Negative pressure occurs when the air pressure inside a home or a duct is lower than the outside atmospheric pressure, often caused by a leaking Return Plenum. This condition can suck unfiltered air, dust, and odors through the cracks in your walls and windows, severely degrading your Indoor Air Quality (IAQ). In Mobile Home applications, negative pressure can pull moisture from the crawlspace into your living area through the Belly Board. A properly balanced ACIq system should maintain a slightly "positive" pressure to keep pollutants out. Technicians use a Manometer to identify and fix negative pressure issues during a home energy audit. Sealing your ductwork with high-quality mastic and tape is the best way to prevent this common efficiency killer. Understanding air pressure is key to creating a healthy, clean-smelling, and comfortable home. It is a fundamental concept in the science of building envelopes.