Black Streaks Near Windows and Sockets: Mould or Thermal Ghosting?

Here is a detail that throws a lot of people off: when dark marks appear on a wall in suspiciously neat, evenly spaced lines, building specialists warn that they are often mistaken for toxic indoor mould when in reality they are something else entirely. The reference guide on the subject calls the phenomenon thermal tracking, also known as ghosting or soot tracking, and describes it as a deposit of airborne particles rather than a living growth, as the inspection resource InspectAPedia explains in its overview of thermal tracking.

If you have noticed dark streaks creeping up beside a window, along a corner or fanning out above a radiator in your home in Bettembourg or anywhere across the south of Luxembourg, this article is for you. We will explain what ghosting really is, where the particles come from, the one simple test that tells it apart from mould, and how to clean the marks without making them worse.

Those neat black streaks usually aren't mould

The first instinct, when a dark stain shows up on a wall, is to think "mould". It is a reasonable fear, and damp homes do get mould. But the streaks we are talking about here have a very particular look: they are dry, sooty and remarkably regular. They run in straight vertical lines, cluster in the upper corners of a room, fan out above heat sources and frame windows and sockets almost like a stencil.

That geometric tidiness is the giveaway. Mould is opportunistic and irregular; it spreads wherever moisture and a food source happen to meet. Ghosting, on the other hand, follows the bones of the building. It traces the cold lines created by the timber studs behind the plasterboard, and it dots the wall exactly where the drywall nails and screws sit. Once you know what to look for, the difference is hard to unsee.

Thermal ghosting: why soot sticks to the cold spots

To understand ghosting you have to think about temperature, not dirt. Inside a wall, some areas are colder than others. The timber studs and the metal fasteners conduct heat away from the room more readily than the insulated cavities between them, so the plasterboard is slightly cooler right over those structural lines. Beside a window, in a corner where two outside walls meet, or on an under-insulated patch, the surface also runs colder than the rest of the room.

Fine particles floating in the indoor air are drawn to exactly those colder spots, and several mechanisms work together to make them stick. InspectAPedia lists condensation, electrostatic deposition, "plating out" and thermophoresis as the main culprits: warm, moisture-laden air gives up a little dampness to the cooler surface, the slightly tacky film catches passing particles, an electrical charge difference pulls more in, and thermophoresis nudges the tiniest particles toward the cold. The result is a sooty shadow that maps the temperature of the wall.

This is not just a contractor's hunch. A peer-reviewed study published in Science of the Total Environment, Strojecki's model of particle deposition onto surfaces in buildings, proposes a refined thermophoretic term precisely to capture how large temperature differences between the indoor air and colder surfaces drive particles to settle on them. In plain terms, the science confirms what you see on the wall: particles prefer the cold.

Candles, incense, cooking: where do the particles come from?

Ghosting needs a steady supply of fine particles in the air, and modern homes provide plenty. The usual suspects are all things we burn or heat indoors. InspectAPedia's FAQ on the subject points to scented candles, a fireplace or wood stove, a poorly tuned oil burner and everyday cooking as common particle sources behind these stains.

If you love a scented candle in winter, burn incense, light a wood stove on cold evenings or fry without an extractor running, you are quietly seeding the air with soot and fine dust. None of this is dramatic on its own, but over a season the particles add up and the wall keeps the receipt. Households in the Sud canton that heat with oil or wood through the long Luxembourg winter tend to see ghosting more clearly, simply because there are more particles in circulation for more of the year.

There is also a safety footnote worth knowing. The same FAQ stresses that while the stains themselves are not the hazard, an oil burner producing heavy soot can signal a fault that deserves a heating engineer's attention. So if the streaks appear suddenly and aggressively near a boiler, treat it as a prompt to have the appliance checked, not just the wall cleaned.

The simple test: ghosting or mould?

Here is the quick, practical way to tell the two apart before you reach for any product.

• Look at the pattern. Are the marks geometric, in straight lines or evenly spaced dots about a hand's width apart? That regularity matches the studs and fasteners behind the wall, the signature of ghosting. Mould only rarely tracks building features so neatly.
• Touch it (gently). Ghosting is a dry, sooty deposit, more like a smudge of fine dust. Mould tends to feel damp, fuzzy or slimy, and it often clusters where condensation actually pools, such as the bottom of a cold window reveal.
• Wipe a small area. A dry, sooty deposit will lift onto a damp cloth and smear like soot. A biological growth tends to leave a stain behind and may simply return.
• Check the smell. Mould usually brings a musty, earthy odour. Ghosting is odourless, or carries only the faint smell of smoke or candle wax.

If the marks are dry, regular and odourless, you are almost certainly dealing with thermal ghosting. If they are damp, fuzzy or musty, treat them as mould and address the moisture problem first.

Why poor ventilation makes it worse

Ventilation matters here for two reasons. The first is about the particles themselves: stale, unventilated air keeps soot and dust in suspension longer, giving them more chances to find the cold spots and settle. Open a window after cooking or burning a candle and a good share of those particles leaves with the air instead of plating onto your walls.

The second reason is about the cold spots. When humid indoor air cannot escape, it raises the dew point and makes condensation on cool surfaces more likely, which is exactly the slightly damp film that helps particles stick. Damp, stagnant air also tips the balance toward genuine mould, so good airflow protects you on both fronts. We go deeper into this in our guide on how to ventilate your home properly, which is worth a read if your walls collect marks every winter.

How to clean the streaks the right way

Because ghosting is a surface deposit, it usually cleans up, but technique matters or you can make the wall look worse.

• Test a hidden patch first. Soot can smear and matte paint can burnish, so try a small, out-of-sight area before tackling the visible streaks.
• Dry first, then damp. Lift the loose soot with a dry microfibre cloth or a soft vacuum brush before adding any water, otherwise you turn dust into smears.
• Use a gentle degreaser. A mild dilution of black soap or washing-up liquid in warm water cuts the greasy soot film. Work from the bottom up to avoid drip marks, and rinse with a clean damp cloth.
• Don't scrub hard. Aggressive rubbing polishes the paint and leaves a shiny "clean" ghost that is as visible as the stain. Light, patient passes win.

Cleaning treats the symptom, not the cause. If the streaks return within a few months, the real fix is to cut the particle sources (fewer candles, a serviced burner, an extractor when you cook), ventilate better and, where the cold spots come from weak insulation, look at improving it. A good deep clean of the room is a sensible reset point before you change those habits, and it is the kind of detailed work our residential cleaning service handles for private homes across the south of the country.

When it's more than a cleaning job

Most ghosting is harmless and cosmetic, but a few situations call for more than a cloth. If you cannot confidently tell ghosting from mould, if the marks are damp and musty, or if they keep spreading despite better habits, get a professional eye on it rather than guessing. The same goes for streaks that flare up suddenly around an oil or solid-fuel appliance, which is a signal to have the heating checked for safety.

There is also the practical question of an even, repaint-ready finish. Once you have reduced the sources and improved ventilation, a thorough wall and surface clean removes the embedded film so the room genuinely looks fresh again, which is especially useful before redecorating or at the end of a tenancy. A little preparation goes a long way here, and our guide on getting your home ready before a clean sets out how to make that work pay off.

Sources

• InspectAPedia, Thermal Tracking, Ghosting & Soot Tracking on Walls and Ceilings (accessed June 8, 2026).
• InspectAPedia, Thermal Tracking / Ghosting FAQs: distinguishing ghosting from mould and identifying particle sources (accessed June 8, 2026).
• M. Strojecki, "Modelling particle deposition onto surfaces in historic buildings", Science of the Total Environment (2023) (accessed June 8, 2026).