Both model codes used by most U.S. jurisdictions create a clear trigger: a domestic kitchen exhaust hood that is capable of exhausting in excess of 400 CFM requires makeup air to be provided at a rate approximately equal to the exhaust air rate[1][2].
The model‑code text is commonly quoted as: “Exhaust hood systems capable of exhausting in excess of 400 cfm ... shall be provided with makeup air at a rate approximately equal to the exhaust air rate” (IMC §505.4)[1].
How to interpret the “capable of exhausting” threshold
Jurisdictions and AHJs differ in how they determine whether a hood is “capable” of >400 CFM. Some accept the hood's tested HVI or manufacturer certified rating at 0.1" static, others use a listed/max capacity or measured installed flow[3][2][11]. Manufacturers such as Broan recommend using the HVI/0.1" static tested rating as a defensible baseline to demonstrate compliance[3].
Because local practice varies, document which rating you relied on (HVI/0.1" rating, listed maximum, or measured flow) and verify expectations with the local building department before finalizing equipment and controls[3][2][11].
How much makeup air should you plan for?
The model codes require makeup air at a rate approximately equal to the exhaust air rate; in practice plan makeup‑air capacity roughly equal to the hood's rated CFM for the operating configuration you expect to use most (for example, a 600 CFM hood => ~600 CFM makeup air)[1][2][3].
The phrase “approximately equal” allows engineering judgment; many specifiers match the hood's tested 0.1" static rating or the hood speed that will be used most and document those assumptions for the AHJ[3][4].
Why makeup air matters (safety and performance)
Supplying makeup air prevents hazardous negative pressure that can cause backdrafting or spillage from combustion appliances and reduces uncontrolled infiltration that can harm indoor air quality—this safety intent underlies the makeup‑air requirement in the code[1][4][5]. If you have combustion appliances in the house, addressing makeup air is part of preventing spillage and combustion‑air problems; for more on appliance backdrafting and testing see our piece on Combustion Appliance Backdrafting & Spillage where testing and common causes are covered in detail.
Common practical solutions
Passive (barometric/gravity) dampers
Spring‑closed passive dampers open when house depressurization overcomes the spring. Pros: low cost and simple. Cons: uncontrolled infiltration when the hood runs, poor control of intake airflow direction and temperature, and they may not meet interlock/automatic‑operation requirements in some adopted codes or municipal interpretations[4][5].
Motorized automatic dampers with pressure‑sensor interlocks
Motorized dampers wired to a pressure sensor or electrical interlock meet many code interlock requirements, reduce unwanted infiltration when not running, and allow the intake to open only when the hood operates. These systems require wiring or pressure sensing and must be sized to match the required CFM. Manufacturer product offerings and pressure‑sensor kits are widely available[3][7].
Powered Makeup Air Units (MAUs)
Dedicated MAUs are packaged fans that deliver makeup air equal to the exhaust and can include tempering or heating. MAUs are the most robust choice for occupant comfort, for very high CFM hoods, or in cold climates—but they are more costly and require more ductwork, electrical work, and space[6][4][9].
Controls and interlocks
Many adopted codes and municipal interpretations require makeup‑air systems to be interlocked or automatically controlled so the makeup damper or fan opens/runs when the exhaust hood runs; automatic operation and a means of closure are common code expectations[1][11]. Choose a control approach that the AHJ will accept (documented pressure sensing, electrical interlock, or manufacturer kit) and retain that documentation for inspections[3][7].
Where to terminate the intake
Intake location and termination must follow intake‑opening location rules to avoid re‑entrainment of exhaust, contamination, or interference with other openings—follow the IMC intake opening location requirements and your local code specifics when planning and installing the intake termination[10][11].
Typical cost guidance and budgeting
Retail and installed cost guidance aggregated from manufacturer listings and buyer guides show wide ranges: passive damper hardware retailing roughly $100–$400 (installed totals vary); motorized damper plus pressure sensor retail commonly about $200–$600 with installed totals often roughly $500–$1,400; and powered/tempered MAU turnkey installed projects commonly reported at roughly $1,500–$4,000+ depending on complexity[7][8][9].
Costs vary by local labor, roof/wall penetration difficulty, duct runs, required heating/tempering, permitting, and integration with existing HVAC—get local contractor quotes for final budgeting[8][9].
How to proceed as a homeowner or installer (practical checklist)
Identify the hood rating you will use to determine the trigger: HVI/0.1" tested rating, listed/max rating, or plan for measured installed flow—document that choice for the AHJ[3][2][11].
Plan makeup‑air capacity approximately equal to the hood CFM at the operating configuration you expect to use most (document the operating speed/assumption)[1][3].
Decide on solution type (passive damper, motorized damper with interlock, or powered MAU) based on CFM, climate, comfort expectations, and budget[4][6][9].
Confirm required controls with the local building department—many jurisdictions require an automatic interlock or documented means of closure when not in use[1][11].
Size ducts, pickups, and intake termination per IMC intake opening rules and manufacturer guidance; avoid routing that could re‑entrain exhaust or pick up contaminants[10][6].
Get at least two competitive contractor bids that include permitting, penetrations, tempering (if needed), and controls—costs vary widely by local conditions[8][9].
Final notes
Model code language gives a clear threshold and safety rationale, but implementation is partly dependent on local interpretation and the rating method you choose—document your assumptions, discuss the plan with the AHJ early, and budget for installation variables like penetrations, duct runs, and any required tempering or electrical work[1][3][9].
Abodivo Tool
Do you need makeup air for your range hood?
Quick check
A short homeowner/installer checklist for determining makeup-air requirements and next steps.
If none of the checklist factors apply (for example the hood is not capable of exhausting in excess of 400 CFM), the model codes do not require makeup air.
Confirm plans and documented assumptions with your local building department or a licensed professional before installation.