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Makeup air is the controlled replacement air a tight home needs when exhaust fans remove indoor air. A large range hood, clothes dryer, bath fans, fireplace, or exhaust-only ventilation system can depressurize the house. That pressure difference may pull air through garages, crawlspaces, attics, wall cavities, or neighboring units and can reverse the draft of a natural-draft furnace, water heater, or fireplace. The safe solution is not simply opening a window or installing a damper: measure the house, check the locally adopted code, select an approach that preserves cooking capture, and commission the finished system under worst-case conditions.
What makeup air does in a tight house
Every exhaust device moves air out. In a leaky building, replacement air may enter through many small cracks without an obvious pressure change. In a tight building, those leakage paths are limited, so the same exhaust flow creates a larger pressure difference between indoors and outdoors. The house then seeks air wherever it can find it.
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That replacement air can arrive through unintended routes: a garage door or attached-garage leakage path, a basement or crawlspace, an attic, plumbing and wiring penetrations, or another dwelling in a multifamily building. The air may carry odors, moisture, pollutants, or soil gases. More seriously, negative indoor pressure can interfere with an appliance that relies on natural draft to move combustion products up a flue.
The Building America Solution Center describes large range hoods, clothes dryers, and fireplaces as contributors to depressurization when makeup air is absent. Exhaust-only ventilation and an improperly balanced heat-recovery or energy-recovery ventilator can add to the same effect. A house can therefore have a pressure problem even when its main kitchen hood is modest, particularly when several exhaust devices run together.
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Makeup air is the deliberate path for outdoor air to enter while exhaust operates. A useful design must do four things at once:
- Keep pressure within a range that does not upset combustion venting.
- Provide enough air for the actual installed exhaust flow, not just the fan’s nameplate rating.
- Preserve the hood’s ability to capture smoke, grease, and heat at the cooking surface.
- Control temperature, humidity, filtration, noise, and comfort for the climate and building.
These requirements explain why a motorized damper by itself is not a complete system. A damper needs a correctly sized outdoor-air path, controls, discharge location, and commissioning.
When the 2024 IRC model provision applies
The 2024 International Residential Code (IRC) model text in Section M1503.6 uses a conditional trigger. When one or more gas-, liquid-, or solid-fuel-burning appliances that are neither direct-vent nor served by a mechanical-draft venting system are located within the dwelling unit’s air barrier, each exhaust system capable of exhausting more than 400 cubic feet per minute (0.19 m³/s) must be provided with makeup air at a rate approximately equal to the exhaust rate. The provision also calls for at least one outdoor-air duct and damper meeting the applicable section.
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This is model-code language, not a nationwide rule. Your state, county, or municipality may have adopted a different IRC edition, amended the threshold, or used another residential or mechanical code. The authority having jurisdiction (AHJ)—usually the local building department—controls permit interpretation. Ask which edition and amendments apply before choosing equipment or drawing a duct route. Historical guidance, such as the 2012 IRC discussion in HUD’s Durability by Design, explains earlier practice but does not establish today’s local requirement.
The 400-cfm threshold should not be treated as a safety guarantee below that number. A 250- or 350-cfm hood can still depressurize a very tight house, especially when a dryer, bath fan, fireplace, or another exhaust runs at the same time. The U.S. Environmental Protection Agency (EPA) advises considering makeup air for larger fans and recommends worst-case testing when relevant non-sealed-combustion appliances are present.
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EPA materials also mention 100 cfm as a typical kitchen range recommendation attributed on that page to ASHRAE and the Home Ventilating Institute. The passage does not identify the edition or year of those recommendations, so 100 cfm is a reference point, not a universal hood-sizing or makeup-air rule. Hood selection should begin with effective capture at the appliance and the cooking style, then address the resulting airflow and pressure.
How depressurization can become a combustion hazard
Natural-draft appliances depend on buoyancy and available combustion air to move flue gases outdoors. If exhaust fans make the house sufficiently negative, the pressure difference can weaken, stall, or reverse that draft. EPA describes the resulting condition as spillage or backdrafting: combustion products, potentially including carbon monoxide, enter the living space instead of leaving through the vent.
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A free scan shows the junk files, broken settings and background clutter dragging Windows down - then fixes them in one click.Free scan · Windows 10 & 11Direct-vent appliances draw combustion air from outdoors and exhaust combustion products outdoors through a sealed system. Induced-draft and power-vented equipment use a fan to move flue gases. EPA recommends considering sealed-combustion, induced-draft, or power-vented equipment when replacing heating equipment because these designs reduce dependence on indoor pressure for combustion. They do not eliminate the need to design kitchen exhaust and whole-house ventilation correctly.
After installing or altering combustion or ventilation equipment, EPA says the installer should test the combustion equipment. For a home with non-sealed-combustion appliances, EPA specifically advises a worst-case depressurization test using an established procedure such as Appendix D of the International Fuel Gas Code or ASTM E1998. EPA’s wording is direct: “It is important that your installer conducts a worst-case depressurization test.” A carbon-monoxide alarm is an important backup, but it is not a substitute for correcting a pressure or venting defect.
Assess the house before selecting a makeup-air method
A reliable design starts with an inventory and measurements rather than a product catalog. Use this sequence with a qualified HVAC contractor, home-performance professional, energy rater, or other technician trained in airflow and combustion-safety testing.
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- List every exhaust source. Record the rated and, where possible, measured airflow for the kitchen hood, bath fans, central vacuum exhaust, clothes dryer, fireplace-related exhaust, and whole-house exhaust ventilation. Note which devices may operate simultaneously. A hood’s high setting and a dryer running together can create a different condition from either device alone.
- Identify combustion appliances. Document fuel type, location, venting method, and whether each appliance is direct-vent, mechanical-draft, or natural-draft. Pay particular attention to appliances located inside the dwelling’s air barrier, because that is the condition used in the 2024 IRC trigger.
- Characterize the enclosure. Note attached garages, basements, crawlspaces, attics, neighboring units, and obvious leakage paths. Tightness testing can quantify leakage, but an assumed leakage rate is not proof that a combustion appliance will remain safe under exhaust.
- Measure operating pressure and airflow. The technician should measure pressure with exhaust devices off and in defined worst-case combinations, then verify actual fan and supply airflow. Installed airflow can differ substantially from a catalog rating because of duct length, elbows, filters, grilles, dampers, and terminations.
- Test combustion safety. With the specified exhaust combination operating, check draft, spillage, and carbon-monoxide conditions using an established procedure. Record the result and the operating setup so it can be repeated after adjustments.
- Ask the AHJ about code. Confirm the adopted code edition, local amendments, permit requirements, and any interpretation of makeup-air provisions before construction.
An open window during a casual check does not establish a safe design. It changes the test condition, may not be available to occupants, and does not show that the installed controls will respond whenever exhaust operates.
Compare the main design approaches
No single approach works for every house. The correct choice depends on measured airflow and pressure, the combustion equipment, the hood’s capture geometry, climate, humidity, duct route, controls, and the owner’s tolerance for complexity.
| Approach | Where it fits | Design questions |
|---|---|---|
| Passive outdoor-air inlet | Exhaust-only arrangements where a planned inlet can provide replacement air. | Will pressure actually drive the required flow? Is the inlet protected from rain, snow, pests, and wind? Will cold drafts or unfiltered air create comfort or indoor-air-quality problems? |
| Dedicated powered, interlocked makeup air | Homes with substantial exhaust or a code-triggered system that needs a controlled outdoor-air path. | What airflow is delivered at operating pressure? Does the fan start whenever the hood or other exhaust runs? Is incoming air tempered and filtered? Where does it discharge, and how is short-circuiting into the hood avoided? |
| Balanced ventilation with HRV or ERV | Whole-house ventilation, including multifamily applications where pressure balance and energy recovery are priorities. | An HRV transfers heat; an ERV transfers heat and moisture. Is the unit sized for routine ventilation rather than the hood’s high flow? Does the climate make moisture transfer desirable? |
| Improved hood capture with lower exhaust demand | Projects where hood geometry can capture contaminants with less total airflow. | Will an overhang, side panels, or enclosure improve capture for this appliance and cooking pattern? This is a design observation, not a guaranteed performance formula. |
PNNL notes that HRV/ERV heat exchangers commonly have a general sensible-effectiveness range of 55% to 95%, varying by system. That range is not a promise for a particular product, and heat recovery does not mean a routine ventilation unit can automatically replace a high-volume hood makeup-air system.
Passive inlets: simple hardware, conditional performance
A passive inlet is an outdoor-air duct and opening that relies on pressure differences to move air. It can be useful in an exhaust-only strategy, but its flow changes with wind, temperature, duct resistance, grille size, and house pressure. A small inlet may provide far less air than expected; an oversized or poorly located one can create cold drafts and still fail to protect a natural-draft appliance under a particular operating combination.
Locate the inlet so that incoming air does not blow directly across the cooktop and disrupt the hood’s capture plume. Consider filtration, condensation, freeze protection, pest screening, and a damper that closes when the system is off if the design requires it. Measure delivered airflow with the exhaust running rather than assuming the nominal duct diameter proves balance.
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Powered and interlocked makeup air
A powered system uses a supply fan or air handler to deliver outdoor air when an exhaust device operates. An interlock can start the supply fan with the hood, open a motorized damper, and apply a time delay or proving switch where the design calls for one. The 2024 IRC provision’s duct-and-damper language supports this type of component under its stated trigger, but it does not specify a universal fan size, discharge location, or control sequence.
Tempering is often necessary in cold or hot climates. Introducing unconditioned outdoor air at high volume can cause discomfort, condensation, or excessive heating and cooling loads. A designer may route air through a heating or cooling coil, mix it with return air where permitted, or discharge it in a location that allows mixing before it reaches occupants. Filtration protects the duct and indoor air from outdoor particles, but filters add resistance that must be included in the airflow calculation.
Do not place a supply outlet where it blows directly into the hood canopy or across the cooking surface. That shortcut can create turbulence and carry contaminants past the capture zone. Verify capture with the makeup-air system operating, not only with the hood on its own.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Balanced ventilation, HRVs, and ERVs
Balanced ventilation supplies and exhausts comparable airflows through a dedicated system. In multifamily work, PNNL identifies balanced approaches as a way to address pressure issues while allowing heat or energy recovery. An HRV transfers sensible heat between airstreams. An ERV transfers sensible heat and some moisture, which can be useful where humidity control is a concern.
Balanced ventilation is normally designed for continuous or scheduled whole-house ventilation. A range hood may operate at several times that flow for short periods. Unless calculations and controls specifically provide the hood’s demand, the HRV or ERV should not be assumed to serve as its makeup-air source. The system also needs commissioning so supply and exhaust flows match their design values after filters and grilles are installed.
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What practitioner examples show—and what they do not prove
Building Science Corporation’s BSI-070 describes a project approach for a 1,000-cfm hood. It introduces direct makeup air at floor level at approximately 60% to 70% of hood exhaust, then supplies the remainder plus a small additional amount as preconditioned air around the kitchen perimeter. The systems are interlocked. In that example, limiting the direct stream helps preserve a negative capture zone at the cooking surface.
The same article describes a 200-cfm hood with an interlocked supply fan delivering approximately 10% more air than the hood, supplied to an adjacent zone so transfer air reaches the cooking area. These figures illustrate how a designer may separate a low-level direct stream from tempered transfer air; they are not code prescriptions, universal percentages, or substitutes for site-specific calculations.
The examples reinforce an important principle: pressure balance and contaminant capture can conflict. Supplying exactly the hood’s measured exhaust at the canopy may destroy the airflow pattern that captures smoke. A successful design distributes and conditions air so the house remains acceptably balanced while the hood still captures at the appliance.
Commissioning: verify the installed system
Commissioning turns a design assumption into a measured result. PNNL recommends assessment by a trained home-energy rater or HVAC contractor and says new ventilation systems should be commissioned by a trained technician or rater.
- Measure each exhaust device at its actual grille, hood, or termination, with filters and dampers installed.
- Measure makeup-air flow at the supply outlet or duct under the same operating condition.
- Record indoor-outdoor pressure with all relevant exhaust combinations, including the specified worst case.
- Verify that interlocks start the supply system reliably and that dampers open fully without excessive delay.
- Repeat combustion-draft, spillage, and carbon-monoxide checks with natural-draft appliances operating.
- Check that supply air does not impair hood capture, create drafts, or cause condensation.
- Give the owner written operating instructions, filter locations, maintenance intervals, and the conditions under which the system must not be altered.
If a test fails, do not mask the result by propping open a door or window. Rebalance the system, reduce exhaust demand through better capture where appropriate, improve the appliance venting strategy, add or resize controlled makeup air, or replace a natural-draft appliance with a sealed or mechanically vented model. Then repeat the full test.
A practical decision path for homeowners and builders
- Start with the operating scenario. Decide which fans and appliances can run together in normal use and in a credible worst case.
- Check the code trigger and local rules. Determine whether a qualifying natural-draft appliance is inside the air barrier and whether the locally adopted code has a makeup-air requirement.
- Prioritize capture. Select hood geometry, overhang, side panels, and duct design that capture contaminants efficiently before adding airflow solely to compensate for poor capture.
- Select the air path. Compare passive, powered/interlocked, and balanced options using measured airflow, pressure, temperature, humidity, filtration, duct routing, and controls.
- Design for service. Provide access to filters, dampers, fans, sensors, and balancing points. A hidden damper that cannot be inspected is difficult to maintain.
- Commission and document. Test airflow, pressure, hood capture, and combustion safety after installation, then retain the readings and control sequence for future service.
For a project involving a large hood, a tight enclosure, or any natural-draft combustion appliance, the useful next step is to schedule an evaluation with a qualified HVAC or home-performance professional and ask specifically for measured airflow, worst-case depressurization testing, and commissioning. Verify the provider’s credentials and service area, and confirm the AHJ’s requirements before work begins.
Quick Recap
Common mistakes to avoid
- Treating 400 cfm as a universal safe limit. It is a conditional 2024 IRC model-code trigger, not proof that a lower-flow system cannot cause backdrafting.
- Buying a damper without a system design. A damper does not size the duct, temper air, choose a discharge location, or prove interlock operation.
- Using an open window as the permanent solution. Occupants may not open it, weather may prevent it, and the opening does not provide a repeatable commissioned flow.
- Assuming the fan rating equals delivered airflow. Duct resistance, filters, grilles, and terminations change the installed result.
- Letting supply air blow into the hood. Poor outlet placement can reduce capture even if pressure readings look acceptable.
- Counting an HRV or ERV as hood makeup air automatically. Whole-house ventilation and high-volume intermittent exhaust are different design loads.
- Skipping combustion testing after a remodel. New windows, air sealing, fans, and appliance changes can alter pressure relationships.
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