Can a range hood vent through a soffit vent?
Short answer. A kitchen range hood should not discharge into a soffit vent or attic; the residential code requires kitchen exhaust to go outdoors, and grease-laden air pulled back into a vented soffit can coat the eaves and attic and create odor and fire concerns. The preferred route is a wall cap or roof cap sized to the hood's duct with a damper, and the code generally requires makeup air for hoods that exhaust more than 400 cubic feet per minute.
Why kitchen exhaust is different
Cooking exhaust carries grease, smoke and steam. A bath fan's moisture is bad enough in an attic, but grease condensing on the soffit, fascia and roof deck is worse. It builds up, collects dust and can feed a fire. Soffit outlets also tend to stain and drip.
Code basics
- Domestic kitchen exhaust must discharge outdoors, not into an attic, soffit or crawl space.
- Range hood ducts are generally required to be smooth-wall metal such as galvanized steel, stainless steel or copper, airtight, with a backdraft damper.
- Hood systems able to exhaust more than 400 cubic feet per minute generally need makeup air.
Best routes
- Wall cap: ideal when the range sits on an exterior wall. Short and easy to service.
- Roof cap: for islands and interior walls. Use a cap designed for kitchen exhaust, sized to the duct (often 6 to 8 inches, more for high output hoods) and flashed into the roof like any penetration.
Have an existing soffit outlet?
Look at the soffit near the kitchen for grease stains, drips or a darkened vent, and check the deck near that eave from the attic. Rerouting to a proper cap is usually worth it, and a roof replacement is the easiest time to add one. See roof flashing explained and roof ventilation basics.