It's a mystery. Your home is only a year old and the creamy-colored carpeting has taken on a dark line at the perimeter of every room and even at the base of every door.

You've tried vacuuming it and washing it, but it won't go away. Your next step is to contact the carpet company and start complaining big time, but your homebuilder's warranty department feels certain that it is not the carpeting itself that is at fault here. What could be causing this "ghost" in your new, expensive carpeting?

According to the National Association of Home Builders, discoloration is caused by one or a combination of culprits, but probably has nothing to do with the quality of the carpet itself. An accumulation of small particles in the interior air of your home may be at work, and only a professional carpet cleaning with enzyme-based agents may help.

Just as we look for the root of health problems to find a cure, researchers have tried to find out why this ghosting phenomenon occurs even in brand new (sometimes even unoccupied and model) homes. The source of these particles is generally identified as compound-like substances such as tobacco smoke, wood-burning smoke, drywall dust, house dust, and home projects such as sanding, grinding and finishing. Also included may be airborne pollution particles, organic material, automotive exhaust, and road dust.

Air movement not only within the house but also throughout the home's wall structure may be contributing to the phenomenon.

"Air movement and temperature gradients are present in every home," says NAHB. "These mechanisms cause attraction of airborne particles to exterior walls or leakage points in the home. When particles are drawn to a leakage 'path,' for example at the base of a wall, the carpet can act as a filter that retains the particles. Over time the particles build up and become visible."

"The most recent and increasingly common form of staining is caused not by dirt or dust but by soot," says Frank Vigil. "Ghosting from soot is seen primarily in more recent construction, but diagnosticians have detected soot stains in older residences as well. Typically, newer homes -- often still under warranty -- are the focus of attention." See: Black Stains in Housing: Soot, Dusts or Ghosts?, Home Energy Magazine

Vigil says three known forces can be responsible for the deposits: impaction (forced air), gravity, and attraction (electrostatic forces and moisture). According to Vigil, the location of the deposits gives a good indication of which of the three forces may be responsible.

According to the NAHB, some discoloration in both carpeting and walls is likely to occur in all homes, but steps can be taken by homeowners to address it. Among them:

  • Reduce or eliminate tobacco smoke.
  • Cut down on candle burning.
  • Limit the use of solid fuel-burning appliances, which can produce soot.
  • Perform automotive refinishing, woodworking and dust-producing activities well away from your home's doorways and open windows.
  • Try not to let cars idle in attached garages.
  • Always use an exhaust fan when cooking.
  • Clean your carpeting regularly, and use a crevice tool each time around your baseboards.
  • Replace your central heat/air vents on a regular basis.

If you're having a home built, you may want to head-off future carpet problems by taking these steps.

  1. Have a tight duct system is installed, which will help to minimize air leaks.
  2. Make sure the filter on the furnace is changed before you occupy.
  3. Ask your builder not to use the HVAC system during construction activities.
  4. Use qualified contractors to install proper vents on appliances, wood stoves and fireplaces.
  5. Make sure the draft on the fireplace flue is adequate.
  6. Have your builder minimizes air leakage from the garage to the house by sealing door openings tightly with weather-stripping. (Alternatively, homes which are too tightly sealed can have other problems, so there must be some balance.)
  7. Have your builder actively looks for leakage routes at door thresholds and stair treads and risers.
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