Airtightness

Airtightness describes how strongly a building envelope resists uncontrolled air leakage through gaps, cracks and service penetrations when intended openings and ventilators are closed. In home energy work, it can affect heat loss, ventilation needs, retrofit design and how new cable or pipe routes are sealed after PV, battery, EV charger or heat pump installation work. It is a building-fabric property, not the same as insulation, ventilation, weatherproofing, a heat pump heat-loss calculation, an air-permeability test result, planning permission, Building Regulations compliance, DNO approval or MCS certification.