Ductwork airtightness

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Ductwork airtightness can be defined as the resistance to inward or outward air leakage through the ductwork envelope (or ductwork shell). This air leakage is driven by differential pressures across the ductwork envelope due to the combined effects of stack and fan operation (in case of a mechanical ventilation system).


For a given HVAC system, the term ductwork refers to the set of ducts and fittings (tees, reducers, bends, etc.) that are used to supply the air to or extract the air from the conditioned spaces. It does not include components such as air handlers, heat recovery units, air terminal devices, coils. However, attenuators, dampers, access panels, etc. are a part of the ductwork even if they have more functions than conveying the air and are therefore also referred to as technical ductwork products.

Ductwork airtightness is the fundamental ductwork property that impacts the uncontrolled leakage of air through duct leaks.


There are two major systems to classify ductwork airtightness, one based on European standards, the other based on ASHRAE standard 90.1-2010. Both are based on the leakage airflow rate at a given ductwork pressure divided by the product of the ductwork surface area and the same ductwork pressure raised to the power 0.65.

Classification of air distribution tightness [7]
Airtightness classesAirtightness classesAir leakage limit (fmax) according to the test pressure (pt) [m3.s−1.m−2]
Previous nameNew name
ATC 7Not classified
ATC 60,0675 x pt0,65 x 10−3
AATC 50,027 x pt0,65 x 10−3
BATC 40,009 x pt0,65 x 10−3
CATC 30,003 x pt0,65 x 10−3
DATC 20,001 x pt0,65 x 10−3
ATC 10,00033 x pt0,65 x 10−3

In future revisions of EN 12237, EN 1507, EN 1751 and EN 15727 these new names will be used; they have already been introduced in EN 17192. [10]

Distribution of ductwork airtightness classes. Number of measurements: 21 in Belgium, 21 in France, 69 in Sweden Distribution of ductwork airtightness classes.jpg
Distribution of ductwork airtightness classes. Number of measurements: 21 in Belgium, 21 in France, 69 in Sweden
Comparison between European (Eurovent and AMA) TightVent Classes A-D and American (ASHRAE) TightVent classes CL3, CL6, etc. Comparison between European (Eurovent and AMA) TightVent Classes A-D and American (ASHRAE) TightVent classes CL3, CL6, etc.jpg
Comparison between European (Eurovent and AMA) TightVent Classes A-D and American (ASHRAE) TightVent classes CL3, CL6, etc.

Power law model of airflow through leaks

The relationship between pressure and leakage air flow rate is defined by the power law model between the airflow rate and the pressure difference across the ductwork envelope as follows:



This law enables to assess the airflow rate at any pressure difference regardless the initial measurement. Threshold limits in ductwork airtightness classifications usually assume an airflow exponent of 0,65.

Pressurization test

The ductwork airtightness level is the airflow rate through ductwork leakages divided by the ductwork area. It is recommended to test at least 10% and 10 m2 of the duct surface including all duct types and a variety of sizes and components. The ductwork surface area is estimated according to EN 14239. [11]

The airflow rate through leakage can be measured by temporarily connecting a device (sometimes called a duct leakage tester to pressurize the ductwork including duct-mounted components. Air flow through the pressurizing device creates an internal, uniform, static pressure within the ductwork. The aim of this type of measurement is to relate the pressure differential across the ductwork to the air flow rate required to produce it. Generally, the higher the air flow rate required to produce a given pressure difference, the less airtight the ductwork. This pressurization technique is described in standard test methods such as EN 12599 and ASHRAE standard 90.1-2010. In principle, it is similar to that used to characterize building airtightness.

Impact of ductwork airtightness

An airtight ductwork has several positive impacts: [12] [13] [14] [15] [16]

Duct leakage affects more severely the energy efficiency of systems that include air heating or cooling.

Duct sealing or duct tightening

At construction stage, the airtightness of individual components depends on the design (rectangular or round ducts, pressed or segmented bends, etc.) and assembly (seam type and welding quality). Components with factory-fitted sealing devices (e.g., gaskets, clips) meant to ease and accelerate the installation process are widely used in Scandinavian countries. [15] A variety of techniques are widely used to tighten duct systems on site, including gaskets, tapes, sealing compound (mastic), internal duct lining, aerosol duct sealing. So-called "duct tapes" are often not suited for sealing ducts, [17] [18] which explains why, in the US, the International Energy Conservation Code (IECC) requires any tape used on duct board or flexible ducts to be labeled in accordance with UL 181A or 181B.

Typical reasons for poor ductwork air tightness include: [19] [20] [12]

Ductwork airtightness requirements

Sweden is often considered as a reference for airtight ducts: the requirements introduced in AMA (General material and workmanship specifications) [21] starting in 1950 have led to excellent ductwork airtightness on a regular basis in Sweden. [13] [22]

In the US, there has been a significant amount of work showing energy saving potentials on the order of 20-30% in homes; [23] and 10-40% in commercial buildings with airtight ducts [24]

The ASIEPI project technical report [25] on building and ductwork air tightness estimated the heating energy impact of duct leakage in a ventilation system on the order of 0-5 kWh per m2 of floor area per year plus additional fan energy use for a moderately cold European region (2500 degree-days).

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Blower door

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Air handler

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Duct leakage testing

A duct leakage tester is a diagnostic tool designed to measure the airtightness of forced air heating, ventilating and air-conditioning (HVAC) ductwork. A duct leakage tester consists of a calibrated fan for measuring an air flow rate and a pressure sensing device to measure the pressure created by the fan flow. The combination of pressure and fan flow measurements are used to determine the ductwork airtightness. The airtightness of ductwork is useful knowledge when trying to improve energy conservation.

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TightVent Europe is a platform, formed in January 2011, with a focus on building and ductwork airtightness issues. The creation of the platform was triggered by the need for a strong and concerted initiative to meet the Directive on the energy performance of buildings ambitious targets for the year 2020 and overcome the challenges in relation to the envelope and ductwork leakage towards the generalization of nearly zero-energy buildings. The platform’s main activities, among others, include the production and dissemination of policy oriented publications, networking among local or national airtightness associations, as well as the organization of conferences, workshops and webinars.

Indoor Environmental Quality Global Alliance

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Ventilative cooling

Ventilative cooling is the use of natural or mechanical ventilation to cool indoor spaces. The use of outside air reduces the cooling load and the energy consumption of these systems, while maintaining high quality indoor conditions; passive ventilative cooling may eliminate energy consumption. Ventilative cooling strategies are applied in a wide range of buildings and may even be critical to realize renovated or new high efficient buildings and zero-energy buildings (ZEBs). Ventilation is present in buildings mainly for air quality reasons. It can be used additionally to remove both excess heat gains, as well as increase the velocity of the air and thereby widen the thermal comfort range. Ventilative cooling is assessed by long-term evaluation indices. Ventilative cooling is dependent on the availability of appropriate external conditions and on the thermal physical characteristics of the building.


  1. EN 12237:2003: "Ventilation for buildings - Ductwork - Strength and leakage of circular sheet metal ducts", 2003.
  2. EN 1507:2006: "Ventilation for buildings - Sheet metal air ducts with rectangular section - Requirements for strength and leakage", 2006
  3. EN 17192:2018: "Ventilation for buildings - Ductwork - Non-metallic ductwork - Requirements and test methods", 2018
  4. EN 1751:2014: "Ventilation for buildings - Air terminal devices - Aerodynamic testing of damper and valves", 2014
  5. EN 15727:2010: "Ventilation for buildings - Ducts and ductwork components, leakage classification and testing", 2010
  6. EN 1886:2007: "Ventilation for buildings - Air handling units - Mechanical performance", 2007
  7. 1 2 EN 16798-3:2017: “Energy performance of buildings - Ventilation for buildings - Part 3: For non-residential buildings - Performance requirements for ventilation and room-conditioning systems (Modules M5-1, M5-4)”, 2017
  8. EN 12599:2012: “Ventilation for buildings - Test procedures and measurement methods to hand over air conditioning and ventilation systems”, 2012
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  10. EN 17192:2018: “Ventilation for buildings - Ductwork - Non-metallic ductwork - Requirements and test methods”, 2018
  11. EN 14239:2004: “Ventilation for buildings - Ductwork - Measurement of ductwork surface area”, 2004
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  15. 1 2 C. Delmotte. "Airtightness of ventilation ducts". Air Infiltration and Ventilation Centre (AIVC) Ventilation Information Paper 01, 2003.
  16. V. Leprince, N. Hurel, M. Kapsalaki “AIVC Ventilation Information Paper no40: Ductwork airtightness - A review", 2020
  17. M. Holladay. "Sealing Ducts: What’s Better, Tape or Mastic?". Green Building Advisor, 2010
  18. M. Sherman and I. Walker. "Can Duct Tape Take the Heat?". Home Energy Magazine Online, 1998
  19. Lawrence Berkeley National Laboratory (LBNL). "An Introduction to Residential Duct Systems". LBNL, 2003
  20. F. R. Carrié, J. Anderson, P. Wouters. "Improving ductwork—A time for tighter air distribution systems". Air Infiltration and Ventilation Centre. Coventry, UK., 1999.
  21. AMA VVS & Kyl 12. Allmän material- och arbetsbeskrivning för VVS- och Kyltekniska arbeten (General Material and Workmanship Specifications of HVAC installations). AB Svensk Byggtjänst, Stockholm 2012. (in Swedish).
  22. Peter G. Schild, Jorma Ralio. "Duct System Air Leakage - How Scandinavia tackled the problem", European project ASIEPI Paper 187, 2009, URL: http://www.buildup.eu/sites/default/files/content/P187_Duct_System_Air_Leakage_ASIEPI_WP5.pdf
  23. Energy Star. "Duct Sealing". Retrieved 5 May 2015.
  24. Lawrence Berkeley National Laboratory - Building Technology and Urban Systems Division (BTUS). "HVAC System Technologies". Retrieved 5 May 2015.
  25. G. Guyot, F. R. Carrié and P. Schild, “Project ASIEPI – Stimulation of good building and ductwork airtightness through EPBD,” 2010