Black Streaks on Walls: Ghosting or a Moisture Problem?

Black streaks beside a window, above a socket or in a corner can have several possible causes. One is thermal tracking, also known as ghosting, in which airborne particles settle more heavily on certain cooler surfaces. Similar-looking marks can also be associated with moisture, a leak, a heating appliance or another issue within the building.

Appearance alone is not enough to decide which explanation applies. A photograph, a regular pattern or the colour of a mark cannot replace moisture readings and an inspection of the property. This guide explains what to record without touching the area and which professional to contact for the circumstances you observe.

What the pattern can show — and what it cannot prove

The shape and location of the marks are useful details to pass on. Regular lines may coincide with cooler parts of a wall, while diffuse staining may appear in a damp setting. Even so, those observations do not identify the material or its cause. More than one factor may be present, and a hidden building problem may not be visible at the surface.

It is therefore unsafe to assume that a geometric pattern means ghosting, or that every black mark is mould. Reference photographs can help describe the issue, but reliable assessment may require moisture readings and checks of ventilation, heating, thermal bridges and possible water ingress.

Thermal ghosting: one possible explanation

With thermal ghosting, fine airborne particles settle more heavily on some surfaces than on others. Temperature differences, air movement, electrostatic effects and thermophoresis can all play a part. A study of particle deposition in buildings shows that differences between indoor air and surface temperatures can influence where soiling develops.

This makes ghosting a reasonable possibility when marks appear around cooler building elements, but it does not diagnose a particular mark. The property, moisture conditions and possible particle sources must be considered together before reaching a conclusion.

What causes might a professional investigate?

Possible lines of enquiry include particles from cooking, candles or combustion; a heating appliance that needs attention; a thermal bridge or gap in insulation; high indoor humidity; condensation, a leak or water ingress; the condition of the paint; and air movement around the surface. This list only helps you choose the right first contact; it does not identify the cause.

Marks that appear suddenly near a heating appliance or flue should be referred promptly to a qualified heating engineer. Visible dampness, a persistent odour, a known leak or water damage point towards a moisture or building specialist. Repeated lines over cold areas may justify an assessment of insulation and thermal bridges by a building professional.

What to record without testing the mark

Do not wipe, dampen, scrape or touch the area in an attempt to identify it. Wiping cannot reliably distinguish particle deposition from a moisture-related problem, and may remove evidence or damage the finish.

• Photographs: take wide and close views, with the date and a size reference.
• Changes over time: note when the marks appeared, whether they spread, return seasonally or followed a specific event.
• Context: record nearby windows, outside walls, heating equipment, cooking areas, known leaks, condensation or recent building work.
• Other signs: report visible dampness, a persistent odour or discomfort in the room without assigning a cause yourself.

These details help the professional prepare an assessment. They do not turn an observation into a diagnosis.

Ventilation helps, but does not identify the cause

Regular air exchange can help limit indoor moisture and particles in many homes. Our guide on how to ventilate your home properly covers useful everyday routines.

Ventilation cannot repair a leak, a thermal bridge or a faulty heating appliance. Even if the marks develop more slowly after airing, that change does not prove what caused them.

Why to wait before cleaning or redecorating

Cleaning straight away can hide how the mark is changing, spread a deposit or damage delicate paint. Redecorating before the cause is understood and, where necessary, corrected may also lead to the marks returning. The appropriate professional should assess the situation first; any restoration of the surface can then follow that advice.

If the surface is damp, the marks are spreading or a combustion appliance may be involved, avoid improvised cleaning. Keep your photographs and seek the relevant professional advice first.

Which professional should you contact?

• Dampness, a leak or persistent odour: a moisture specialist, leak-detection service or building surveyor, depending on the context.
• A cold wall or suspected thermal bridge: a building, insulation or building-physics professional.
• Sudden marks near heating equipment or a flue: a qualified heating engineer for the appliance concerned.
• Possible biological contamination: a qualified professional to assess the moisture and substrate before restoration.

Fast Clean does not diagnose the source of these marks. The work required depends on the assessment of the building or appliance and any remedial action identified.

Sources

• InspectAPedia, Thermal Tracking, Ghosting & Soot Tracking on Walls and Ceilings (accessed August 10, 2026).
• InspectAPedia, Thermal Tracking / Ghosting FAQs: possible particle sources and assessment questions (accessed August 10, 2026).
• M. Strojecki, "Modelling particle deposition onto surfaces in historic buildings", Science of the Total Environment (2023) (accessed August 10, 2026).