable doorway to temporarily install our blower door fan system (depending on building size).
  2. We then undertake the first series of environmental measurements such as internal and external temperatures as well as barometric pressures and internal and external temperatures.
  3. We then turn on the fan equipment and take measurements between 25-70Pa in 5Pa increments, recording the pressure differential at each step.
  4. Once the pressures have been taken we undertake the second set of environmental measurements.
  5. Finally, our technicians will calculate the total air flow required to achieve a pressure differential of 50 Pa, divided by the total building envelope area – this calculation will show leakage rate in m³/h.m² @ 50 Pa.
  6. Once the building air leakage rate has been established, we then pressurise the building (blows air into) to around 50 pascals of pressure. This forces air to flow through cracks or air leakage paths throughout the building envelope.
  7. We then turn on the smoke machine and walk around the house or commercial building, highlighting, and recording all the air leakage paths through the building envelope.

We usually allow two to three hours to complete each air tightness smoke test; however, we can attend site all day if instructed. If you have the necessary manpower, it allows your operatives to seal all the necessary air leakage paths on the day of the test. If you are planning on undertaking remedial sealing works on the day of the test, your operatives will need to have mastic, expanding foam, draught excluders, plasterboard, grip fill and suitable safe access to undertake the sealing works.

What’s a good level of air tightness for your project?

However you process the data, you still need to ascertain what is a good level of air tightness for your new build or refurbishment project. In many existing Victorian houses or commercial buildings, the air leakage can be as high as 15m3/hr/m2, which when you consider new dwellings are getting between 3-5m3/hr.m2, and commercial buildings below 3m3/hr/m2, it’s a truly terrible result. So, what’s a “good” level of airtightness for your building.  As we previously stated, most new houses that don’t have mechanical ventilation, a good air tightness target would be between 3-4m3/hr/m2. For a dwelling with mechanical ventilation its best if your figure is below 3m3/hr/m2.

With buildings becoming more air-tight, we are seeing the increased impact of known holes, such as air leakage through grills and openings of mechanical systems. In many cases it’s not uncommon to see almost 50% of the buildings total air leakage through the mechanical system.  This means designers and installers have to ensure that backflow dampers are working properly as they should seal tight in the closed state, providing an air-tight barrier. If dampers do not work properly, it will have a negative effect on the overall tightness of the building under test and later in service.

Many modern commercial buildings, constructed since 2006, when the air testing regulations came into force have quite an air-tight envelope, such commercial buildings that has utilised sealed cladding etc. which has elements which are slower to degrade, however, other elements such as external doors and windows, can become air leakage areas due to degraded seals or structurally damaged frames.

Another main area of air leakage can be around the mechanical ventilation system due to poorly maintained or damaged dampers. Even with dampers closed we have seen air leakage double when temporary sealing is removed from the associated internal ventilation louvres. Another common issue is often down to mechanically operated roof lights what don’t shut tightly against the seals and damaged fireproofing that was used as the air tightness barrier in the building eaves and/or to isolate unconditioned areas, such as plant rooms from the main building.


How does airtightness tests fit into the design process? 

With the ever tightening of building emissions, the trend is for greener and more air-tight buildings. The building industry is going to be required to meet more stringent airtightness targets, either because they are added to new building codes, or because owners are looking for better buildings with less energy consumption, more comfort, better indoor air quality, and much cheaper to run. In our experience a robust air-tight building is often a good indictor to client’s that the building contractor has delivered a good, airtight building enclosure.

Airtightness testing can also be used to check the building at various stages. For new construction of commercial buildings, we often undertake an air test approx. one month before handover, just prior to final decorations so any ‘hard to detect’ air leakage paths can be identified, recorded and shown to the site team to undertake remedial sealing works. Thereafter, approximately one week before handover the final precompletion testing can be undertaken to achieve Part L compliance.

On a similarly note, if you are about to do a major refit to improve the building energy efficiency for BREEAM etc, such as adding insulation, replacing windows and doors, replacing electrical rooflights the mechanical ventilation system, it is usually advisable to do a test before you get too far into the design and build process. In many cases it makes good financial sense to undertake an air test as part of the initial assessment of the existing building, to check the amount of air leakage so a more informed decision can be made on remedial basic air sealing works in comparison to more expensive and time-consuming works such as the replacing of new doors and windows within the external envelope.

Thermal imaging and air tightness testing is a powerful combination.

For even better results, we often use thermal imaging surveys at the same time as a blower door test running. The blower door helps exaggerate air leaking through defects in the building shell. Such air leaks appear as black streaks in the infrared camera’s viewfinder depending on the colour palate as well as identifying missing insulation etc.

Combining the best skills in air leakage testing and thermography allows our engineers to gather amazing results, accurately highlighting air leakage points around a building big or small. Thermography uses specially designed infrared video or still cameras to make images (called thermograms) that show surface heat variations. Thermography can also detect damaged or missing insulations in walls, floor, and roofs within building envelopes. Also, as thermal imaging can pick up damp within the building envelope, it can be used to find wet insulation, as its conducts heat faster than dry insulation

Air tightness testing can help mechanical designers

We have found that by using airtightness tests regularly, architects, M&E consultants and building contractors are getting better at designing for strong airtightness performance. Furthermore, once you have established the air leakage target for the building this can then be forwarded to the mechanical systems designer, so heating/cooling systems can be more accurately designed around the buildings air leakage rate.

When mechanical engineers don’t know the air leakage rate of the building they tend to aim on the safe side, which often results in a measure of ‘over design’. Its’ all about having the correct building information at design stage, if you have already ascertained the air tightness target the mechanical engineer will be better informed and will be able to better job of sizing the mechanical systems for heating, cooling, and dehumidification.

Another problem encountered with airtight buildings, is the proposed mechanical systems can over pressurise them, forcing conditioned air out through the building envelope; again, measuring the airtightness of a building allows the mechanical engineer to be better informed when finalising the values to use in system design so over pressurisation is reduced to a minimum.

Reducing air leakage across the building envelope is fundamental to make buildings more comfortable, healthy, durable and to lower energy consumption. In the end, airtightness testing is basically the most important quantitative quality control tool to help lower emissions from new and refurbished buildings, with the added benefit of helping M&E designers to specify the correct mechanical system for the building.

We can help with your air tightness test

APT Sound Testing we have large amount of experience in understanding the requirements of Approved Document L, along with extensive experience of carrying out thousands of successful air tightness tests on a wide range of developments across London and the UK.

In order to achieve the required air tightness target (which is getting lower all the time) and to avoid the risk of a failed air tightness test, we strongly recommend that early consideration is given to the design of the air leakage line within your project.

By working with our customers throughout their design and construction stages, we can provide advice and guidance on the most feasible ways to avoid air leakage and attain compliance for the air tightness test. Please download our air tightness checklist to help you prepare for your test.