1) What “black mould around window frames” usually means

Most common: condensation

Warm indoor air holds moisture from showers, cooking, drying clothes, and even breathing. When that moist air touches cold surfaces (glass, frames, and the plaster “reveal” around the window), it cools and water forms. Mould then grows on the damp film (often feeding on dust and paint).

Also common: rainwater ingress from outside

Cracked/missing sealant, damaged pointing/render, or leaking gutters/downpipes above a window can allow wind-driven rain to track into the opening. This can look similar to condensation at first, but it behaves differently (use the decision tree below).

2) Quick Decision Tree: What’s Actually Causing the Mould?

Use this “if this, then that” flow to identify whether you’re dealing with condensation, rainwater ingress, or a hidden internal leak.

1. Step 1: Do you regularly get water droplets on the glass (especially in the morning)?

   • Yes → go to Step 2 (condensation likely).
   • No → go to Step 4 (ingress/other source more likely).

2. Step 2: Is it worse in winter/cold snaps, mainly on cold edges/corners of the frame/reveal?

   • Yes → Condensation is the primary cause (see Common Causes + Stop It Coming Back).
   • No → go to Step 3 (could be mixed causes).

3. Step 3: Does it flare up after showers/cooking/drying clothes indoors?

   • Yes → Condensation driven by indoor humidity (extraction/ventilation/moisture load issue).
   • No → Mixed cause likely (condensation + cold bridging/draughts, or a small external leak).

4. Step 4: Does it get noticeably worse after rain or wind-driven rain?

   • Yes → Rainwater ingress likely (sealant/pointing/render/gutter/downpipe).
   • No → go to Step 5.

5. Step 5: Is plaster soft/blown, paint bubbling, or staining spreading beyond the window edge?

   • Yes → Ingress or persistent saturation likely (inspect/repair before decorating).
   • No → go to Step 6.

6. Step 6: Is the damp area near a radiator pipe, bathroom wall, or plumbing run?

   • Yes → Hidden plumbing leak possible (check valves, rad tails, pipework, bathroom seals).
   • No → Condensation on a cold/poorly ventilated window area remains the most likely cause.

3) Why it happens around windows specifically

Thermal bridging at reveals and lintels

The wall around a window opening is often less insulated than the rest of the wall. That area becomes a colder strip, so moisture condenses there first (especially on the plaster reveal).

Air leakage and draughts around frames

Tiny gaps at trims, plaster lines, or around the frame allow cold air to cool the inside edge. Colder surface = more condensation.

Reduced airflow behind curtains/blinds

Tight curtains/blinds create a still, cold pocket of air next to the glass. Moisture can’t disperse and condenses along the edges, feeding mould around the frame line.

4) Common Causes Around Window Frames (UK Homes)

Even when the headline cause is “condensation”, it’s usually triggered by one (or several) of the issues below. Work through these — they’re the most common real-world reasons mould forms around window frames and reveals.

Condensation triggers (most common)

• High indoor humidity: long showers, frequent cooking, drying clothes indoors, lots of occupants in a small space.
• Weak extraction: bathroom fan not effective, rarely used, or not venting outside properly.
• Trickle vents closed/blocked: humidity stays trapped and gets dumped onto cold windows.
• Cold bridging: uninsulated reveals/lintel areas run colder than nearby walls.
• Intermittent heating: cold room + warm humid air = quick condensation on cold surfaces.
• Curtains tight to glass: poor airflow behind them increases edge condensation.
• Furniture blocking airflow: wardrobes/tall units near windows create stagnant cold zones.

Window-specific issues (often overlooked)

• Blocked weep holes (uPVC): drainage channels can clog with dirt/paint, keeping areas damp.
• Mould on silicone: in bathrooms/kitchens the silicone often stains deeply and may need replacing.
• Draughts at the frame-to-plaster line: concentrated mould in a thin strip can indicate edge cooling.
• Cold glazing: older/poorer glazing runs colder, increasing condensation risk.

External moisture (rain ingress) causes

• Failed external sealant: cracked, missing, or detached sealant lets rain track in.
• Cracked render/pointing: especially above the window; water finds the easiest path inside.
• Leaking/overflowing gutters/downpipes: even small drips can saturate the area over time.
• Damaged sill/cill or gaps beneath the frame: water can track under and show internally near corners.

Timber frame note (if your frames are wood)

Timber frames can hold moisture if paint fails or joints open. If you see peeling paint, blackening, softness, or staining at bottom corners, treat it as both a mould problem and a wood repair/protection problem (clean, dry, repair, prime, repaint).

5) How to stop it coming back (the real fix)

The goal is to stop moisture sitting on cold surfaces long enough for mould to grow. In practice that means controlling humidity, improving air movement, and reducing cold spots — plus fixing any external defects.

Step 1: Reduce moisture at the source

• Use extractor fans properly: during showering/cooking and for a good period afterwards.
• Keep trickle vents open (if present) and avoid blocking vents/air bricks.
• Contain steam: close kitchen/bathroom doors while producing moisture.
• Be careful drying clothes indoors: ventilate hard or dry elsewhere if you can.

Step 2: Keep the window area warmer

• Avoid big temperature swings: consistently cold rooms are mould magnets.
• Improve airflow at the window: don’t trap curtains tight to the glass overnight.
• Longer-term options: reveal insulation, secondary glazing, or improving overall insulation/ventilation balance.

Step 3: If it’s rain ingress, fix the defect first

• Replace failed external sealant correctly (don’t just smear more over the top).
• Repair cracked pointing/render around the opening.
• Fix leaking/overflowing gutters and downpipes above the window.

6) Cleaning & Aftercare (so it doesn’t re-seed and return)

Cleaning removes visible growth, but aftercare stops leftover spores and damp films from re-establishing. Use this approach for small to moderate areas around frames and reveals.

Before you start (safety + setup)

• Ventilate the room (open window and/or run extractor).
• Wear gloves; consider a mask if sensitive or the area is dusty.
• Use disposable cloths/paper towels where possible (bag and bin afterwards).
• Keep kids/pets away until surfaces are dry and the room is aired.

Step-by-step: frame + reveal cleaning

1. Pre-wipe gently (damp cloth): removes dust/grime that mould feeds on.
2. Apply a mould remover / fungicidal wash: follow label instructions for dwell time.
3. Wipe away residues: work top-to-bottom; replace cloths often to avoid smearing.
4. Dry thoroughly: wipe dry, then allow airflow/heat until fully dry to the touch.

If the mould is on silicone sealant

• Light staining: cleaning may work if humidity is controlled.
• Deep black staining or repeated return: cut out and replace silicone after the area is cleaned and dry.

If the mould is on painted plaster (reveals)

Plaster is porous and can hold moisture and staining. If redecorating, use the right order:

1. Clean/treat mould and allow the area to fully dry.
2. Apply a suitable stain-blocking primer where discolouration remains.
3. Repaint with a durable finish. “Anti-mould” paints help most when humidity is already controlled.

Aftercare: the first 7–14 days (the make-it-stick period)

• Wipe morning condensation daily if you still get droplets on the glass.
• Run extraction properly during and after showers/cooking.
• Keep background ventilation going (trickle vents open, regular air changes).
• Inspect weekly for early specks and deal with them immediately.

7) What NOT to Do (mistakes that make it come back)

These common “quick fixes” either spread mould, hide the problem, or make condensation worse. Avoid them if you want a long-term result.

• Don’t paint over mould (even with “anti-mould paint”) without cleaning/treating it first.
• Don’t wipe with water and leave it damp — drying is critical.
• Don’t dry brush/sand mouldy areas while dry — it can spread spores.
• Don’t block ventilation (trickle vents, extractors, air bricks) to “stop draughts”.
• Don’t rely on a dehumidifier alone if you suspect an external leak.
• Don’t over-seal internally as a “damp fix” — it can trap moisture and move the problem elsewhere.
• Don’t ignore mould in silicone — if it’s deeply stained, replacement is often the real fix.
• Don’t mix cleaning chemicals — use one system and follow the label instructions.
• Don’t keep curtains tight to cold glass all night if you wake up to wet windows.

8) When to escalate (and when it’s a landlord issue)

Get professional help if:

• The mould covers a large area or returns quickly despite improving ventilation and cleaning properly.
• You see significant plaster damage (soft/blown/crumbling), or clear “after rain” patterns.
• There’s a suspected external defect (sealant/pointing/gutters) you can’t safely inspect or repair.
• There are health concerns (asthma/respiratory issues, very young/elderly occupants).

If you rent

Report it. A “wipe and repaint” isn’t a real fix if the underlying cause remains (ventilation provision, leaks, defects, cold bridging). Photograph the area, note when it worsens (morning vs after rain), and keep basic humidity notes if you can.

Helpful sources (UK)

• RICS: Damp and mould (consumer guide)
• NHS Inform: Damp and mould indoors
• Citizens Advice: Damp repairs and responsibility