Ghosting results from heat transfer

Inspector's Eye

May 04, 2003

A reader writes about a recent home inspection of a 1950 Cape Cod that he is considering for purchase.

At the second floor, the reader noticed several faint black streaks at each rafter/joist in the room that had the only air conditioning return in it. The stains started a foot or so above the floor and followed the angular rise but stopped short of the ceiling. It was only in this room that the reader noticed these streaks.

The reader's inspector said the streaks were a result of a combination of the radiator and air conditioning return creating air turbulence, which caused dirt and dust to settle on the wallboards. He did not write it up in the home inspection, as he thought this was normal for Cape Cods.

The reader wants to know if the condition is cause for alarm?

Dear reader:

Your home inspector's explanation that the streaks consist of dirt and dust that have settled on the wall is accurate, but is only half of the explanation.

This phenomenon is called ghosting. It is not normally an indication of a serious problem. It occurs because heat transfer through wall and ceiling framing, including studs, joists and rafters, causes cool spots on insulated walls and ceilings during cold weather. Under some conditions, dust and dirt collect at the cool spots, darkening them.

You may have never had a reason to consider it, but wood conducts heat. Wood framing in an outside wall of a house allows some of the heat in the house to flow to the exterior through the wood. This is known as thermal bridging. And while wood doesn't conduct as much heat as some other structural materials used in buildings, such as steel and masonry, it conducts a lot more than the insulation between the studs in the same wall.

The degree to which a material resists transfer of heat is expressed as an R-value, with better insulators having higher R-values. The fiberglass batt insulation installed between the two-by-fours in a typical 3 1/2 -inch stud wall has an R-value of 11. In contrast, the two-by-four studs in the wall have an R-value of 4.38.

This thermal bridging at the wood studs causes the surface of the wall to be cooler at the studs than it is between them.

At those cooler areas, particles in the air are more likely to "plate out" onto the surface - a process that has been attributed to moisture condensation on the wall or to reduced kinetic energy in the air next to the wall. The plated-out particles gradually darken those parts of the wall.

The same principle applies to ceilings below an unheated attic and, in a Cape Cod, to the sections of wall or ceiling attached to the rafters.

The degree to which ghosting occurs depends in part on the air quality in the house. Smoke from cigarettes or a smoky fireplace will contribute to ghosting. The frequent burning of candles also is a contributing factor, due to the tiny soot particles produced by the flame.

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