If a room sits over an attached garage, it has an unconditioned space below rather than another heated room. Its supply duct may be long or poorly balanced, although the actual airflow depends on the design. Gaps at penetrations can also let garage air reach adjoining cavities. Those conditions can contribute to temperature swings or smells of exhaust, stored products, dust, or damp materials; they are possibilities to inspect, not assumptions about any Lowry house.

Because the rest of the house smells fine, owners often assume something in the room is at fault and start washing carpet or replacing a mattress. Sometimes that is right, but more often the smell is arriving from below or simply building up because the room gets so little fresh air.

Matching the smell to a likely cause

What you notice
Faint exhaust or gasoline smell, worse after a car comes in
Likely cause
Garage air leaking up through ceiling gaps or around ducts
First thing to check
Ceiling penetrations, attic hatch, gaps around pipes and ducts in the garage
What you notice
Chemical or solvent smell
Likely cause
Paint, fuel, pesticides or lawn products stored in the garage
First thing to check
Shelves directly below the room
What you notice
Stale, stuffy smell with no single note
Likely cause
Too little supply air and no return in the room
First thing to check
Register airflow with the door closed versus open
What you notice
Musty smell near exterior walls
Likely cause
Cold floor and walls causing condensation in winter
First thing to check
Carpet edges and closet corners on outside walls
What you notice
Sweet-rotten smell that appeared suddenly
Likely cause
A source inside a floor or wall cavity is one possibility
First thing to check
Where the smell is strongest; see the guide on decay smells

Working through it

  1. Treat the garage as part of the room

    Move fuel cans, open solvents and lawn chemicals away from the area under the room or out of the garage. Avoid idling cars inside.

  2. Look up from the garage

    With a flashlight, look for missing drywall, unsealed holes around ducts or pipes, and an attic hatch that does not close tightly. These openings should be sealed with materials appropriate for the wall between a garage and living space.

  3. Test the airflow

    Close the room's door with the system running. If the door pulls or pushes noticeably, or the register barely moves a tissue, the room is short on air. A door undercut or a transfer grille can help.

  4. Then clean the room

    Once air leaks and storage are handled, launder bedding and wash or extract the carpet. Cleaning first usually just postpones the smell.

Questions for an HVAC contractor about this room

  1. Is this room getting the airflow the system was designed to give it?
  2. Does the room have a return path when the door is closed?
  3. Are there duct connections in the garage ceiling that are leaking?
  4. Is the floor over the garage insulated and air-sealed?
  5. Would a small change, such as a damper adjustment, help before anything bigger?

Common questions

Is a car exhaust smell in the room dangerous?

Exhaust contains carbon monoxide, which itself has no smell. A smell of exhaust means garage air is getting in, so make sure a working carbon monoxide alarm is installed in or near the room and fix the air leak promptly.

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