Ventilation consists of replacing stale air with fresh air, a process referred to as air renewal, and this is achieved by two means: air infiltration (uncontrolled ventilation) and purpose-built ventilation.
Air infiltration consists of draughts. A draughty house will need to be heated to a much higher degree than a home that is considered airtight, which is why building an energy efficient house often means putting in lots of insulation, then making it all airtight and ventilating it in a controlled manner.
Purpose-built ventilation systems control how fresh air enters the home, either passively through openings in the house envelope (natural ventilation) or mechanically with fans.
