Makeup Air Explained: Why Tight Homes Need a Balanced Air System
Modern building codes nationally, but especially in San Diego,California, are pushing homes to be far more airtight than ever before. This is a good thing—it saves energy, improves comfort, and helps create a more controlled indoor environment. But building a tight home also introduces a new challenge that many homeowners (and even some builders) overlook: negative air pressure.
If negative pressure isn’t handled correctly, it can lead to comfort issues, air-quality problems, and even safety concerns. That’s where makeup air comes in.
Why Airtight Homes Create Negative Pressure
In a high-performance home, the building envelope is carefully sealed. Systems like ZIP sheathing, sealed wall plates, continuous air barriers at the ceiling, and well-detailed penetrations dramatically reduce air leakage. When done correctly, the house doesn’t “breathe” on its own anymore.
The problem arises when appliances exhaust air out of the house.
Common examples include:
- Range hoods or over-the-range microwaves
- Clothes dryers
- Bathroom exhaust fans
- Combustion appliances
Anytime air is pushed or pulled out of the home, the same amount of air has to come back in. If there’s no intentional path for that air, the house goes into negative pressure.
Building codes typically require makeup air when a kitchen exhaust hood exceeds 400 CFM, but even smaller systems can cause issues in very tight homes.
What Is Makeup Air?
Makeup air is simply a controlled way to let outdoor air enter the house to replace the air being exhausted. The goal is to balance pressure, not to let air leak in randomly through windows, doors, or wall assemblies.
In this home, a makeup air vent was installed even though it wasn’t strictly required by code. Because the house is built extremely tight, adding makeup air helps prevent pressure imbalances and improves overall comfort.
How a Barometric Damper Works
The system used here includes a barometric damper, which is a passive device that only opens when it’s needed.
Here’s how it functions:
- A rigid duct (in this case, 4 inches) brings air from the exterior into the house
- The duct is wrapped with R-8 insulation to prevent condensation when cold outdoor air meets warm indoor air
- Inside the duct is a barometric damper with a butterfly-style flap
When the home experiences negative pressure, the damper automatically opens just enough to allow the required amount of air inside. When pressure equalizes, it closes back up.
The damper doesn’t swing fully open—it modulates based on demand. This allows only as much air as necessary to enter the home.
Sizing the Makeup Air Duct
Makeup air systems assume that outdoor air is clean, but that isn’t always the case.
Situations where makeup air can be a concern include:
- Agricultural areas where pesticides are sprayed
- Wildfire-prone regions
- Poor outdoor air quality days
Because barometric dampers open automatically, homeowners need to be aware of when exhaust systems are running. In some cases, having a way to disable or manage makeup air may be important for health and comfort.
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Outdoor Air Quality Matters
Makeup air systems assume that outdoor air is clean, but that isn’t always the case.
Situations where makeup air can be a concern include:
- Agricultural areas where pesticides are sprayed
- Wildfire-prone regions
- Poor outdoor air quality days
Because barometric dampers open automatically, homeowners need to be aware of when exhaust systems are running. In some cases, having a way to disable or manage makeup air may be
Exterior Vent Covers: Intake vs. Exhaust
The exterior vent cover used for makeup air is different from an exhaust vent.
Makeup air intake covers:
- Include louvers and insect mesh
- Prevent pests like wasps and bees from entering
- Allow easy maintenance by removing the screen for cleaning
Exhaust vents (such as for dryers, bathrooms, or crawl space fans):
- Use flappers that stay closed until positive pressure pushes air out
- Prevent backdrafting when systems are off
Understanding the difference ensures air moves in the correct direction and only when intended.
The Goal: A Balanced, Comfortable Home
A well-designed makeup air system keeps your home:
- Pressure balanced
- More comfortable
- Healthier
- More energy efficient
The key is proper design and sizing. Always work with an experienced HVAC professional who can run the calculations and design the system correctly for your specific home.
Need Help?
If you need help building a custom home, ADU, or completing a whole-house remodel, visit freemansconstruction.com.
For assistance with buying or selling real estate, visit freemansrealty.com. You can also explore our comprehensive collection of similar videos on our YouTube channel, The Real Property Show.
Thanks for reading—and for caring about building homes the right way.
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