Ventilation is one of the key elements in maintaining healthy indoor air quality and overall comfort in a building. Adequate ventilation can remove moisture, odors, and other pollutants in a space, leading to a cleaner, healthier environment for the occupants. It is a common topic in local building codes or energy-efficiency requirements.
What is makeup air
Makeup air is fresh outdoor air brought into the building to replace the air that has been exhausted by equipment or appliances such as a cook range hood, a bathroom fan, or industrial vents. It balances air inside the house, prevents drafts, and often stops contaminated exhaust air from being pulled back into our occupied space.
Why does makeup air matter
When the building runs a powerful exhaust fan, it pushes a large volume of air out of the space. Such a mechanism creates negative pressure, pulling unconditioned air through cracks and holes around the exterior envelope. Then, when a building is depressurized, these unwanted gases could be withdrawn back into our occupied space and cause a health hazard.
It matters ...
If you use a cook range
In a residential setting, a massive loss of indoor air happens when we use a range hood. Any standard residential range hood that is 30 to 48 inches wide can easily exceed 400 cubic feet per minute (CFM) of air. These high-powered exhaust hoods are typically wall-mounted, undercabinet, or island models and are specifically designed to vent heavy smoke, steam, and grease from large cooktops. When suddenly losing a large quantity of air, the house is depressurized, resulting in the exhaust fan struggling to pull more smoke and grease out.
If you are a code-follower
Due to this indoor-outdoor-air imbalance phenomenon, building codes require us to install a makeup air system alongside them. The system replaces the exhausted air with fresh outside air to prevent depressurization and ensure the air pressure is balanced on both sides of exterior walls.
If your project wants to get LEED certified
Under the Indoor Environmental Quality Category (IEQ) in recent LEED Residential: Single Family, a project can fulfill most of the prerequisites if it comprehensively addresses issues related to the cook range. This could mean designing a dehumidification mechanism when drawing makeup air to reduce moisture problems, using airtight ducts to limit leakage of combustion gas, installing a proper filter (MERV 8 and higher) to reduce occupants' exposure to indoor pollution, and applying a suitable barrier to minimize air transfer between units.
By the way...
Makup air requirement has stated in 2009 IRC 1503.4, "Exhaust hood systems capable of exhausting in excess of 400 cubic feet per minut shall be provided with makeup air at a rate approximately equal to the exhaust air rate. " similar language repeated again in 2024 IRC 1503.6, "where one or more gas, liquid or solid fuel-burning appliance that is neither direct-vent nor uses a mechanical venting system is located within a dwelling units' air barrier, each exhaust system capable of exhausting in excess 400 Cubic feet per minute shall be mechanically or passively provided with makeup air at a rate approximately equal to the exhause air rate."
Makeup air is not required for exhaust systems operated only when windows are open. So whether you live in a detached single-family home or an attached family home, open your kitchen window when cooking could help save some operational energy.
To understand the importance of a balanced indoor/outdoor pressure, check out this video provided by This Old House! (starting 1:24)