Air tightness is the control of the flow of air into and out a building. It is the opposite of air leakage – the uncontrolled flow of air through gaps and cracks in the external walls of a building. A draughty house is not air tight because of the excessive infiltration of air. It is a leaky house. Too much air leakage leads to unnecessary heat loss and discomfort to the occupants from cold draughts.
Ventilation is the controlled flow of air into and out of a building through purpose built ventilators that is required for the comfort and safety of the occupants. It is best achieved by having a heat recovery ventilation system.
Air tightness could be represented as how well the various components of the building have been put together. In an airtight building air leaks do not occur at ceiling or wall junctions, plaster board joints, doors, windows, service entry points etc. Air is not getting in or out at any point in the building except where it is designed to go in or out.
Draughts or leaks create an unpleasant environment. They are also expensive in that heat is lost. The result is that the heating system has to operate at a greater capacity to compensate for the losses. Air tightness is absolutely essential in maximising the effectiveness of thermal insulation, ensuring vast savings over a lifetime.
In an air tight house the movement of air is best controlled by a heat recovery ventilation system. If air movement is not controlled air quality can be unpredictable. In winter in a draughty building air may come in at 3⁰ C. A comfortable indoor temperature is 21⁰ C. The householder has to pay to heat that air. A good level of air tightness will mean that only the right volume of air will have to be heated. Similarly an air tight building will not let warm air escape.
The target must be “build tight, ventilate right”.