How a return-air plenum works

Many commercial buildings use the space above the drop ceiling as the return-air path. Air leaves the room through return grilles in the ceiling, travels through the open cavity, and is drawn back to the air handler. It is efficient and cheap to build, and it means that cavity is full of moving air.

Problems start when that open cavity is shared. In older strip centers and some office buildings, demising walls between tenants were built only to the ceiling height, or were extended to the roof deck but later cut open for pipes, conduit and ducts. A restaurant, salon, nail studio or fitness room on one side can then push odor into the cavity, where the neighbor's return grilles pick it up.

Pressure decides direction. If one suite exhausts a lot of air, such as a kitchen hood or a restroom fan bank, without bringing in matching makeup air, it runs at negative pressure and pulls air from its neighbors. If a suite is positively pressurized, it pushes air outward. A suite with weak supply and strong exhaust will often receive odor from both sides.

Two types of fix

Both usually involve the landlord, since walls above the ceiling and shared systems are normally building elements.

Close the path

  • Extend or complete the demising wall to the roof deck
  • Seal gaps around pipes, conduit and ducts with appropriate firestop or sealant
  • Add a dedicated return duct so the suite no longer draws from the open cavity
  • Seal shared chases and abandoned openings left by earlier buildouts

Change the pressure

  • Balance the neighbor's exhaust with makeup air so it stops pulling from adjacent suites
  • Adjust the affected suite's outside air so it runs slightly positive
  • Check that exhaust fans discharge outdoors, not into the ceiling cavity
  • Rebalance after any tenant improvement that changes airflow

Evidence that helps a landlord act

  • Dates and times the odor appeared, and what it smelled like
  • Whether it matches the neighbor's operating hours or activities
  • Photos above the ceiling showing gaps, open walls or penetrations
  • Which return grilles or areas of the suite smell strongest
  • Any report from an HVAC contractor about pressure or airflow
  • Copies of written notices sent to the property manager

Fire separation is a separate question

Some walls between tenants are also rated fire separations. Openings in a rated wall are a building safety issue, not just an odor issue, and any repair has to meet the requirements of that assembly. That is another reason the work belongs with the landlord's contractors and may involve a permit from the city's building division. A tenant who finds large openings should report them in writing rather than sealing them with foam from a hardware store.

Once the path is closed, residual odor in ceiling tiles, insulation or duct liner near the wall can be evaluated. Replacing a handful of stained or saturated tiles is often more effective than treating them.

Common questions

Can the landlord make the neighbor stop cooking or smoking?
That depends on the leases involved. More often the practical path is closing the air pathway and fixing pressure, which the landlord can usually do without changing what the neighbor is allowed to do.

General information for Westminster, not a quote or professional advice. In an emergency, call 911.