How to Check Whether Garage Air Reaches the House

Why this matters

An attached garage stores the things a house should never breathe: engine exhaust, fuel vapor, mower and generator exhaust, pool chemicals, solvents. The wall and ceiling between the two are supposed to keep them out, and the house's own pressures decide whether they do. A customer who asks "is the garage getting into the house" after a CO alarm, a gasoline smell in a bedroom, or headaches that only happen on winter mornings deserves an answer built on a reading, not a reassurance. This procedure takes about an hour with a digital manometer, a smoke pencil, a flashlight and a personal CO monitor. It looks for the two things that both have to be present for garage air to come in: a hole, and a pressure pushing through it.

Conditions that stop the test before it starts

• A CO alarm is sounding, or someone has symptoms that ease outdoors (headache, nausea, dizziness): everyone out of the house, call 911 or the fire department from outside, and nobody re-enters until they clear it. This procedure diagnoses a past event. It is never run during a live one.
• Nothing burns fuel in the garage during the test. No car, generator, mower, pressure washer or space heater. You trace the path with pressure and smoke, never with real exhaust.
• Step 3 deliberately depressurizes the house. If the house has an atmospheric (draft hood) gas water heater, furnace or boiler, that can pull flue gas back into the house. Either set those appliances to off or pilot at their own controls for the test window (off, never pilot, for any appliance that sits in the garage where fuel vapor can reach it), or have a qualified gas technician watch for spillage during step 3. See related: How to Screen for Spillage After You Add an Exhaust Fan.
• Wear the personal CO monitor at your breathing zone throughout. If it reads above its normal background in the garage before you begin, stop and find out why (a car parked warm, an appliance running) before going further.

The reading convention

Every pressure in this procedure is the house with reference to the garage: manometer in the house, its reference hose run under or beside the shared door into the garage. A negative number means the house is at lower pressure than the garage, so any hole between them carries garage air into the house. A positive number means the flow runs the other way. One inch of water column is about 249 pascals (Pa), so the readings here are a few pascals at most; average each one over 10 to 30 seconds on the manometer rather than reading a single flicker. See related: What a Pascal Is and Why House Pressures Are Tiny, and How to Read House Pressure With a Manometer.

The steps

1. Close the garage overhead door, the garage's other exterior doors and windows, and every house window and exterior door. Leave interior house doors open. A closed garage is the condition the complaint usually happens in (the car idling with the door down, fuel cans overnight), and it is the condition where garage air has nowhere to go but the house. Skip it and an open overhead door ties the garage to outdoors, the readings describe the house against the street instead of against the garage, and the pollutant concentration you are worried about never builds in real life the way it does with the door shut.

2. Take a baseline with every fan and the air handler off. Note outdoor temperature, wind speed and which side the wind is on. This reading is stack effect and wind alone. In heating season the lower floor of a heated house usually runs slightly negative to anything near outdoor temperature, which usually includes an unheated garage. See related: Stack Effect and the Neutral Pressure Plane. Skip it and you cannot tell later whether a negative reading came from the weather or from a fan, which is the difference between an air sealing fix and a fan or makeup air fix.

3. Turn on every exhaust device the house has, and read again. Bath fans, the range hood on high, the clothes dryer on an air-only cycle, a vented central vacuum. This is a practical worst case for house depressurization from the devices the owner actually runs. See related: Counting Every Exhaust Device in the House. Skip it and a garage that tests tight on a calm afternoon gets a clean bill, then leaks every Sunday when the hood and dryer run together.

4. With the exhaust still running, turn the air handler fan on, read again; then shut the exhaust off and read with the air handler alone. A furnace or air handler moves several hundred to over a thousand cubic feet per minute, and any of its ducts that leak outside the house shift the house pressure. Watch the direction. A supply leak outside the house pushes the house more negative. A return leak inside the garage does the opposite at the shared door: it pulls garage air into the duct and delivers it to the house, which nudges the house reading positive while sending garage air straight to every register. Skip it and you miss the one path that needs no house pressure at all to work.

5. Smoke the shared door, under the step 3 condition. Puff the smoke pencil from the garage side along the door's latch side, hinge side, head, bottom sweep and threshold. Smoke drawn into a gap is garage air moving into the house. Then check whether the door closes and latches on its own from fully open; a door that needs a push is a door that sits open. See related: The Garage to House Fire Separation, which covers what the door has to be. Skip it and the commonest single path, a worn sweep on a door nobody can see the bottom of, never gets named.

6. Smoke the ceiling and the other penetrations. In the garage: the attic hatch or pull-down stair, light boxes, the opener's ceiling mount, wire and pipe holes in the ceiling and common wall, outlets on the common wall, the bottom plate where it meets the slab. Over a garage with a room above it, every hole in the ceiling opens into the floor cavity of that room. Over a garage whose attic is shared with the house attic, the hatch connects the garage to every attic bypass the house has. Skip it and you seal the door, the house still runs negative, and the air simply finds the ceiling instead.

7. Inspect any duct or air handler in the garage, without opening it. Note ducts passing through, any register or return grille in the garage, panned joist cavities, flexible duct, and an air handler cabinet sitting in the garage. With the blower running, smoke the duct seams and the cabinet seams from outside. Smoke drawn into a return seam or a cabinet joint is garage air entering the system. The IRC requires ducts in the garage and ducts through the separation to be at least 26 gauge sheet steel or other approved material with no openings into the garage (R302.5.2), and its mechanical chapter prohibits taking return air from a garage, in substance; both as adopted by your jurisdiction, check the current edition. Do not open the cabinet or touch wiring. This finding goes to the HVAC contractor. Skip it and you hand the owner an air sealing quote for a problem a sealed door cannot fix.

8. Record CO alarm coverage. Note where each CO alarm is, its age from the date on the back, and which sleeping areas have none. For new construction, and for existing houses when permitted work triggers it, the IRC requires CO alarms in dwellings with fuel-burning appliances or an attached garage that communicates with the dwelling, located outside each separate sleeping area in the immediate vicinity of the bedrooms (R315, 2021 edition, as adopted). Replacement age is on the alarm's own label or manual. Do not mount one in the garage itself unless its manual says it is suited to that environment. Skip it and the owner leaves with a sealing plan and no warning device near the bedroom over the garage.

9. Restore everything and confirm it. Exhaust devices off, dryer off, air handler back to its normal mode, thermostat to its original setting, gas appliances returned to their original control settings. Confirm each gas appliance relights and its burner runs; if anyone saw spillage in step 3, the gas technician confirms draft before you leave, not you. Confirm your CO monitor is back to its starting background. Skip it and you leave a water heater on pilot in February.

Worked example: the record from one test

A two-story house, outdoor 24 F, wind light from the north (the garage side is east). The air handler is in the basement; its return trunk to the bonus room crosses the garage ceiling. Water heater atmospheric, set to pilot for the test with the owner's agreement. The filled-in record:

GARAGE-TO-HOUSE TEST       house WRT garage, Pa
------------------------------------------------
Garage closed, house closed, interior doors open
Outdoor 24 F, wind light N, garage on E side

Condition                         Reading (avg)
1 fans off, AH off                    -1.2
2 exhaust on (2 bath, hood, dryer)    -5.8
3 exhaust on + AH fan                 -5.1
4 AH fan alone                        -0.4

Smoke, garage side, under condition 2
  door bottom sweep          drawn in, strong
  door latch side            drawn in, slight
  attic pull-down stair      drawn up into gaps
  wire hole by opener mount  drawn in, slight
Smoke at return duct seams, AH running
  2 transverse joints        drawn in
CO alarms: upstairs hall only, 9 yr old
  ground floor bedroom (wall shared
  with garage): no alarm nearby
Door self-closes from open: no, stops 6 in
Restored: WH relit 10:42, burner on, CO bkgnd

Reading it. The baseline of -1.2 Pa is the stack effect on a 24 F day: small, steady, and enough to move air through a bad sweep. The exhaust devices took the house to -5.8 Pa, 4.8 times the baseline (5.8 divided by 1.2), so on cooking-and-laundry mornings the flow through every hole a little more than doubles (flow through a crack goes up with something like the square root of the pressure, and the square root of 4.8 is about 2.2).

The number that lies is condition 4. The air handler alone moved the house from -1.2 to -0.4 Pa, which reads like the air handler is helping. It is helping the shared door, by pulling garage air into its return seams and blowing it into the house. Condition 3 tells the same story: adding the air handler to the exhaust made the house 0.7 Pa less negative (5.8 minus 5.1), and the smoke at the two return joints is where that air came from. A tech who read only the door would call the air handler harmless.

Who owns what. The door sweep, latch strike and the closer that stops 6 in short go to the handyman or door installer. The pull-down stair needs a gasketed, close-fitting cover that also satisfies the separation, and the wire hole needs an approved fill; see related: The Garage to House Fire Separation. The return trunk in the garage goes to the HVAC contractor under R302.5.2, either sealed to standard or rerouted out of the garage. The missing CO alarm near the ground floor bedroom goes to the owner today.

The retest. After the repairs, condition 2 still reads near -5.8 Pa, because sealing a door does not change the house's exhaust. What changes is smoke: nothing drawn in at the door, the stair or the wire hole, and the return joints are sealed or gone. If the exhaust reading itself is the concern, that is a makeup air question for the HVAC side. See related: Kitchen Hood Makeup Air.

What changes the answer

• The air handler sits in the garage. Then step 7 is the whole test. Its cabinet and return run negative to the garage every minute it runs, and the shared door readings can look fine.
• The garage is heated or has its own exhaust fan. A garage exhaust fan pulls the garage negative to the house, so step 2 can read positive, and that is the condition you want. A garage heater burning fuel inside the garage is its own combustion and venting question for a gas technician.
• The garage attic is open to the house attic. Then the attic stair is effectively a hole into the whole house's bypass network, and the garage ceiling deserves as much attention as the door.

References

• International Residential Code, 2021 edition, R302.5 (garage openings, ducts and penetrations) and R315 (carbon monoxide alarms), as adopted by your jurisdiction
• See related: The Attached Garage as a Pressure Neighbor
• See related: The Garage to House Fire Separation
• See related: How to Screen for Spillage After You Add an Exhaust Fan
• See related: Counting Every Exhaust Device in the House