Good ventillation is an important requirement of a building to supply fresh air and avoid condensation. However, it also removes heat from a building in the colder months.
Most older houses have natural ventillation where vents in walls, chimneys, cracks and gaps in the floors, windows and roofs create a route for air to enter and leave the house. Further ventillation is assisted by fans such as in bathrooms and kitchens.
An air tightness test can be used to identify cracks and gaps, where heat is being lost. These areas can then be sealed while at the same time ensuring that these is still adequate ventillation to the house. The test involves temporarily sealing all of the existing vents and blowing air into the property from the front door. Using smoke the tester can then see where the air is flowing.
In recent years heat recovery ventillation systems have become available. These require the house to be sealed very well. The systems uses vents in areas around the house to remove air, extract the heat from this air and use it to heat the fresh air supply. They are more suitable for new houses which are air tight.