If your fireplace smells like a campfire on humid summer days, the cause is almost always humid air drawing downward through the flue and across creosote deposits. It is not a sign of fire damage, and it is fixable with a sweep and a top-sealing damper.
Why humidity turns creosote into an odour
Creosote is hygroscopic, meaning it absorbs moisture from the air. When warm humid air enters the flue and contacts those deposits, it releases the sharp acrid smell you notice in the room. The effect is strongest in July and August, exactly when nobody is thinking about their chimney.
In Oswego this is noticeably worse for homes near the Fox River corridor, where ambient humidity runs higher. Households a few blocks from the water report it far more often than those out in the drier subdivisions to the east.
The airflow problem behind it
The odour only reaches you if air is moving down the flue rather than up. That happens when your house is at negative pressure relative to outside, which modern tightly sealed homes do constantly. Running bathroom fans, a kitchen hood, a clothes dryer, or air conditioning all pull air out of the house, and the chimney is often the easiest replacement path.
So you have two contributing factors: something for the air to smell, and a reason for the air to travel the wrong way. Removing either one solves it, but removing both solves it permanently.
The two fixes that actually work
First, remove the source. A thorough chimney sweep takes out the creosote that is producing the odour, including the deposits on the smoke shelf and in the smoke chamber where the heaviest accumulation sits and where most quick cleanings never reach.
Second, stop the airflow. A top-sealing damper mounts at the top of the flue with a silicone gasket and seals far more completely than the cast-iron throat damper at the bottom, which on most homes has warped and no longer closes tightly. It also keeps the entire flue dry, which slows future buildup. Our caps and dampers page explains the difference.
What does not work: air fresheners, closing the existing damper harder, or a chimney cleaning log. None of them address either cause.
Talk to a local chimney pro
If your fireplace has been smelling all summer, now is the ideal time to deal with it. Spring and summer are our quietest months in Oswego, so appointments are easy to get, and you go into heating season with a clean flue instead of joining the October queue.
Written by Josh Amber, co-owner of Ember Chimney Sweepers, Oswego IL. Published September 19, 2026.