What causes a smell in the attic?
Short answer. The type of smell usually points to the cause. Musty means mold or wet insulation from a leak or condensation, ammonia or a rotten odor means animals or a dead animal, a sewer smell often means a cracked or disconnected plumbing vent in the attic, and a hot asphalt or chemical smell is common on very hot days, after a new roof, or with poorly cured spray foam.
Match the smell to the source
- Musty or earthy: mold and mildew on decking or damp insulation. Check around penetrations, along the eaves, and above bathrooms.
- Ammonia or urine: squirrels, raccoons or rats using the insulation as a latrine. Look for droppings, trails in the insulation, and chewed vents.
- Sweet and rotten: a dead animal, often near the eaves or the tops of walls. It fades over a few weeks, but the spot needs cleaning.
- Sewer gas or rotten egg: a plumbing vent that ends in the attic instead of through the roof, a cracked vent pipe, or a joint that came apart. This is a plumbing fix and worth doing promptly.
- Hot tar or asphalt: normal for a few days after a new roof, and on very hot afternoons when shingles and underlayment heat up.
- Chemical, fishy or burning: spray foam that did not cure properly, or overheating wiring, which needs an electrician right away.
Why the smell reaches the house
Attic air gets into living space through the same gaps that let house air into the attic: light fixtures, the attic hatch, and duct leaks if the air handler is up there. Air sealing helps, but find the source first.
Roof-related causes worth checking
Slow leaks around pipe boots, chimneys and valleys can soak insulation for months before a ceiling stain appears, and the first sign is often the smell. A plumbing vent that was cut short or left disconnected during earlier roof work is another. See pipe boot leaks and common roof leak causes.