Negative Pressure

Negative pressure (or depressurization) occurs when the air pressure inside a building is lower than the outdoor air pressure. This pressure difference is created when exhaust ventilation removes more air from the building than is supplied. A slight negative pressure of 0 to 5 pascals is considered normal and beneficial, as it prevents moisture-laden indoor air from being pushed into wall cavities where it could condense and cause damage.

Problems arise when negative pressure becomes excessive, typically above 10 to 15 pascals. In this case, the building draws air aggressively through any available opening — not just through intended ventilation paths but also through cracks in the foundation, gaps in the building envelope, and penetrations in walls and floors. This uncontrolled air intake can pull radon gas from the soil, mold spores from crawl spaces, and mineral fiber dust from insulation into living spaces.

Common causes of excessive negative pressure include an overpowered kitchen range hood (which can extract 200 to 600 liters per second), a fireplace or wood stove competing for combustion air, sealed makeup air valves, or an unbalanced mechanical ventilation system. The problem is amplified in modern, tightly sealed buildings where there are fewer natural air leakage paths.

If you notice whistling sounds near windows, doors that are hard to open, cold drafts at floor level, or a fireplace that smokes when the range hood is running, excessive negative pressure may be the cause. A ventilation professional can measure the pressure difference and recommend corrective actions such as adding makeup air supply, reducing exhaust rates, or installing a dedicated combustion air duct for the fireplace.