The Musty Fireplace Smell

A Smell That Comes With the Weather

There is a particular smell some fireplaces give off on damp days. People describe it as sour, smoky, musty, like a wet campfire or an old ashtray. It is often strongest on the first humid evening of fall or the day after a storm, and it can fill a room even when the fireplace has not been used in months. In Thousand Oaks we hear about it most in the weeks after the first real rain of the season, when moisture comes back after a long dry summer.

The smell is not random. It is almost always caused by moisture meeting the deposits inside the chimney, and finding out where the moisture comes from is the way to stop it.

Where the Odor Comes From

Wood fires leave creosote and soot on the inside of the flue. Dry, those deposits have a faint smell. Wet, they release much more of it. Creosote is made of compounds from burned wood, and when humidity rises or rainwater soaks into it, those compounds come out into the air. If air is flowing down the chimney into the house rather than up and out, the smell comes with it.

Several things can add to the odor.

• - Rain getting into the flue. An uncapped flue lets rain fall directly onto the creosote on the walls and the debris on the smoke shelf.
• Water soaking through the masonry. A cracked crown or failed flashing keeps the brick around the flue damp, and that dampness reaches the deposits.
• Debris on the smoke shelf. Leaves, nesting material and fallen creosote collect on the ledge behind the damper and hold water like a sponge.
• Animals. A bird or animal that got in and died in the chimney is a different, unmistakable smell, and it often follows a missing cap.
• Negative pressure. Modern, tightly sealed houses with kitchen and bath fans running can pull air down the chimney, carrying whatever smell is there into the room.

Is It a Safety Problem?

The smell itself is mostly a nuisance. What it says about the chimney can matter more. Water that is reaching the flue is also rusting the damper, eroding the mortar joints in the flue liner and working into the masonry. Over time that leads to the repairs nobody wants: a damper that will not close, open joints that let heat and gases reach the brick, and crumbling mortar that loosens the stack. A strong odor is also a sign that the chimney has more deposits than it should, which is a fire concern when the fireplace is next used.

If the smell is more like rotten eggs, that is not a chimney odor. Gas companies add that smell to natural gas so leaks can be detected. Leave the house and call the gas company.

Quick Checks You Can Make

Before calling anyone, a few simple observations help.

• - Does the smell come only on damp days, or all the time?
• Is it stronger when the bathroom or kitchen fans are on?
• Is there water, rust or dark staining in the firebox?
• Can you see a cap on top of the chimney from the street or the yard?
• When was the chimney last swept?

Closing the damper, if it seals well, reduces air flowing down the chimney and often cuts the smell. Opening a window slightly when exhaust fans run helps balance the pressure. These are short-term measures. They do not remove the moisture.

Fixing It for Good

Getting rid of the smell has two parts: clean out the deposits and stop the water.

Clean. A chimney sweep removes creosote from the flue and clears the smoke shelf of debris. That alone often reduces the smell a great deal, at least until moisture comes back. A sweep with us is from $210, and while the flue is clean we look for the signs of where water is getting in: streaks down the tile, wet deposits, rust on the damper.

Keep the rain out. A cap is the simplest and most effective step if the flue is open to the sky. It stops rain falling into the flue and keeps out animals and leaves. Installed caps run $155 to $635. If the fireplace is rarely used, a top-sealing damper, a damper mounted at the top of the flue that closes with a cable from the firebox, seals the flue against both rain and air flow when the fireplace is not in use.

Fix the masonry. If the crown is cracked or the flashing has failed, the brick around the flue stays damp and the smell will return. Those repairs are quoted in writing after an inspection. The chimney masonry repair page explains crown work, and chimney flashing repair covers the roof joint.

Balance the house. In tight homes, a fresh air supply or simply opening a window when fans run can stop the chimney acting as an air intake.

Odor Removers and Their Limits

Some products sold as chimney deodorizers can mask the smell for a while, and baking soda or charcoal in the firebox absorbs some odor. None of them remove the water. If you have a musty smell that keeps coming back with the weather, the chimney is telling you something about where rain is getting in, and that is worth listening to before the next wet season. A Level 1 inspection with us costs $52 and is usually enough to find the source. Book through our contact page.