Dark marks or stripes on your ceiling: what they mean

Dark lines running across a ceiling in evenly spaced stripes, or dark rings around downlights, are almost always ghosting — fine airborne particles settling on the coldest, dampest parts of the ceiling. It looks like mould and gets treated like mould, and that is why it keeps coming back. Ghosting is a thermal problem. Clean it without addressing the cold line beneath it and it will return in the same place next winter.

The stripes are not random. They follow the ceiling joists, because the timber conducts heat more readily than the insulated cavity either side of it. That regularity is the tell, and it is what separates this from a mould problem or a leak.

What is actually happening

Three things combine.

A cold surface. Wherever a solid element bridges from the roof space to the ceiling lining — a joist, a batten, a steel member, the edge of a downlight housing — heat escapes faster. That strip of plasterboard runs a degree or two colder than the surface either side of it.

A thin film of moisture. The colder surface reaches the dew point sooner, so a microscopically thin layer of condensation forms on it. Not enough to see or feel. Enough to make the surface slightly tacky.

Airborne particles. Every house has soot, dust, cooking oils, candle smoke and general fines moving through the air. On a dry wall they bounce off. On a slightly damp cold strip they stick.

Repeat that nightly through a winter and the deposit becomes visible. The pattern maps the thermal structure of your ceiling exactly.

Why the rings around downlights

Downlights create their own version of the same effect, and in Australia they are everywhere.

A recessed fitting is a hole in the ceiling. It requires clearance from insulation for fire safety, so there is a deliberate gap in the insulation blanket around each one. That gap is a cold spot. The fitting itself also generates heat and drives a small convection current, drawing air up through and around the housing, which delivers a steady supply of particles to the cold ring surrounding it.

The result is a dark halo around each light. On a ceiling with a dozen downlights, twelve of them.

How to tell it from mould

This is the distinction that decides what you do next, and getting it wrong costs money.

Ghosting is geometric. Straight parallel lines at regular spacing, or neat circles around fittings. It follows the structure.

Mould is organic. Blotchy, irregular, spreading outward from a source. It concentrates in corners, behind furniture, in wardrobes and in wet areas rather than in stripes across an open ceiling.

Other differences worth checking: ghosting has no smell, while mould is usually musty. Ghosting is a dry, sooty deposit that smears when wiped; mould is often slightly slimy when damp. Ghosting appears in cold months and stops in warm ones. And ghosting wipes off relatively easily but returns in exactly the same pattern — which is the single most reliable indicator.

If the marks form a grid, they are almost certainly not mould.

What is feeding it

The particles have to come from somewhere, and reducing the supply is half the fix.

• Candles, particularly scented and paraffin ones, which produce a great deal of fine soot.
• Open fireplaces and wood heaters, especially where the flue draws poorly.
• Unflued gas heaters, which release combustion products directly into the room — and moisture along with them, which makes them a double contributor.
• Gas cooking without effective extraction.
• Smoking indoors.
• An attached garage, where exhaust fumes migrate into the house.
• General household dust, which is enough on its own in a house with pronounced cold bridging.

The Australian specifics

Two things make this more common here than the overseas literature suggests.

Downlight prevalence. Recessed lighting is close to a default in Australian housing, and each fitting creates a mandated gap in the insulation. A ceiling with many downlights has many cold spots by design.

Roof cavity dust. Australian roof spaces, particularly under older unsarked tile roofs, accumulate a large volume of fine dust over decades. Every ceiling penetration — downlights, exhaust fans, manholes, cornice gaps — is a route for that dust to move into the room below, drawn by the pressure differences that normal heating creates.

Add an exhaust fan discharging into the roof cavity and you have the moisture supply as well — see where does your bathroom exhaust fan actually vent.

What to do about it

In this order. Cleaning first is the common mistake.

1. Confirm it is ghosting rather than mould, using the pattern test above. If it is irregular and smells musty, treat it as a moisture problem instead.
2. Get into the roof space with a torch and look directly above the marks. You are looking for gaps in the insulation, batts that have been pushed aside, insulation that stops short at the edges, and areas that have been disturbed by anyone working up there.
3. Check the insulation coverage. Batts butted tightly together with no gaps between them, and continuous coverage right out to the wall plate. Gaps at the perimeter are extremely common and they are exactly where the ceiling runs coldest.
4. Check where exhaust fans discharge. If they terminate in the roof cavity, that is a moisture source sitting directly above the affected ceiling.
5. Seal the ceiling penetrations. Sealed downlight fittings or covers, damper-fitted exhaust fans, and a gasketed manhole cover. This reduces both the airflow and the particle supply. Anything involving light fittings is work for an electrician, and clearances around fittings exist for fire safety.
6. Reduce the particle sources. Fewer candles, better extraction over the cooktop, and getting advice from a licensed gasfitter about any unflued gas heater.
7. Manage the humidity. Ventilate deliberately, extract at source, and do not dry washing indoors. Less moisture means no film for the particles to stick to.
8. Then clean. Once the cold spot and the moisture are addressed, wash the ceiling with a mild detergent, and repaint if the staining has set into the paint film.

Why cleaning alone always fails

The marks come off readily, which makes cleaning feel like a solution. But nothing about the ceiling has changed — the joist still conducts, the insulation gap is still there, the downlight is still surrounded by a cold ring.

The following winter the same strips run cold, the same film forms, and the same particles land. People conclude they have persistent mould, escalate to remediation, and spend real money on the wrong problem.

The stripes are a map of your insulation. Read them rather than washing them off.

Common questions

Is ghosting harmful?

The deposit itself is soot and dust rather than a biological growth, so it does not carry the health considerations mould does. What it indicates — cold bridging, insulation gaps and elevated indoor humidity — is worth attending to on its own merits. If you are unsure whether you are looking at ghosting or mould, and anyone in the household has asthma or a respiratory condition, your state health department publishes guidance on mould in homes.

Why did it appear this year and not before?

Usually because something changed. New downlights, insulation disturbed by a tradesperson working in the roof space, a new heater, a colder than usual winter, or a change in how the house is ventilated.

Does it mean my insulation is bad?

Not necessarily bad, but it means it is not continuous. Thermal bridging at joists happens to some degree in every ceiling; pronounced ghosting suggests the insulation is thin, gapped, compressed, or has been displaced.

Will more insulation fix it?

It helps if the current coverage is inadequate, but continuity matters far more than depth. Perfectly laid batts with a gap at every downlight will still ghost around the lights.

Can I just paint over it?

You can, and it will look fine until the following winter. Painting without addressing the cold spot buys you one season.

Why is it worse in some rooms?

Rooms with more downlights, more candles, an open fire, or higher humidity. Bedrooms and living rooms with recessed lighting are the usual worst cases.

Where to go next

Ghosting is one of the few faults that draws you a diagram of the problem on your own ceiling.

Read next: where does your bathroom exhaust fan actually vent, or winter condensation and mould if you are still not certain which of the two you are looking at. Or start at Preventative Maintenance for the wider picture.

Those stripes are the shape of your ceiling joists, drawn in dust. Fix what makes them cold and they stop appearing.