Can a kitchen exhaust be vented through the soffit?
Why kitchen exhaust is different
A bath fan moves damp air. A range hood moves damp air plus grease, smoke and heat, often at much higher flow. Grease condenses on the first cool surface it meets, which at a soffit means the panels, the intake vents beside them, and eventually the inside of the attic if the intake draws it in. Grease buildup is a fire concern, it attracts pests, and a grease-stained soffit is hard to clean.
What the code expects of a range hood duct
- Discharge to the outdoors, never into an attic, crawl space or other space inside the building.
- Single-wall duct of galvanized steel, stainless steel or copper, smooth inside, with airtight joints.
- A backdraft damper.
- Makeup air for hoods able to exhaust more than 400 CFM, so a big hood does not pull air down a chimney or starve gas appliances.
The plastic flex duct sometimes used for bath fans does not belong on a range hood.
Better terminations
A roof cap made for kitchen exhaust, with a larger opening, a damper and no fine screen to clog with grease, flashed into the shingles. Or a wall cap when the range sits on an outside wall. The duct size comes from the hood maker's installation sheet and should not shrink anywhere along the run. Recirculating hoods avoid the question but only filter grease and odor; they do not remove moisture or heat.
If you are adding a hood during a remodel and the roof is due soon, plan the cap with the roof work so it is flashed in with the new shingles. See roof flashing explained and roof ventilation basics.