Should a house have positive or negative air pressure?

A house should generally have balanced or slightly positive air pressure rather than strong negative air pressure for comfort, health, and energy efficiency, though the ideal depends on climate. Slight positive pressure of 2 to 5 pascals means pressure inside is slightly higher than outside, so air leaks outward through cracks rather than inward, which helps prevent unfiltered outside air, dust, pollen, humidity, and radon from being sucked into living spaces and walls. This is especially beneficial in hot humid climates because it reduces moisture intrusion that can cause mold inside walls, and for allergy sufferers because it keeps pollen out. Slight positive pressure also prevents backdrafting of combustion appliances like furnaces and fireplaces that can pull carbon monoxide back into the house when negative pressure is strong. However, in cold climates, too much positive pressure can push warm moist indoor air into wall cavities where it condenses and causes mold, so balanced pressure is better there. Strong negative pressure is generally undesirable in homes because it causes cold drafts, increased energy bills as outside air is sucked in, difficulty opening doors, and dangerous backdrafting. In tightly sealed modern homes, the goal is balanced ventilation with controlled fresh air via an energy recovery ventilator or makeup air system for large range hoods, providing filtered fresh air without excessive positive or negative pressure. Hospitals use negative pressure intentionally for infection isolation, but homes should aim for neutral to slightly positive pressure with good filtration.