What is ductwork airtightness?

Ductwork airtightness can be defined as the resistance to inward or outward air leakage through the ductwork shell. The limit values for the maximum air leakage volume flow in duct systems are defined in the 4 tightness classes A to D which is now transformed into 7 new Air Tightness Classes ATC 1 – ATC 7  in different European standards, e.g. EN 12599, EN 16798-3, EN12237or EN1507.

See also

How is the building airtightness quantified?

The airtightness of a building is quantified by means of the measured air leakage rate through the building’s envelope in m³/h at a given reference pressure difference, often 50 Pa. The building airtightness may be expressed by other quantities, derived from the air leakage rate at a reference pressure difference, and normalised using, for example, one of the following measures for the size of the building:

  • Internal building volume V. At 50 Pa, the associated indicator is called air change rate at 50 Pa and noted n50 [unit h-1].
  • Envelope area AE. At 50 Pa, the associated indicator is called air permeability at 50 Pa and noted qE50 (former: q50 in EN 13829) [unit m3/(h.m2)[.
  • Net floor area AF. At 50 Pa, the associated indicator is called specific leakage rate and noted qF50, (former: w50 in EN 13829) [unit m3/(h.m2)].

Other indicators of airtightness exist in national regulations.

What is calibration?

Calibration is the comparison of measurement values delivered by a device under test with those of a calibration standard of known accuracy. Such a standard could be another measurement device of known accuracy, a device generating the quantity to be measured such as a voltage, a sound tone, or a physical artifact, such as a meter ruler. 

A calibration laboratory establishes whether the value given by a measuring instrument is correct in relation to the international unit of measurement. An ISO-17025 accreditation offers the best guarantee that this is indeed the case. Accreditation is granted for a specific area of work based on a specific, defined method. In accreditation, this is called the 'scope'.  

Each European country has its own accrediting body. These organisations have a so-called Mutual Recognition Agreement (MRA) with each other, in Europe, and often beyond. In Europe there is an umbrella organisation called the European Cooperation for Accreditation (EA). This organisation ensures that accreditation is dealt with in the same way in all countries and that the same requirements apply everywhere. Certificates issued under accreditation in all these countries are therefore fully recognized and accepted in countries that have signed the MRA.

A guarantee of flawless work can never be given. Accreditation guarantees that "work is performed with the highest possible reliability". In order to continue to test this reliability, audits are carried out, with some regularity internal audits and at least once a year an external, independent audit is carried out by the accrediting body. Accredited laboratories are obliged to participate in proficiency testing, studies in which the test results of different laboratories are compared with each other. 

The calibration of a blower door consists of 2 parts: a pressure calibration and a flow calibration. It is always recommended to make sure how these calibrations are performed. What kind of measurement setup is used and what are the measurement points? The scope (measured quantities, measuring method, ranges and uncertainties) of accredited labs can be found on the internet.


See also

What are relevant CEN standards related to building airtightness?

TightVent Europe has published a list of applicable standards for building and ductwork airtightness. The detailed list can be found here.

What are relevant CEN standards related to ductwork airtightness?

TightVent Europe has published a list of applicable standards for building and ductwork airtightness. The detailed list can be found here.

The graph below shows ductwork airtightness standards in relation to the design and construction process.

Ductwork airtightness standards in relation to the design and construction process (Source: CEN TC156/WG3)

How do you design for ductwork airtightness?

Leakage in ventilation duct systems is important to keep to a minimum in order to save energy and keep the designed performance for a good indoor air quality. The tightness can be built by quality products and a professional installation or be sealed off afterwards by a sealant which is transformed into a gaseous state using heated compressed air. The aerosolized particles then seal the whole leakage of the ventilation system.

To design ventilation duct fittings in the proper way it is important to have the right shape on each product both after production but also during transport and handling. Each product has to have purpose designed seals, all seals like tape or mastic made on the installation site are extremely hard to achieve in a professional manner.

The ducts and duct fittings like bends, connectors or transformations are normally not so difficult to achieve and there are some certification programs that guarantee that the system is the promised quality. Technical components like dampers, silencers, filter boxes  are normally a bit more difficult and are sometimes produced by a company only focusing on this specific component where no certification program is available. In 2010 the standard EN 15727 Duct and ductwork components, leakage classification and testing was developed to help the ventilation business focus on airtightness for the whole system even if the components are produced by different suppliers.

The installation will be focused on in the standard EN 12599 handing over and the design and calculation will be addressed in EN 16798-3 performance requirements of ventilation.

At the installation phase it is important to follow the supplier installation instructions and use the proper recommended sealings, screws and clamps the right way. To test air tightness during the installation before the system is built in or insulated is also wise to be able to fix any possible problem. To have the installer himself test the system during the work will be very educational and will lead to professional, fast and insightful staff. 



What is an airtightness/air leakage testing? What is fan pressurization?

A method of quantifying how much air leaks into or out of an enclosure. EN 13829 gives a standard test method for buildings. Several standards apply to ductwork systems (see also “How is the ductwork airtightness quantified?“).

Building airtightness levels can be measured by using a fan, temporarily installed in the building envelope (a blower door) to pressurize the building. Air flow through the fan creates an internal, uniform, static pressure within the building. The aim of this type of measurement is to relate the pressure differential across the envelope to the air flow rate required to produce it. Generally, the higher the flow rate required to produce a given pressure difference, the less airtight the building [1].

[1] M. Limb, “Technical note AIVC 36- Air Infiltration and Ventilation Glossary,” International Energy Agency energy conservation in buildings and community systems programme, 1992.

What is TAAC? How can I be involved?

TAAC  is the TightVent Airtightness Associations Committee, launched in September 2012 by the TightVent Europe platform, with the primary goal to bring together national associations and experts in order to promote reliable testing/inspection and reporting procedures.

The scope of TAAC includes various aspects such as: building & ductwork airtightness requirements in the countries involved; competent tester schemes in the countries involved; applicable standards and guidelines for testing; inspection of ventilation systems; collection of relevant guidance and training documents; share of knowledge and experience; and information on ongoing research work in the field of building and ductwork airtightness.

At present the participants are from Belgium, Czech Republic, Estonia, France, Germany, Hungary, Ireland, Latvia, Netherlands, Poland, Portugal, Spain, Sweden, Switzerland, UK and the US.

In case you are interested to join this initiative, please write an email to:


For more information please see:


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