Airtightness describes how well the building envelope resists uncontrolled air leakage through gaps, joints and penetrations. An airtight house loses less heat to draughts and allows ventilation to be delivered deliberately, through designed openings or a mechanical system, rather than randomly through cracks.
Uncontrolled air leakage carries heat out of a building and can drive moisture into the construction, where it may condense. Airtightness is achieved with a continuous barrier — membranes, parge coats, tapes and sealants — that wraps the heated envelope without gaps.
Airtightness must always be paired with proper ventilation. The principle used in modern construction is 'build tight, ventilate right': seal the fabric to eliminate uncontrolled leakage, then provide a designed ventilation system to supply fresh air and remove moisture.