Air tightness is an increasingly important part of the ‘whole house’ approach to building dwellings. Airtight houses are more energy efficient and more comfortable for families to live in than conventional homes.
It is important for homes to achieve a high level of air tightness in order to maintain energy efficiency and provide comfort for occupants. Successful airtight building stems from an attention to detail at both the design and construction stages of the project. If warm air is allowed to leak out of a building, the benefits of improved insulation and other energy efficiency measures will be lost.
Air Leakage Testing
Testing for air tightness in dwellings is an important part of UK building regulation compliance. Close attention must be paid to the reduction of air leakage throughout the building design and construction phases. It is much simpler to design and build an airtight dwelling than to carry-out remedial measures in a draughty home.
Air infiltration occurs when air leaks through cracks and gaps in the building fabric. The amount of infiltration is influenced by the type of construction and by the speed and direction of the wind. Wind blowing against the building causes pressure differences between the inside and the outside. Air is drawn into the dwelling through holes on the windward face and leaves the dwelling on the leeward side.
Warm air inside the dwelling is more buoyant than the colder air outside. Buoyant air rises by convection and, in doing so, draws-in cool air from outside. As buoyant air rises, it increases the pressure inside the dwelling which pushes warm air out of holes in the building envelope
Air Leakage Paths
Air can leak through the ceiling below a roof void, particularly around the loft hatch cover and around its frame. It can escape through open joints in the brickwork and concrete blocks that make up the external walls. Draughts commonly occur at window and door locations, particularly between the frames and the brickwork. They can also occur between the frames and the doors or opening sashes. In the case of suspended floors, air leakage can occur where timber joists are built-into masonry walls. Air can pass through cracks between floorboards and through gaps at the junction between the floor and surrounding walls. Generally, significant problems can arise from the gaps left around electricity and plumbing services penetrations in walls, floors and ceilings.
Air Barrier Construction
‘Air barrier’ is a term used to describe a specific layer created within the thermal envelope that separates heated and unheated spaces. The air barrier will effectively restrict the passage of air between the internal and external environments. At an early stage, the designer should identify the exact position of the air barrier and mark it on a drawing. Its exact position will depend very much on the design of the building and the materials used in the construction.
Air Testing Procedure
When the building work is complete, an air test will usually be carried out by a specialist company. A temporary airtight screen is fitted into an external door opening of the dwelling. Water traps are filled, trickle vents closed and extract vents sealed. An electric fan is then mounted in the airtight screen and operated to blow air into or out of the dwelling to create a pressure difference between inside and outside. The air tightness of the dwelling is calculated by measuring the rate of airflow through the fan, while a range of pressure differences are maintained between the inside and outside of the dwelling.
Air Test Failure
In the event of an air barrier failing an air leakage test, smoke testing and thermal imaging can determine the main areas of leakage. Failure of the air barrier at the air testing stage can be very costly in both time and money.
Air tightness is the concern of all members of the design and construction teams. Communication between all workers is vital to make sure that practical working details are produced and that the air barrier is constructed with the utmost care.
When constructing an airtight dwelling, it is important to provide adequate controlled ventilation to ensure occupant comfort and safety.