Smell coming from the attic? Match the odor to the cause

Two smells come first

Before the table, the two that aren't a research project.

Rotten egg or sulfur. PG&E lists a sulfur or "rotten egg" type of smell as a sign you may have a gas leak. Many houses have a gas furnace, a water heater flue or a gas line running through the attic, so the smell can come from up there. PG&E's instructions are short: leave the area, alert everyone in the household, go somewhere safe away from the immediate area, then call 911 and PG&E at 1-800-743-5000. PG&E also says to avoid matches, flashlights and other possible ignition sources, which is one more reason not to climb into the attic with a work light to investigate.

If you live in Palo Alto, the gas utility is the City of Palo Alto Utilities, not PG&E. The city says to leave and call its dispatch center at (650) 329-2579 for a hint of gas, and to leave on foot and call 911 if there's a significant smell of gas all around the area.

Burning, hot plastic or smoke. A burning smell coming out of the ceiling can mean wiring, a light fixture or an attic fan overheating. If you see smoke, or the smell is strong or growing, treat it as a fire: Ready.gov's guidance is to leave the home and call 911 or the fire department, and not to go back in until the fire department says it's safe. If the smell is faint and you can trace it to one fixture or appliance, NFPA's electrical safety tip sheet (the San Francisco Fire Department hands it out) says to call a qualified electrician if you have a burning or rubbery smell coming from an appliance, flickering or dimming lights, or warm or discolored outlets.

Neither of these is an insulation job, and neither should wait for one.

Match the smell to the cause

Once the emergencies are ruled out, most attic odors fall into a handful of types. The nose is a decent first diagnostic tool, as long as you use it from the hallway and not with your head in a pile of droppings.

The last row has a catch. Sewer gas and natural gas can be hard to tell apart, so if you aren't sure which one you're smelling, follow the gas steps above.

Ammonia or urine: rodents

That sharp, cat-litter smell, strongest on warm afternoons when the attic heats up, is rodent urine soaked into insulation. It usually comes with other signs: scratching after dark, droppings by the hatch, tunnels through the insulation. Don't sweep or vacuum anything up there; California's Department of Public Health warns that it can stir contaminated material into the air. The full sequence, from getting the animals out to new insulation, is on our page about rats or mice in the attic. On our side, the work is rodent proofing the entry points and cleaning out the soiled insulation once the animals are gone.

Musty or damp: moisture

A musty smell means something stays wet. EPA puts it simply: "The key to mold control is moisture control," and it says you may suspect hidden mold if a building smells moldy but you can't see the source. In a Bay Area attic the usual water sources are a slow roof leak, a bathroom fan ducted into the attic instead of outdoors, and warm, humid house air leaking up through gaps in the ceiling and condensing on a cold roof deck in winter.

The fix starts with the water, not the smell. Our page on attic mold and condensation walks through the causes, and attic ventilation covers the airflow side. Masking a musty smell with a fan or an air freshener just hides the clue.

Stale smoke: wildfire residue

If the attic smells like an old campfire or an ashtray, and especially if it started after a fire season with heavy smoke, a likely cause is smoke and ash drawn in through the attic vents and caught in the insulation. Insulation is fluffy by design, which makes it a good trap for whatever the air carries. Our page on wildfire smoke in the attic covers how to tell, what to do first and why residue needs an assessment before anything is removed.

Something dead

A heavy, sweet, rotten smell that appears suddenly and gets worse for a few days is almost always a dead animal: a rat, a mouse, sometimes a bird or a squirrel. It may be in the attic, inside a wall or under the floor, which is why the smell can seem to come from a room rather than from above.

Finding and removing it is a job for a pest control company or a wildlife service, depending on the animal. Don't handle it bare-handed. Once it's out, the insulation around the spot may be soiled and need to come out, and the way the animal got in should be closed, or you'll be back here next season.

What to tell whoever you call

A few details save time, whoever comes out:

• When the smell is strongest. Morning, afternoon heat, after rain, when the heater runs.
• Where it's strongest. One room, the hallway under the hatch, a closet, the whole upstairs.
• What changed recently. New roof, a storm, a smoke event, work in the attic, a pest treatment.
• What's in the attic. A furnace, a water heater flue, a gas line, an attic fan, ducts.

Once the emergencies are off the table and the source is known, we can help with the attic side of it: removing soiled or smoky insulation, closing the openings animals use, and fixing the airflow. Start with a free inspection, or send photos and video for an estimate. Book it here.