Design and Detailing for Airtightness

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Key Principles

1. Testing procedure is set out in CIBSE TM 23 and in BS EN 13829: 2001.

2. A pressure test involves sealing all ‘normal’ gaps such as vents and pressurising or depressurising the building. The level of fanpower required to maintain the pressure differential indicates the ‘leakiness’ or ‘permeability’ of the building.

3. Pressure tests are typically followed by an audit (using smoke pencils, for example) to expose and make visible the various air leakage routes.

4. Where projects comprise large quantities of a single component, component testing in the laboratory may be appropriate as well as on site element testing.

5.1 Climatic conditions

As mentioned in Chapter 1, the raised pressure differential of 50 Pascals created during an airtightness test is quite small. Whilst this is adequate to overcome most of the common pressure differential anomalies, such a small differential is vulnerable to larger pressure differences created by climatic conditions.

Air leakage tests require calm days – i.e. a reading on the Beaufort Scale of 3 or less (3.4 to 5.4 metres per second wind speed at 10 m above ground) This corresponds to a gentle breeze with leaves and small twigs in constant motion. In winter conditions, and on exposed sites therefore testing may not be possible, although it is often possible to make allowances, so long as these are carefully recorded.

5.2 The Test itself

Guidance on testing buildings for airtightness is contained in CIBSE Technical Memorandum TM23 Testing Buildings for Airtightness and in BS EN 13829: 2001. All UKAS accredited testers test to the guidelines contained in the BS EN.

Essentially the process is one of de-pressurising or (less commonly) pressurising the inside of the whole building, and of measuring the rate at which air needs to be blown or sucked to maintain that pressure differential; a leaky building will equalise readily and require a greater measurable effort to maintain the 50 Pascal differential, while a tight building will easily contain the enforced differential and require little additional input.

The pressure difference is induced by one or more calibrated fans that are normally mounted within a suitable doorway. An adjustable door panel system, sealed around the edges is used which can also be connected to large external fans via collapsible ductwork if required. The rate of the fan, or the volume flow of air through the fan can be understood as the rate of air entering / escaping throughout the remainder of the building envelope.
Buildings are tested in such a way as to recreate ‘normal’ conditions. Doors and windows are closed, trickle ventilators closed, extract fans and such like are closed but not sealed. Internal doors are wedged open.

If the building is under construction, testing is ideally undertaken outwith working hours, but sometimes this is not practical so some scheduling of work needs to be thought through in advance. With all external doors and windows sealed shut, some work becomes impossible (such as work with solvents requiring ventilation) and internal trades are normally ‘sealed in’ for a short time, where they can carry on undisturbed.

In existing buildings, tests are normally carried out when the building is unoccupied if possible because of the disruption.

5.3 Air Leakage Audits

The air leakage test quantifies the rate of air leakage through the envelope as a whole, but it cannot locate the air leakage paths. Where remedial work is required therefore, tests are followed by a range of auditing techniques designed to identify the specific places where air is leaking.

In many cases a simple visual inspection may be sufficient – especially if undertaken by someone with experience of the likely locations of leakage.

However, most leakage routes are difficult or impossible to spot without visual aids. One common technique is to use smoke tracers – smoke pencils or smoke machines. These render the air paths visible in certain situations. The building may be positively pressurised and the leaks witnessed externally, or, more usually, negatively pressurised while a smoke pencil is drawn over likely gaps and defects which become visible as the smoke is sucked inwards.

Another technique, which has certain advantages and disadvantages compared to smoke tracing, is the use of an infrared camera. (see page 4) Used either externally or internally, these thermographic cameras register the radiant heat levels of surfaces and so are able to ‘see’ for example, where cold air is cooling the fabric around a gap internally, or conversely where warm air is escaping and heating the colder materials on the external face.

To work effectively, there needs to be a recognisable difference between the internal and external ambient temperature, so before any heating has been installed and on a warm summer’s day thermography may not be effective. Similarly on warm and sunny days, sunshine on external surfaces can distort the true situation so it is better on such days to wait until early evening. Conversely, rain on external surfaces can be equally distorting of the true thermal situation. However, these cameras are useful in identifying problems at high level or difficult to reach areas, and are also very helpful in identifying other construction defects such as poorly installed (or non-existent!) insulation within the fabric.

On larger commercial buildings, airtightness testing may be undertaken at the same time as ‘standard’ ventilation system commissioning and associated studies, but these are not discussed as part of this guide.

5.4 Component Testing

A distinct aspect of overall airtightness testing is the individual component test. This may be undertaken quite separately, in the laboratory or by the manufacturer of a particular component. Such tests may be deemed necessary on a large project where large areas of one particular type of component, for example curtain walling, are to be specified,

Insitu element testing involves isolating the area within a temporary sealed compartment, which is then pressurised, and the air leakage related to the area of interest assessed. In this way sample areas of a building may be pressure tested using smaller fans as required.

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