Carbon Monoxide Risk in Attached Garages - What You Need to Know

Had a customer last year who mentioned his family had been having headaches in the mornings for a few weeks. Dismissed it as seasonal allergies. It wasn't. They had a small CO leak coming from an older water heater in the attached garage seeping into the house through gaps around the interior door.

They were lucky. CO is odorless, colorless, and the symptoms look like a lot of other things before they look like poisoning.

Attached garages are the most common source of carbon monoxide intrusion into homes. Understanding why - and what to do about it - is genuinely important.

Why attached garages are a specific CO risk

Carbon monoxide comes from incomplete combustion. Any gas-powered engine or appliance produces it. In an attached garage, the sources include: vehicles being started or warmed up, lawn equipment running briefly before being moved outside, gas-powered tools, and appliances like water heaters or furnaces if they're located in the garage.

The problem with an attached garage is proximity. There's a shared wall and usually a shared door between the garage and the living space. CO is slightly lighter than air and disperses, but in an enclosed space it builds up fast. And it migrates through gaps - around the door frame, through utility penetrations where wiring or pipes pass through the shared wall, through any gap in the drywall.

A detached garage with CO buildup is dangerous to whoever is in it. An attached garage with CO buildup is dangerous to the entire household, including people sleeping in rooms adjacent to or above the garage who have no idea anything is burning.

The car idling situation

This is the scenario that causes most acute CO poisoning from garages. Someone starts the car and lets it warm up in the garage. Even with the garage door open, CO levels can build up faster than they dissipate, especially in cold weather when the door is only cracked or when the exhaust is pointed toward the house wall.

Modern cars produce significantly less CO than older vehicles but they still produce it. Electric vehicles produce none, which is one of the less-discussed practical advantages.

The rule: start the car and move it outside immediately. Don't idle in an attached garage, period. Even with the door fully open. The garage door open means the garage is ventilated - it doesn't mean CO isn't reaching concentration levels near the car and house wall before it clears.

Garage door gaps and CO intrusion

Here's something most homeowners don't think about. The door between the attached garage and the house - the interior door - is a barrier, but only as much as its gaps allow.

Standard interior doors have gaps under them, around the frame, and through any cracks in the drywall. CO doesn't need much space to migrate. Even a gap under the door is enough for CO to seep into the living space when levels in the garage are elevated.

The building code-compliant solution: the shared wall between an attached garage and living space should be sealed. The door should be a solid-core, fire-rated door with weather stripping, including a door sweep at the bottom. If your interior garage-to-house door is a hollow core door with no sweep - it's not providing the barrier it should.

This isn't about garage door operation. But it comes up in garage safety conversations because the garage door condition affects how vehicles can be operated and how quickly CO clears. A garage door that doesn't seal properly at the bottom - bad seal, frozen to the ground in winter, warped - can make it harder to fully clear the garage before going back inside. Our garage door bottom seal guide covers what a proper seal looks like and what replacement costs.

CO detectors - placement matters

A CO detector on the ceiling of the living room doesn't do much for garage-source CO. CO from the garage enters the living space at low levels, near the floor, through the gaps under the interior door. By the time it reaches a ceiling-mounted detector in a distant room, levels are already elevated where people are sleeping.

The detector that protects against garage-source CO should be: in the room immediately adjacent to the garage or in the ceiling of the garage itself near the shared wall, and in any bedroom above the garage.

Battery-operated CO detectors work fine. Combination smoke/CO detectors are convenient. Hardwired with battery backup is the most reliable. Replace them every 5-7 years - the sensors degrade and the device will stop detecting accurately before the alarm stops sounding.

Appliances in the garage

If there's a gas water heater, furnace, or other fuel-burning appliance in the attached garage - it needs proper venting to the outside. These appliances produce CO as part of normal operation. The venting system is what keeps it from building up inside.

Have gas appliances in the garage inspected annually. Cracked heat exchangers, blocked flues, and failed vent connections are the kind of failures that slowly expose a household to CO over months.

If you're adding appliances to an attached garage, have the installation done by a licensed professional and ensure proper venting is part of the job.

What CO poisoning symptoms actually look like

Mild exposure: headache, fatigue, shortness of breath, nausea. These are easily attributed to other things. People sleep through mild CO exposure and wake up feeling like they have a cold.

Moderate exposure: severe headache, drowsiness, confusion, vomiting.

Severe exposure: loss of consciousness. At high enough levels, death.

The specific pattern that should trigger immediate suspicion: multiple people in the household experiencing similar symptoms simultaneously, symptoms that improve significantly when people leave the house, symptoms worst in the morning after sleeping.

If this pattern sounds familiar - get everyone out, leave the door open, call the fire department. They can test CO levels and identify the source. Don't go back in until it's cleared and the source is identified and fixed.

GarageDoorRepairz - garage safety, door sealing, or anything else about your attached garage setup. Give us a call.