Makeup Air

Makeup air (also called replacement air or compensation air) is outdoor air that flows into a building to replace the air removed by exhaust ventilation. In buildings with mechanical exhaust only, makeup air enters passively through dedicated wall vents, window trickle vents, or gaps in the building envelope. In balanced systems, makeup air is actively supplied by the air handling unit.

Adequate makeup air is essential for maintaining proper pressure balance in the building. When exhaust ventilation removes more air than enters the building, negative pressure develops. Slight negative pressure (a few pascals) is normal and actually helps prevent moisture from being pushed into wall structures. However, excessive negative pressure causes problems: doors become hard to open, cold drafts appear near windows, and — most critically — contaminated air can be drawn from the soil, crawl space, or wall cavities into living spaces.

Common situations that create makeup air deficiency include running a powerful kitchen range hood without a window open, using a fireplace without a dedicated combustion air supply, or sealing a drafty building without upgrading the ventilation system. Each of these scenarios can dramatically increase negative pressure beyond safe levels.

If you experience draft problems, difficulty opening exterior doors, or persistent odors that seem to come from the building structure, your makeup air supply may be insufficient. A ventilation technician can measure the building's pressure balance and recommend solutions such as installing makeup air valves, adjusting exhaust fan speeds, or adding a supply air fan.