A few weeks back, a rowhouse dweller asked for advice about dealing with cigarette-smoke odor finding its way into her living space from the other side of the party wall. The odor was most noticeable, she said, when the neighbors turned on their central air conditioning.

Harris Gross, a licensed professional engineer and home inspector, says older homes retrofitted with ductwork for central-air systems often lack sufficient return ductwork, especially on the second floor.

Lawrence Weintraub, a Philadelphia restoration architect for 30 years, explained that if bedrooms do not have ducted return vents, only supply vents, “then the room receives great positive air pressure when the A/C is on.”

Thus, if there is a lot of smoke in the room, air conditioning will force the odor through gaps or cracks in the party wall, Weintraub said.

In a brick firewall, Gross said, gaps are present in the mortar, and the areas where the beams pocket into the “double wythe” (two layers of brick) are “thin” spots susceptible to smoke penetration.

Pocketed joists are often at the same elevation as adjacent houses, Weintraub said.

“Through decades of shrinking and movement, gaps form around these joist ends, allowing for air transmission between dwellings,” Gross said. “If one or both homes have exposed wood flooring, perhaps an original tongue-and-groove plank floor, then for sure, smoke can work its way through the floor-joist cavity and into the room on the other side.”

“If the smoke is working its way through the floor-joist cavity, they would have to open the ceiling below the room, adjacent to the party wall, inspect the cavities between the joists, and apply new mortar or grout, or use spray foam to seal the gaps,” he said. “I would install insulation between each cavity against the exposed brickwork between the joists.”

Other possibilities, Gross said: Paint unfinished brick walls with a color-matched mortar/sand/lime mix; inject gaps with clear silicone to seal the wall, or coat the wall with a masonry sealant after caulking.