Humid Air, Damp Brick and Your St. Augustine Chimney

Where the Moisture Comes From

A chimney is a tall, open-topped column of masonry that sits outside the air-conditioned envelope of the house. Warm, wet outdoor air drifts down an open flue and meets the cooler surfaces of the firebox and smoke chamber, where it condenses. Rain adds more water through an uncapped flue, a cracked crown or open mortar joints.

Brick and mortar are porous. They take on water easily and give it up slowly, especially on the shaded sides of a stack. In a climate where the air stays humid for much of the year, a chimney can stay damp from one rainy spell to the next.

Chimney Moisture Problems You Can See

Rust is usually the first sign. Steel throat dampers, firebox grates, galvanized caps and chase covers all corrode faster when they rarely dry out. Rust flakes in the firebox or a damper that grinds when it moves are common in fireplaces that sit unused through the summer.

On the masonry, look for white efflorescence, dark patches that stay after the rain stops, mortar that turns sandy, and brick faces that flake off. Inside, peeling paint or a soft spot in the drywall beside the chimney points to water getting through.

Odors and Chimney Mold

Old soot and creosote in a damp flue give off a sour, smoky smell that gets stronger in humid weather. The smell is often worse when the air conditioning runs, because the house pulls air down the chimney.

Chimney mold is less common inside the flue itself, but wet insulation, drywall or framing next to a leaking chimney can grow it. That calls for stopping the water first. Removing mold from finished rooms is a job for a remediation contractor; our work is keeping the chimney from feeding it.

What Keeps a Chimney Dry

Four things do most of the work. A cap keeps rain out of the flue. A damper that actually seals, often a top sealing damper, stops humid air drifting down when the fireplace is off. Sound mortar joints and a crown with an overhang keep rain out of the masonry. A breathable water repellent helps porous brick shed rain while still letting trapped moisture escape.

Avoid paint-like sealers on brick. A film that traps moisture inside the masonry makes spalling and efflorescence worse, not better.

A Simple Routine

Have the chimney inspected once a year, ideally before the cooler months. Keep the damper closed when the fireplace is not in use. Sweep the flue when deposits build up, since clean flues smell less. After a long, wet spell, look for new stains or rust streaks and call before small problems spread.

• Cap on the flue, screen intact
• Damper closes and seals
• Crown and joints free of open cracks
• No paint-like coating on the brick