What the Code Says and Why It Matters
Estimated reading time: 4 minutes
Not long ago, a kitchen remodel turned into an unexpected code conversation. Picture this: a shiny new range hood proudly installed above a natural gas stove—but instead of a rigid metal pipe venting to the outside, a crinkled flex duct snakes upward and disappears into the attic like a rogue tunnel.
For seasoned inspectors, it’s a red flag. For a new homeowner? It’s just a fan that “seems to work fine.” So what’s the problem?
Let’s clear the air—literally.
Image Credit: Premier Inspection and Consulting
The International Residential Code (IRC) states that kitchen exhaust must vent to the exterior. Not to the attic. Not to a soffit. Not into wall cavities. And not using flexible foil duct. Specifically, IRC M1503.3 requires cooking exhaust equipment to terminate outside the building. IRC M1503.4 adds that ducts must be made of galvanized steel, stainless steel, or copper, with a smooth interior surface. A backdraft damper must also be present to keep outside air (and critters) from sneaking back in.
These stringent rules apply to ducted exhaust systems and are distinct from the requirements for listed and labeled “ductless” or “recirculating” hoods, which filter air instead of venting it outside. The code also provides other allowances, like using a functional kitchen window, but the nuances of natural ventilation and other mechanical ventilation methods are a topic for another article.
Picky Picky Picky
Why is the code so picky? The consequences are significant when kitchen air is vented improperly into an attic. Over time, grease particles coat the insulation and framing, creating a fire hazard. Moisture builds up, potentially leading to mold and rot that can compromise the roof structure. With a gas range below and foam insulation above, as in our example image by Premier Inspection, we’re looking at a real-world setup where carbon monoxide buildup and degraded air quality could become serious concerns. The sealed nature of a modern attic keeps that contaminated air—and its risks—trapped inside the home’s envelope.
Today’s powerful appliances amplify this problem. Range hoods are rated by how much air they move, measured in cubic feet per minute (CFM). High-CFM fans (600 CFM or more) can accidentally turn your kitchen into a depressurization zone, especially in tightly sealed, energy-efficient homes. Without properly balanced ventilation, these powerful fans can trigger backdrafting, where combustion gases from other appliances like a water heater or furnace are pulled back into the living space instead of safely venting outside.
It’s common to find vent kits that include flexible ductwork. The box may even show a “loop up and out” illustration. But just because it came from the factory doesn’t mean it is code-compliant for your installation. Manufacturers often list multiple installation options. Always check with your local adoption office, the Authority Having Jurisdiction (AHJ), as it is the ultimate building code authority. When in doubt, follow the most restrictive path—your attic and lungs will thank you.
During a home inspection or consultation, here is how to spot and fix the issue.
Look For:
- A flexible or foil duct connected to the range hood.
- No visible vent termination cap on the roof or an exterior wall.
- A greasy, musty, or “kitchen-y” smell in the attic.
- A poor seal or excessive duct length and bends.
The Fix: To correct the issue, the flex duct must be replaced with a smooth, rigid metal duct. It must be routed on the most direct path to an exterior wall or roof cap, with all joints sealed. It’s straightforward work—but only when you know what to look for.
If you’re buying a home or renovating a kitchen, this detail matters. You may not see it on the surface, but it shows up later—in roof deck rot, poor air quality, or costly upgrades. Inspections exist to find the things behind the things—and this is one of them.
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