If you’re seeing black mould on paint, grout, silicone or window reveals, you’re dealing with a moisture-plus-cold-surface problem. Below you’ll find safe, proven cleaning methods, a step-by-step process for different surfaces, and the long-term fixes that keep mould away in UK homes.
What is black mould and why does it appear?
“Black mould” is a common term for visible mould growth—often dark green-black—on cold or damp surfaces. It thrives where warm, humid air contacts a cooler surface (external corners, window reveals, behind furniture, around showers). Bathrooms, kitchens and north-facing rooms are typical hotspots. Fixing the cause is as important as cleaning the stain: improve ventilation, reduce moisture, and limit cold surfaces.
Before you start: safety, prep & PPE
PPE & room prep
- Nitrile gloves, eye protection, and a properly-fitting mask (P3 or Half Mask Respirator preferred).
- Open windows for cross-ventilation; keep doors closed to limit spread.
- Protect floors with dust sheets. Remove or cover soft furnishings.
- Have two clean microfibre cloth piles: “apply” and “wipe-off”.
- Use a dedicated bucket for contaminated cloths and dispose of them safely.
General approach
- Treat small areas first (patch-by-patch) to avoid spreading spores.
- Work from clean to dirty. Change cloths frequently.
- Never dry-brush mould (it aerosolises spores). Always wet-clean.
- Allow sufficient contact time (per product label) before wiping.
Best cleaners for black mould (pros & cons)
Several products can remove the staining. Biocidal products are best for killing the mould and its spores. Household options can help on light growth but may not prevent regrowth if the underlying moisture problem continues.
| Cleaner | Where it shines | Pros | Cons / cautions |
|---|---|---|---|
| Biocidal mould wash (ready-to-use or concentrate) | Painted walls, ceilings, tiles, window reveals | Designed to kill mould/spores; reduces regrowth; usually wipe-on, leave, then remove | Follow contact time; may need repainting if staining has penetrated |
| Hydrogen peroxide (3–6%) | Light-to-moderate growth on non-porous and some porous surfaces | Good oxidiser; less odour than bleach; breaks down to water/oxygen | Can lighten dyes; test first; observe contact time; store safely |
| White vinegar (undiluted) | Light growth, bathrooms/kitchens | Household staple; helps disrupt biofilm | Strong odour; can etch some stones/grouts; not as effective on deeper colonies |
| Borax solution (~1 tbsp per 250 ml warm water) | Tiles, grout, painted masonry | Cleans and leaves a mild residue that inhibits some regrowth | Keep from children/pets; rinse residues on food-contact areas |
| Steam cleaner (with care) | Tiles, glass, silicone, hard bathroom surfaces | Heat helps dislodge film on non-porous surfaces | Not for painted plaster or moisture-sensitive materials; condensation risk |
| Bleach-based sprays | Tile grout and silicone only | Visibly whitens stains quickly | Often bleaches rather than kills; odour; can damage finishes; avoid on porous paint/plaster |
Step-by-step removal
1) Painted walls & ceilings (emulsion on plaster)
- Prep: Ventilate. Put on PPE. Lightly mist the area with clean water to avoid spore drift.
- Apply cleaner: Use a biocidal mould wash per label instructions. Flood the surface enough to wet the patch and a 5–10 cm margin.
- Contact time: Leave for the specified period. Don’t rush—this is when spores are neutralised.
- Wipe: Use disposable or washable microfibres. Replace cloths as they soil.
- Repeat if needed: Staining may need a second pass. Allow to dry fully.
- Prime & repaint (if stained): Once dry, spot prime with a stain-blocking primer, then repaint. Consider adding an anti-mould additive to the emulsion.
2) Tile grout & ceramic surfaces
- Apply: Use a biocidal cleaner or, where whitening is needed, a careful application of a bleach-based gel on grout lines.
- Dwell: Let it sit per instructions. Keep it wet; reapply rather than scrubbing dry.
- Agitate: Use a nylon brush (avoid metal). Rinse thoroughly.
- Seal: When fully dry, apply a grout sealer to slow future penetration.
3) Silicone around baths, showers & windows
- Assess: If mould is deep inside the bead, surface cleaning won’t last.
- Remove: Cut out old silicone carefully with a plastic scraper and knife.
- Clean the joint: Degrease, treat edges with biocidal wash, allow to dry.
- Re-seal: Apply a high-quality sanitary silicone (mould-resistant). Let it cure fully before use.
4) Window reveals, frames & trickle vents
- Treat: Wash with biocidal cleaner; use cotton swabs for rebates and vents.
- Dry: Keep reveals dry and ensure trickle vents aren’t blocked.
- Thermal upgrade (optional): Consider insulating liners or thermal plasterboard at persistently cold reveals.
5) Behind furniture & wardrobes
- Pull away: Move furniture 50–100 mm from cold external walls.
- Clean: Treat the wall with biocidal wash; allow to dry.
- Airflow: Add low-profile spacers or slatted backs; avoid cramming clothes tight.
What to avoid
- Dry brushing or sanding mould—this spreads spores.
- Over-reliance on bleach on porous paint or plaster—it can bleach the stain but leave viable spores.
- Mixing chemicals (e.g., vinegar with bleach) due to hazardous gases.
- Soaking plaster with steam or excessive water—risk of damage and deeper penetration.
How to keep mould from returning (the long-term fix)
1) Reduce indoor moisture
- Use extractor fans (with over-run) in bathrooms and kitchens.
- Keep lids on pans, and avoid drying clothes indoors where possible. If you must, use a heated airer with ventilation or a dehumidifier.
- Deal promptly with leaks (roofs, gutters, overflows, seals).
- Maintain background ventilation (open trickle vents, occasional purge opens).
2) Limit cold surfaces
- Maintain a steady, modest background heat in winter to keep surfaces warmer.
- Improve insulation where feasible; use thermal liners at cold corners/reveals.
- Re-arrange rooms to allow airflow around external walls.
3) Finish with protective coatings
- Add an anti-mould additive to emulsion for problem rooms.
- Consider anti-condensation paint on cold bridges to buffer moisture and raise surface temperature.
- Seal grout after cleaning; use sanitary silicone with mould inhibitors.
Helpful products (DIY Refurb / Biokil Crown range)
- Biocidal Mould Wash (ready-to-use or concentrate) — kills mould and spores on walls, ceilings, and reveals. View Here.
- Anti-Mould Paint Additive — mix into emulsion to inhibit regrowth. View Here.
- Anti-Condensation Paint — for cold corners, external walls, and window reveals. View Here.
- Sanitary Silicone (mould-resistant) — for long-lasting bathroom seals. View Here.
- Grout Sealer — protects cleaned grout from rapid re-soiling. View Here.
- Dehumidifier (portable) — helps dry rooms and laundry in winter. View here.
Use biocides safely: always read the label and product information before use.
FAQs
Is black mould always dangerous?
Any visible mould indicates a moisture problem and can irritate airways. Clean it safely and address the underlying cause. Seek advice if anyone at home is vulnerable.
Will bleach fix black mould?
Bleach can whiten stains on non-porous surfaces like tiles and silicone, but it may not kill mould within porous paint/plaster. Prefer a biocidal mould wash for those areas.
Do I need to repaint?
Not always. If staining remains after cleaning, spot-prime with a stain blocker and repaint, ideally with an anti-mould additive in the finish coat.
Why does it come back each winter?
Cold weather increases condensation on external walls and windows. Balance heating and ventilation, reduce indoor moisture, and protect cold surfaces with suitable coatings.
