Air apparent?

Designing buildings to be airtight has never been more essential than it is today. The impending introduction of Approved Document L (ADL), which sets tough new energy reduction targets, will serve to ensure engineers think carefully about how to reduce air leakage in buildings.

Air leakage should not be confused with natural ventilation. Unwanted air infiltration is the result of poor facade design and construction, allowing air to seep into the structure through gaps and cracks in the building's fabric.

The amount of unwanted air entering a building depends on the differences in internal and external pressures and temperatures, and so varies with changes in weather conditions. Nevertheless, the threat of air leakage is constant and can greatly affect energy use and costs, particularly in winter, as heating systems battle to counterbalance the resultant internal air-cooling.

It can also have a detrimental effect on the comfort levels of occupants. In a well-designed building airflow can be manipulated and comfort levels ensured, whereas in a leaky building airflow is much more difficult to control and can lead to occupants experiencing severe discomfort in localised cold spots.

Wind pressure also plays an important role. The direction of wind hitting the building affects the route the air takes. Air tends to enter via the windward face and exits on the leeward face, and can create a negative internal pressure.

In addition, heat produced from heating systems, people, equipment and solar gain results in an increase in internal air buoyancy and air being drawn inwards. An imbalance between internal and external air pressure causes the warmer air to rise and exfiltrate the building, drawing in cooler external air.

So, where exactly does air leak in through facades? According to the Building Research Establishment (BRE) there are seven common infiltration paths, and they are not confined to any particular area (see box 'Common infiltration paths' on page 44 for more details).

Who's responsible?
It would be easy to blame the construction team for any leaks and gaps in the facade, but as John Beasley, principle facade consultant at the BRE explains, faults in the construction process can arise from a lack of communication between the design team: "In instances of wall to roof junctions not being protected, for example, the problem often comes merely from the fact that throughout the design process this area is neglected by each party, 'that's not our responsibility' and 'someone else will be dealing with that' is often the attitude, and it results in the gap not being filled properly and air being allowed to leak into the structure."

BRE has recently produced building specification advice to help engineers ensure against poor communication leading to poor construction. It states how designers 'need to provide clear workable design detail specifications for an effective, maintainable air-leakage barrier; and to brief builders/contractors and subcontractors from the outset'.

Testing for compliance
The guide sets out to explain how designers should explicitly refer to compliance testing procedures in the specification and state clearly who pays for what. All problem areas should be identified in the contract documents, as well as who is responsible for finishing-off the building. Testing for airtightness on completion is the responsibility of the main contractor, and reference to this should be made in the pre-contractual agreement.

The testing itself is carried out using the fan pressurisation technique, which fires highly pressurised air directly at the most vulnerable areas of the building, such as doors, windows and trickle vents. Smoke extract fans are left closed but not sealed, and other integral openings such as lift shafts are left unsealed.

There are three common terms for specifying air leakage performance: the air leakage index, air permeability, and air leakage rate. It is important not to assume one is like the other, as Brian Webb, airtightness consultant at BRE explains: "Each one is calculated differently, so must be treated differently. Air permeability is probably the one to look for as Approved Document L uses it to specify minimum performance requirements."

Air permeability is defined as the air leakage in m³/h/m² of the building envelope at the reference pressure of 50 Pa (see page 44 for the BRE's table of recommended values for individual building types).

According to research the airtightness layer needs to meet the following criteria: it must be made up of air-impermeable materials; the materials should be made continuous around the envelope; it should have sufficient strength to withstand any pressures created by air controlling systems and wind or stack effects; be easily installed; and must be durable and/or accessible for maintenance or replacement.

It's to your advantage
The benefits to services engineers of a well-designed and 'tight' building are huge. In a tight building predictions can be made about its behaviour under various weather conditions. This means investment in energy saving measures such as heat recovery systems and optimum start/stop heating controls can be worthwhile.

Conversely, a leaky building can negate any theoretical benefit from such systems, as exfiltration of warm air through a leaky fabric will reduce heat available to the heat recovery systems and could depress their efficiency rating by up to 20%. Also, optimum start controls are less likely to ensure the building reaches a suitable temperature soon enough.

Associated problems
A leaky building can also lead to problems with an excessive build-up of moist air. If walls and roofs are not designed to be tolerant of the exfiltration of warm air, then there is an increased risk of condensation and moisture accumulation. This can have dire consequences for moisture-vulnerable materials.

The importance of achieving airtightness in buildings should not be underestimated. BRE believes air leakage can have serious adverse affects on efficiency ratings of services. The soon-to-be-released ADL will leave little time for such inefficiency in construction. You have been warned.