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Q.The homeowner living in a ranch house our company built five years ago is complaining of mysterious carpet staining, showing up as a stripe along the outside walls of some rooms. There do not appear to be any water leaks or problems with the gas appliances. The house is on a slab and has a gas-fired hot-air furnace. The homeowners do not burn any candles. What could be causing this problem?

A.Frank Vigil, senior building science specialist at Advanced Energy in Raleigh, N.C., responds: Dusting, sooting, or ghosting — as these marks have all been referred to — is not uncommon. Markings on walls, carpeting, furnishings, even inside of appliances, are all too common in today’s houses, possibly because we’ve done exactly what we set out to do years ago: build the houses tighter. In tight homes — especially tight homes with insufficient ventilation — particulate has more opportunity to deposit, instead of being flushed away by regular air changes.

The dusting or staining that your homeowner is experiencing requires two things in order to occur: There must be a source for the material, and there must be a driving force to cause the material to deposit. A laboratory test of the material can be helpful, at least to narrow the possible sources. Is the staining gray, black, brown, or yellow? Does it appear on places other than the carpet-to-wall junctions? You mention that while the homeowners don’t burn candles (so they claim), they do have a gas furnace. Has the furnace been tested for draft under the worst-case scenario (with exhaust fans, clothes dryer, and central vacuum cleaner operating)? Is there a woodstove or fireplace (gas or wood)? Investigations over the years have found all of these things can be culprits for staining.

The driving force for stains along the carpet-to-wall junction are typically pressures caused by mechanical fans and/or stack effect (heat rising). Air will always seek the path of least resistance. If the house is pressurized, escaping air will often go up the wall cavity to the attic. The carpet serves as a filter, scrubbing the air of some of the contaminates. Over time, this is what you see on the carpet. On the other hand, negative pressures in the house could be causing attic air to filter down the walls, with the carpet again serving as a filter.

Pressure mapping of the house by a qualified technician can easily pinpoint the pressure dynamics the house experiences while fans are operating. From there, you can begin to trace likely sources for the stain.