Why Does My Neighbor's Chimney Smoke Get Into My House?

How shared stacks are built in attached houses

In the rowhouses, brownstones and twins common across New York City, northern New Jersey and Philadelphia, chimneys were often built into the party wall between two homes. One brick stack holds several flues, some serving one house and some the other, separated inside by a single course of brick or a thin partition called a wythe. On the roof, the outlets end side by side, sometimes only inches apart and often at the same height.

Over a century, those partitions take a beating from heat, acidic exhaust and freeze-thaw cycles. Mortar joints wash out, and in unlined flues the bricks themselves can crumble. A stack that once kept each household's smoke separate can end up with openings between flues that nobody sees from either side.

The usual paths into your rooms

Smoke follows air, and your house is often pulling air in. Bath and kitchen fans, the dryer, and the stack effect that draws air into the lower floors of a tall house all create a slight suction that the nearest open path will satisfy.

• Down your own open flue, when the neighbor's smoke leaves an outlet right beside it
• Through cracks in the partition between two flues inside the shared stack
• Through old fireplace or stove openings into the chimney that were covered but not sealed
• Through a boiler or water heater thimble left open after an appliance was removed
• In through windows or vents near a neighbor's chimney that ends below your roofline

Telling chimney entry from outside drift

Where and when you smell it narrows the path. Smoke entering through the chimney is strongest at the fireplace or the wall with the chimney breast, often on a floor where the house pulls air in, and it worsens when your exhaust fans run. You may find a light dusting of fine soot on the hearth or on the sill of a covered opening. A smell in a closet, a bedroom without a fireplace, or along one wall often traces back to an old stove opening hidden behind plaster or a decorative cover.

Smoke drifting in from outside behaves differently. It is noticeable throughout the house, strongest near open windows and supply vents, and it follows the wind direction. If it is mostly outdoor drift, sealing your chimney will not change much, and window and ventilation habits on burning nights matter more.

A simple check helps on an evening when you know the neighbor is burning. Turn off your exhaust fans and the dryer, and crack a window on the floor where the smell is strongest. If the smell fades within a few minutes, your house's suction is drawing the smoke in through the chimney, and sealing that path will work.

Sealing your own flue

The fastest gains come from stopping your flue from acting as an intake. A damper that closes tightly helps if you have one. If the fireplace is not used, a top-sealing damper or a sealed cap plate on your flue closes it at the outlet, right where the neighbor's smoke arrives. Any flue that will not be used again can be capped permanently, with a small vent so the masonry stays dry.

Inside the house, old openings into the chimney should be closed with masonry or a fitted, sealed cover rather than a sheet of cardboard or a stuffed pillow. Upper floors of older brownstones and rowhouses often had small coal or stove fireplaces in the bedrooms that were plastered over decades ago. A thimble left open in the basement after a boiler was replaced is another frequent leak, closed with a proper thimble plug or masonry.

When cracks between flues are the path

If smoke still arrives after your flue is sealed, it is probably passing through the masonry between flues. An inspection from the top and inside each flue shows where the partition has failed. The usual repair lines the active flue, the one the neighbor uses, with a stainless steel liner, so their exhaust stays inside a sealed pipe and can no longer reach the cracks. Sometimes a crumbling partition or the top courses of the stack are rebuilt as well.

Outlets that sit side by side at the same height invite cross-flow. A common trade fix is to stagger them, extending the active flue upward with liner pipe and its own cap so its smoke leaves above the neighbor's openings rather than beside them.

Shared chimneys, shared water problems

A stack that shares smoke also shares rain. A cracked crown or missing cap on the neighbor's side lets water into masonry that is part of your wall too, and it can show up as damp plaster, peeling paint or a musty smell on your side of the chimney breast. Tracing it means looking at the whole top of the stack, not just your own flue.

Water in a shared stack also feeds the partition problem described above. Wet mortar between flues breaks down faster through freeze-thaw winters, so a leak on one side today often becomes a smoke path between houses a few seasons later. A cap on every flue outlet and a sound, sloped crown across the whole stack protect both homes at once.

Working it out with the neighbor

Because the stack is shared, the lasting fix often involves both households. A written inspection describing which flue serves which house, where the cracks are and what each repair does gives both owners the same facts to work from, and coordinating the top work saves bringing a crew back twice. Rules on shared walls and who pays for what vary by town and by deed, so check with your local building department, or with an attorney if there is a disagreement, rather than assuming.

Boiler exhaust and carbon monoxide

Wood smoke is unpleasant; exhaust from a neighbor's oil or gas boiler drifting into your rooms is a safety issue, because carbon monoxide travels with it and is dangerous. A working CO alarm belongs on every floor and close to where people sleep; check that each one responds to its test button. If an alarm sounds or anyone in the house feels unwell, go outside together and call 911 or the utility from there.

A neighbor's boiler running all winter is a steadier source than an occasional wood fire. An exhaust smell in the basement or along the party wall during heating season deserves an inspection soon, not at the next routine visit.