David Brook is an extension agent with the Oregon State University Extension Energy Program. Wind is one natural force that causes air pressure changes in a building. Even when the wind isn't blowing the building can create its own negative pressure through a process called the stack effect. When there are holes in the building envelope near the top of the structure, warm air escapes through these openings while colder air enters the building through lower openings. The best defense against the stack effect is to carefully seal your homes against air leaks.
Mechanical equipment can also cause negative pressure.
Unbalanced air flows between the return and supply sides of a duct system may be caused by leaky or disconnected ducts, by blockage in the ducts or by closed interior house doors that block air flow between supply registers and returns. In a small or tightly built house, exhaust devices often cause the most powerful depressurization. Even a small fan can create potentially dangerous depressurization. Leaky houses can have problems, too, if there are appliances with powerful fans. These include downdraft ranges, new clothes dryers, attic or whole-house cooling fans, and central vacuum cleaners.