The overall approach a design takes to bringing fresh air into a building and removing stale air - natural, mechanical, or a mix of both. A natural ventilation strategy relies on openable windows and stack or cross-ventilation airflow; a mechanical strategy uses ducted supply and extract systems instead, trading energy use for tighter control over air quality and comfort. As buildings get built increasingly airtight to hit energy targets, the ventilation strategy becomes the counterbalance that keeps indoor air genuinely healthy to breathe.