Understanding your home: Air Flow

The Building Envelope and Airflow

Leakages (uncontrolled airflow) through the building envelope is a major source of heat loss and will also lead to other problems. As warm air can carry a lot of water vapour, airflow is one of the main means by which moisture enters into the building envelope.

During winter, the warmer air inside is forced out of the building envelope. It carries heat and moisture , whilst incoming air will bring in drafts and dry winter air. For the air to move from one side of the envelope to the other, there will be holes in the envelope and the difference in air pressure between the inside and outside. The difference in the air pressure will result from any combination of temperature difference (that creates a stacking effect in the home), wind or combustion appliances.

Wind Effect

Wind effect is when the wind blows against the building envelope creating a high pressure area on the windward side and forces air into the house. The other sides of the house would have a low pressure area where the inside air would be forced out.

Stacking Effect

Stacking effect happens in a heated home. Less dense warm air would rise and expand. It creates a high pressure area at the top of the house. Air will escape through upper storey windows, holes in the ceilings and wall cracks. The rising warm air will create a low pressure at the bottom of the house, which will draw in air through the windows, wall cracks and flooring. Higher leakage rates, taller buildings,  bigger differences between the outdoor and indoor temperatures increase the stacking effect.

Combustion and Ventilation Effect

Appliances that burn fuel, such as wood, oil, gas or even propane need air to support the combustion. Open chimneys and fireplaces exhaust a lot of air and reduce pressure in the house. This air will need to be replaced, hence outside air is drawn in through leaks in the envelope.
Controlling the airflow will protect the building materials from moisture damage. It will improve comfort and will make a healthier, cleaner and quieter home. Controlling the airflow will prevent uncontrolled air leaks through the building envelope. It will provide for fresher air supply.

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