Air
Makeup Air
Also known as replacement air
Outdoor air supplied deliberately to replace air removed by an exhaust device. Needed when a fan removes more air than the enclosure will passively admit, which would otherwise leave the house at negative pressure.
In more detail
The arithmetic is simple and frequently ignored. A professional-style range hood may move 600 to 1,500 cfm. A tight house will not pass that much air inward through its remaining leaks without going substantially negative relative to outdoors, and the air it does pass comes from wherever the pressure field finds a path.
Depressurization has consequences in a specific order of seriousness. It can reverse the draft on a naturally vented combustion appliance, pulling flue gases into the house instead of out of it. It can draw soil gas, including radon, through the foundation. It can pull air from an attached garage, an attic, or a damp crawlspace. And it makes the exhaust fan itself ineffective, since a fan that cannot get replacement air does not move its rated volume.
Makeup air answers this by providing a deliberate inlet. The simple version is a motorized damper interlocked with the hood, opening when it runs. In cold climates that introduces a slug of outdoor air where nobody wants it, so tempered makeup air — ducted, sometimes heated, delivered near the source — is the better-engineered option and the more expensive one.
Code thresholds requiring makeup air above a given exhaust rate exist in most jurisdictions, but the trigger and the required capacity vary by code edition and local amendment, so the applicable requirement has to be confirmed rather than assumed.
Why it matters
Two trends collided: enclosures got tighter and kitchen ventilation got larger. A house built to current airtightness expectations with a 1,200 cfm hood is a pressure problem by design, and the time to solve it is when the hood is selected, because makeup air needs a duct route, an exterior penetration, an interlock and possibly a heater. Discovered after the fact, the usual remedy is to run the hood on a low setting forever, which is a strange outcome for equipment chosen for its capacity.
The common misunderstanding
That it is a paperwork item to satisfy an inspector. Makeup air is the mechanism that keeps a large exhaust fan from reversing a flue or pulling soil gas through the slab. The permit requirement exists because the physics does, and a hood installed without it in a tight house does not simply underperform — it changes the pressure regime the combustion appliances were installed under.
Related terms
On Compound
Where this term does real work — and where the decisions get made.
Sources
Last reviewed July 2026 by Compound Editorial. Definitions are educational and not project-specific engineering or medical advice.