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This article describes how to identify and diagnose ceiling stains in buildings focusing on the cause of various interior wall and ceiling stains and explains how to recognize thermal tracking, thermal bridging stains, building air leaks, and building insulation defects. Often these stains are mistaken for toxic indoor mold.

Our page top photo shows two common thermal tracking or "ghosting" patterns. The narrow dark stains on the ceiling show the location of ceiling joists, and the wide black sooty stain in the upper left of the photo shows where ceiling insulation is less or has been omitted entirely. Readers should see HEAT LOSS in BUILDINGS. Also see AIR SEALING STRATEGIES and AIR LEAK DETECTION TOOLS as well as AIR LEAK MINIMIZATION. For photos of snow melt on rooftops that also show points of building heat transfer and loss, see STAIN DIAGNOSIS on Building Exteriors.

When investigating a building for a mold problem, you can save mold test costs by learning how to recognize Stuff that is Not Mold or is only Harmless Mold but may be mistaken for more serious contamination.

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Building Air Leaks & Thermal Tracking Marks at Interior Ceilings

Sooty or dark smudges or stains appearing near the ceiling on the inside of building exterior walls, especially in older homes whose interiors have not been re-painted or cleaned in some time. Thermal tracking stains may appear at the top of the wall and extend onto the ceiling surface such as shown in this photograph. Note those dark "stripes" extending along the ceiling and into the room? These ceiling stains probably mark the location of ceiling joists (where the in-room ceiling surface temperature was kept a bit cooler since these locations in the ceiling cavity are occupied by a wood joist rather than by insulation).

Thermal tracking or "soot tracking" such as shown in the photograph at the top of this page may be found wherever moisture condenses on cool building surfaces.

Warm moisture-laden air touches the cooler surface of a building wall or ceiling, giving up some of its moisture to the surface as condensation.

As air moves through the building, typically up walls and across ceilings, debris in the air, particularly soot such as that left by burning candles (scented candles may be more of an IAQ issue), adheres more to the damp surfaces than to others, leaving black marks or "tracks."

In a conventionally-framed wood structure, wall and ceiling framing is typically on 16" or 24" centers.

The wall or ceiling interior surface will be cooler where the framing is located than will be the spaces which are not touched by framing and which, perhaps, are insulated. So if you see black streaks up the building wall in a regular 16" or 24" pattern, particularly on cooler exterior walls but potentially anywhere, it may be thermal tracking.

Diagnosing Stain Patterns on Cathedral Ceilings

Cathedral Ceiling Stains: Alan Carson, a home inspection educator and researcher, points out that on cathedral ceilings, where we see dark streaks following exterior studs or roof joists, (on cathedral ceilings very often), this is often associated with condensation on the wall or ceiling surface, and dirt in the air sticking to the condensation. When the condensation dries, you just have the dirt following the stud or joists lines. In the upper left section of our photograph below you can see stains marking the location of ceiling joists.

Ceiling stains over lights: Alan Carson adds that we also see dirt marks on ceilings above light bulbs from chandeliers or other light fixtures using incandescent bulbs. For example, if there are five candelabra type bulbs on a chandelier, we will often see five dark rings on the ceiling above. These relate to airborne dirt being thrust against the ceiling as a result of the thermals that are created by the heat of the light bulb.

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• Thanks to Alan Carson, Carson Dunlop, Associates, Toronto, for technical critique and content suggestions regarding thermal tracking.
• We welcome more thermal tracking, soot tracking, air bypass leaks, and similar photos of indoor stains as well as text suggestions to expand this detail and would be glad to credit contributors.

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