Can an air admittance valve go inside a wall? Only if you can reach it and it can breathe

The question came up this week on Reddit's r/Plumbing. A homeowner gutting the bathrooms of an older house says a relative, a former contractor, is setting air admittance valves inside the walls and laughs at the idea of vented access panels. The model codes side with the panels.

Under the International Plumbing Code and the International Residential Code, an AAV must be accessible in case it needs replacing and must sit where air can get to it, according to an International Code Council CodeNotes guide based on the 2018 editions. A sink or vanity cabinet counts as accessible. For an in-wall valve, the guide calls for a recess box with a grille or an access grille. Individual and branch valves go at least 4 inches above the horizontal branch or fixture drain they vent, and at least 6 inches above insulation. Every drainage system still needs at least one vent pipe running outdoors, because the valve cannot relieve positive pressure. Where the code and the maker's instructions differ, the stricter rule applies.

Oatey's installation guidance adds that a valve must be within 15 degrees of vertical, sized to the drainage fixture units it serves and never permanently covered. Under the Uniform Plumbing Code, the answer depends on the jurisdiction. In Washington's adoption of the 2021 UPC, the AAV section is optional and applies only where a local ordinance adopts it; it repeats the access and airflow rules and sets the valve at least 4 inches above the trap arm.

The valve has one moving part, a seal, and the ICC guide warns that a drain blockage pushing waste up into the assembly can stop it from working. A failed valve behind drywall means cutting the wall to swap it. The relative's other idea fails for the same reason: the National Electrical Code requires boxes to be installed so the wiring inside can be reached without removing any part of the building.