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What Is Air Tightness Testing?

This article relates to the air tightness testing of buildings as commonly conducted on the completion of new dwellings as part of building regulations. This is also known as air pressure testing and air leakage testing

Historically, buildings were constructed to be draughty. Very early buildings were built without a chimney and the smoke permeated through the open roof structure. 
Until the introduction of modern central heating, most houses had open fires so that air and combustion products were drawn up the chimney. This air was replaced in the room by cold fresh air drawn in by either vents or cracks in the building structure. Obviously, this was not a very efficient way of heating and the draughts caused discomfort. 

Build Tight, Ventilate Right

Nowadays air leakage is differentiated from planned ventilation. A common philosophy is, “Build tight. Ventilate right” as ventilation is essential in removing unwanted moisture and unsanitary air. This should be planned as part of the building services and is covered in Part F of the English Building regulations. 

This article describes the regulations as they apply to England - slightly different versions of regulations apply to Wales, Scotland and Northern Ireland although the test method is the same. Testing may also be applied to achieve other voluntary standards, typically Passivhaus as described below.

In the UK the SAP model is applied to measure the thermal performance of dwellings and is used to determine compliance with regulations, to generate EPC’s and other statistics.

A new version of SAP and Part L 2021 has been introduced in England that introduces a significant and overdue tightening of regulations. In Wales Scotland and Northern Ireland, the building regulations are a bit different and they tend to be slower to adopt the latest version of SAP and regulatory standards. 

What Is Air Leakage?

Air leakage is the additional unwanted air flow that occurs in addition to the planned ventilation due to voids and cracks in the external structure.  Additional cold air from leakage reduces the heating system efficiency making it harder to reach the performance targets set by Part L. 

The air leakage rate is one of the inputs into SAP – but it can’t be practically measured until the dwelling is finished. Therefore, an air leakage target is set at the SAP design stage before construction starts and this is included in the Part L design specifications. 

Do you need air tightness testing carried out?

If you are building a new development, yes. Since 2006, an air tightness testing regime has been required on all new developments in the UK as part of the approved document Part L.

When Is Air Tightness Testing Performed?

Air tightness testing, also known as air pressure testing, in the UK is normally performed on new dwellings to determine compliance with building regulations (Part L in England). Dwellings are tested on completion and the results incorporated into SAP calculations that determine the compliance and generate the Energy Performance Certificate. The blower door method of testing is currently used, although a new pulse method is being developed, each has advantages and disadvantages. 

An impulse test may be easier and faster but has at least one significant disadvantage - it is not possible to discover the cause of the leakage.  The remainder of this article assumes a blower door test as commonly used in the UK. 

How Is the Air Tightness Test Undertaken for Building regulations?

The dwelling is normally finished complete and ready to occupy sell or rent. Sealant/mastic should have been applied to a suitably high standard in the correct places according to the build - knowledge is required to avoid common mistakes that cause leakage and these must be applied at the appropriate stage of the build. 

Not all air leakage can be fixed after completion – behind kitchen and bathroom units for example. The electricity supply should be on to power the fan. 

Prior to testing, the tester will have calculated the building envelope from construction plans provided by the client. The tester will also need to know what target the test needs to achieve in order to comply with part L or other requirements. For part L compliance the target is usually taken from the design SAP calculations.  On arrival, the tester will inspect the dwelling to ensure that it is ready for testing. 

The tester applies temporary sealing masks over planned ventilation systems and flues only as allowed in the appropriate procedure (The procedure is slightly different for each version of part L; therefore, the tester requires knowledge of the version of the building regulations being applied. Temporary sealing of other leakage paths with gaffer tape etc is not allowed. 

For blower door tests - The blower door kit is inserted and sealed into and open external doorway. External openings and vents are closed, and all internal doors are opened - planned ventilators paths such as extractor vans chimneys and some types of flue are temporarily sealed. The fan in the blower door is powered and adjusted to create a slight differential pressure compared to outside. 

Typically, air is blown out resulting in a slight depressurisation so that air leaks into the dwelling via cracks and voids etc. Because the fan is fully calibrated the amount of flow is measured. 

Alternatively, the building may be pressurised by blowing air in - so that it leaks out through cracks etc in the envelope but otherwise the method and the results obtained are the same.

How the pressure is measured

At a reference pressure (normally 50 pascals), the building leakage is measured (in cubic metres per hour). In practice several readings are required at different pressures and the results are graphed and improved by compensating for the effects of background pressures, for atmospheric pressure and for variance caused by temperature differences inside and outside the dwelling - all are measured as part of the procedure. 

In addition, the test accuracy is determined by assessing the statistical deviation. If the deviation of a range of points is too high the test is invalidated. Such deviations may be caused by wind gusts interfering with measurements or other undesirable anomalies. If this occurs the tester repeats the procedure until a valid test is obtained.

Obviously larger buildings tend to have greater leakage so the results are normalised by dividing the results by a parameter reflecting the size of the building and then compared.

For UK building regulations this is the building envelope area and the air permeability results are in cubic meters per hour of leakage at 50 pascals per square metre of building envelope (m3/hr-1/m-2 at 50 pa). This can be visualised as the average linear velocity of escape via the building envelope. Typically targets for such leakage are 5 or less (m3/m2/hr at 50 pascals).

The envelope area is normally calculated in advance of the test using plans elevations and sections as provided by the client. The envelope area is the sum of the external wall, floor and roof areas that enclose the heated portion of the dwelling.

For passivhaus properties, the results are divided by a specially calculated dwelling volume and this results in air changes per hour ACH Hr-1. This is a special volume calculation according to passivhaus rules often completed by the passivhaus design team and should be verified by a passivhaus assessor prior to testing.

The test leakage rate at 50 pascals is practical for testing but it is rather higher than the dwelling will experience in practice. Therefore, in order to estimate the likely leakage under non-test conditions - some form of interpolation formula is required. This varies according to dwelling heating model used. 

For SAP, the air leakage test result in cubic metres per metre of envelope at 50 pascals is further divided by 20 and added as an extra air change rate per hour on top of ventilation rates that sap estimates from planned ventilation from services such as fans and flues.

Passivhaus dwellings have different air tightness requirements

Passivhaus properties generally require a much lower air leakage for certification - new houses normally require 0.6 air changes per hour (ACH) at 50 pascals or better. This is an order of magnitude better than current UK building regulations. Such houses typically have ventilation systems with heat recovery, making air leakage very undesirable. 

For passivhaus testing, the overall method is the same, but some details are significantly different. Both depressurisation and pressurisation modes are required and the results averaged. Passivhaus dwellings have a target of 0.6 ACH at 50 Pa. The ACH (air changes per hour) is calculated by dividing the average leakage in cubic metres per hour measured at 50 pascals by a dwelling volume. 

This internal volume is specifically calculated for the test according to passivhaus rules. It is the sum of the net volumes in each room and should be verified by the passivhaus assessor. Tests are typically performed at various stages of construction starting as soon as a sealed envelope is created and whilst the air leakage barriers are still exposed and can be repaired. 

Subsequent tests ensure that the dwelling maintains the high standard required as the building work continues. A high level of discipline and awareness of air leakage is required by all personnel working on the dwelling to ensure that the airtightness is maintained, and barriers are not breached.

What is a good air tightness test result?

For UK Building regulations the air leakage result is only one of many inputs into the SAP model that determines compliance to part L. It is not a standalone test that has a specific target. Before the 2021 part L regulations, a result of about 5 m3/hr -1/m-2 was about the norm used to pass Part L and this figure was frequently assumed in the design SAP calculations.

Under Part L 2021 the required performance is much tighter than before and this will tend to result in lower targets. To benefit from results of less than 3.0 extractor fans in bathrooms and trickle vents are not acceptable and MVHR type ventilation systems are required. Otherwise, SAP will assume a leakage of 3.0 if the test result is below 3 and the performance will not improve.

Why are the air tightness regulations important?

The aim of regulations is to create an efficient dwelling to withstand the rigours of future demands during its lifetime. Various products such as barriers and tapes are available to make a dwelling airtight. Air leakage is often misunderstood, and the extent of leakage paths are not appreciated by builders. 

Experience is required to create an airtight dwelling and use available products to maximum effect. These are designed to last the lifetime of the dwelling, unlike cloth “gaffer” tape etc which dries out after a few months.

In the UK, air leakage testers should be members of either the ATTMA or the IATS approved schemes who provide certificates that are acceptable to building control.

At Tophouse Assessments, we specialise in providing accurate, reliable and trusted air tightness and air pressure testing to builders and developers across Kent, London and the South East. So if you have any questions or want to know more about any of the services we provide, don’t hesitate to contact us