Ventilation is the method by which fresh air enters the home and stale air leaves it. In the past uncontrolled ventilation, known as natural ventilation in the building regulations, was the most common method. It is cost effective as it consists of permanent wall vents or installing windows prefitted with vents. Natural ventilation is complemented by extractor fans in bathrooms and kitchens to extract moist stale air more quickly.
This method allows for plenty of fresh (cold) air to enter the home but also for plenty of heat to escape, which has led to the practice of draught-excluding, or boarding up vents, which in turn leads to moisture problems and the danger of carbon monoxide build up.