Your Place: Finally, an answer on the sooty marks on walls

A tip of the Heavens hat to Medford architect Charles J. Collins Jr., ably assisted by Joe Ponessa, for helping to solve a riddle from a few weeks back: sooty marks on walls above electric baseboard radiators.

Collins has inspected many homes and has renovated houses with similar stains at or near the intersection of wall and ceiling.

"We, meaning myself, my consulting mechanical engineer and my home inspector have all concluded that the stains are the result of the temperature differential between the well-insulated wall cavity and the poorly insulated wall plates that allows a condensation accumulation . . . and then attracts dust," he said.

He's found a wealth of Internet articles about stains above baseboard radiation that "do not seem to reference the burning of dust but rather the same type of attraction of dust due to condensation."

It's called "ghosting," according to what Collins has found. He sent this snippet from an article that explains it:

"Ghosting often occurs higher up on the ceiling, but the cause could also be present just above the baseboard. The combination of humidity in the house plus cold spots on the walls/ceilings attracts the moisture. Then dust in the air inside the house gets stuck to these moist condensation spots."

Rutgers professor emeritus Ponessa said the phenomenon is called "shadowing," usually attributed to the fact that, in a standard wall assembly, the studs are typically colder during the heating season than the adjacent insulated wall cavity.

"The physical theoretical explanation has something to do with the soot particles, bounding around in the air . . . are slowed down at the colder areas, and somehow settle there," Ponessa said. "Another possibility is that moisture condensation on cold spots on the wall promotes mold growth."

The way to prevent shadowing or ghosting is to cut the temperature differential, say through insulation or air sealing.

For an fuller explanation, Ponessa sent this link to Joe Lstiburek's fact sheet, "BSD-150: Black Stains on Carpets and Ghosting of Framing," at www.buildingscience.com.