Air Leakage and Ventilation Testing

Do I need an air tightness test for building compliance?

Part L of the building regulations requires an air tightness test for any new domestic building and all commercial buildings of over 500m² floor area. The airtightness measurements are used in the energy modelling of the building via SAP and SBEM and can affect its energy efficiency rating.

Air tightness tests are carried out after the construction of the building and meeting the required standard with the test result satisfies compliance with Part L of the building regulations in England & Wales and regional variants in Scotland and Northern Ireland.

Part L approved air pressure testing methods

After the introduction of the revision to the building regulations, there will be two possible testing methods for compliance testing. These will be the Low-Pressure Pulse (LPP) method provided by BTS and blower door testing. Both testing methods have to be carried out in accordance with CIBSE TM53.

Pulse testing provides quick and non-disruptive measurements of building airtightness in conditions similar to how a building usually operates. Pulse works by releasing a known amount of air from a pressure vessel and measuring the response in the air pressure within the building. The test is completed in less than a minute after the building and testing equipment has been prepared. Pulse results are given at a pressure difference between the inside and outside of the building of 4Pa, which is similar to how the building normally operates and representative of ambient conditions.

Blower door tests have been used to measure the airtightness of buildings for decades using a blower door fan. The test works by introducing a known volume of air through a fan installed in a temporary frame located in a door opening. The measurement is carried out at an elevated pressure difference between the inside and outside of the building, with results reported at a pressure difference of 50Pa. Because the blower door equipment has to be installed in a door opening, the airtightness of the door is not included in the measurement but can be tested by repeated tests in different door openings.

The elevated pressure difference caused by the blower door equipment can be used to detect leakage paths for air movement as there is a larger than normal volume of air blowing through these paths. BTS' Leak Checker can also be used for this purpose.

Comparison of Pulse and Blower Door methods of air tightness testing →

Air tightness testing in existing buildings

Airtightness is equally important in existing buildings as it is in new buildings. Understanding the airtightness of a building is an important part of understanding how a building is operating, and a key step in planning how to manage and improve the energy efficiency of an existing building.

The importance of understanding airtightness in existing buildings is underlined by the permitted use of air tightness testing in retrofit guidelines such as PAS 2035.

Glossary of air leakage terms

There is a wide range of interchangeable terms used throughout the air testing industry, many of which are described below:

• Air barrier comprises materials and/or components, which are air impervious or virtually so, separating conditioned spaces (heated), from unconditioned spaces (unheated).
• Air change rate is the rate at which outside air enters a space divided by the volume of that space. This is expressed as ACH (air changes per hour, as described below).
• Air change rate volume (m³) is expressed as V and constitutes the volume of air inside the dwelling under test, for all the zones incorporated within the test zone and with deductions made for internal walls and floors but no deductions for furniture.
• Air changes per hour (ACH)* is a measure of air leakage rate expressed as volume flow rate (m³/h) at 50 Pa reference pressure difference per cubic meter (m³) of building volume (i.e. Q50/V or ACH50 and also known as the N50 method). The Passivhaus methodology uses N50 as it considers the volume of air that needs to be heated and therefore internal walls and floors are excluded. Voids within wall and floor constructions also cannot be counted.
• Air exfiltration is the uncontrolled outward leakage of indoor air through cracks, discontinuities and other unintentional openings in the building envelope. Sometimes also expressed as exfiltration rate.
• Air infiltration is the uncontrolled inward leakage of outdoor air through cracks, discontinuities and other unintentional openings in the building envelope. Sometimes also expressed as infiltration rate.
• Air leakage index is measured in the same way as air permeability (expressed as m³/m².h @50Pa) but the building envelope area excludes the solid ground floor area. The lowest floor area is only included when it is not ground supported as in this case there is the potential for air leakage through it.
• Air leakage is the term used to describe both air infiltration and exfiltration under natural pressures. It is the total uncontrolled flow of air through gaps and cracks in the fabric of a building.
• Air leakage path is a route by which air enters or leaves a building or flows through a component.
• Air leakage rate is not very well understood in the construction industry. This is a measure of the actual infiltration heat loss experienced under naturally occurring air pressures. The test pressure of 50 Pa used to derive air permeability and air leakage index is much higher than the pressure differences that drive leakage due to weather conditions and is therefore only used for normalised compliance testing purposes. Nevertheless, from a large number of measurements carried out on dwellings it has been found that the air leakage rate in air changes per hour (ACH) is approximately 1/20 of the 50 Pa air permeability rate. Application of the ‘1/20th rule’ gives an estimated air leakage rate in air changes per hour (ACH).
• Air permeability* is the most commonly used metric to quantify the air leakage rate through a building envelope, expressed as the volume flow per hour (m³/h) of air supplied to the space by air-moving equipment, per square meter (m²) of building envelope area for a specified internal to external pressure difference of 50Pa: for example, 10 m³/m².h @50Pa. It is also expressed as the Q50 method. The building envelope area includes any surface that is a boundary between the conditioned internal space and the external environment, including the solid ground floor area.
• Airtightness is the term used to describe the tightness of a building. The smaller the air leakage for a given pressure difference across a building, the tighter the building envelope indicating a well-sealed building.
• Airtightness layer is the same as an air barrier. It is built into the external envelope to minimise air leakage (infiltration and exfiltration). It may consist of a wide range of materials e.g. sealants, gaskets, glazing or membranes and should be continuous to be effective.
• Breather membrane is a water-resistant sheet that allows transmission of water vapour, but which provides resistance to airflow.
• Conditioned zone is the occupied zone in a building that requires heating or cooling and is normally bounded by an airtightness layer.
• Draught is an excessive movement of air within the conditioned zone, which may cause discomfort
• Draught-proofing is the act of filling in the gaps between opening parts of components and their frames e.g. around external doors, operable windows and letter flaps.
• Effective leakage area (ELA) is the estimated area of one hole that would let in the equivalent amount of air as the fabric of the building i.e. the sum of all gaps and cracks in a building envelope expressed as an area in square meters (m²). It is however only an aerodynamic equivalent area based on a sharp-edged orifice and should therefore be regarded as approximate. Extrapolation of the results from a fan pressurisation test down to a reference pressure of between 4 Pa and 10 Pa is also required.
• Envelope area (m²) is the boundary or barrier (m²) separating the interior volume of the building from the outside environment. This includes the area of the external walls, roof and, depending upon the air leakage parameter specified, the area of the ground supported floor. It is used to determine the total air permeability and expressed as m³/h.
• Fan pressurisation test is a method of testing air leakage of a building. It allows airflow and pressure difference across the envelope to be measured and an estimate of air leakage to be obtained. This is the method a blower door fan uses.
• Low-pressure pulse test is the technique developed by the University of Nottingham and Build Test Solutions. It generates an instant pressure rise in the building and pressure decay is monitored and used to determine the building air leakage. This is the method Pulse uses.
• Quasi-steady is an assumption of steady-state. This can be deduced when a test of variable conditions is undertaken is a period of time that is short enough to not allow particularly significant changes in the system to occur.
• Specific leakage rate is the same as the air permeability. It is the air leakage rate per net floor area at the test reference pressure differential across the building envelope.

* Air Permeability and Air Changes per Hour (ACH), do not have a direct relationship with each other (so you can’t apply a conversion factor to one to get the other). Therefore the building needs to be measured to the right protocol, and the results calculated in the right way (N50 not Q50).