The difference between mould and mildew, and why it matters for removal

Mildew is a surface fungus that sits on top of your wall; mould grows into the wall material itself. Mildew looks like a thin, powdery or slimy coating — usually grey, white, or slightly green — and wipes away easily. Mould appears as darker spots or patches, often black or deep green, and does not come off with a cloth because it has roots in the drywall, plaster, or paint.

This distinction matters because mildew you can treat yourself with basic cleaning supplies. Mould that has penetrated the wall surface usually requires replacing the affected drywall or plaster, and sometimes a professional assessment to rule out structural damage or hidden moisture. If you scrub mould and it returns within days, or if the wall feels soft or crumbly, stop cleaning and call a mould remediation contractor or your local health department for guidance.

Both thrive in moisture and poor air circulation. Before you clean either one, you must address the moisture source — whether that is a leak, condensation, or humidity — or it will return.

Key Takeaways

  • Mildew sits on the surface and wipes away; mould has roots in the wall and requires drywall replacement if it has spread.
  • The moisture source — a leak, condensation, or high humidity — must be fixed or mildew and mould will return within weeks.
  • For mildew, use a 1:10 bleach-to-water solution or a commercial mildew cleaner, explore it, wait 10 to 15 minutes, and scrub with a stiff brush.
  • Wear gloves, eye protection, and a respirator mask rated N95 or higher when cleaning, and may support the room is well ventilated.
  • If mould covers an area larger than 10 square feet, or if you cannot locate and fix the moisture source, contact a professional.

Finding and stopping the moisture source before you clean

Mildew and mould cannot exist without moisture. If you clean the wall but leave the moisture problem untouched, the growth will return in two to four weeks. Before you pick up a brush, identify where the water is coming from.

Check for active leaks: look at the wall from both sides if you can access the space behind it (in a basement, attic, or crawlspace). Feel the wall surface — is it damp to the touch? If the wall is wet or dripping, you have a plumbing leak, roof leak, or foundation crack that needs repair before cleaning makes sense. Call a plumber or roofer depending on the location.

Check for condensation: bathrooms and kitchens produce steam. If mildew appears on walls near a shower, bathtub, or sink, the room may lack adequate ventilation. Run the exhaust fan during and for 20 to 30 minutes after showers or cooking. If there is no exhaust fan, open a window. Wipe down wet walls after bathing. If condensation persists, a dehumidifier can help, but the real fix is moving moisture out of the room.

Check for high indoor humidity: if mildew appears in multiple rooms or on exterior walls in winter, your home may have overall humidity above 50 to 60 percent. A humidity meter costs £10 to £20 and tells you whether the problem is the whole house or one room. Improve ventilation by opening windows on dry days, running exhaust fans, and ensuring your dryer vents outside (not into the attic or crawlspace).

Cleaning mildew with bleach or commercial cleaners

For surface mildew on painted drywall, tile, or grout, a bleach solution is effective and inexpensive. Mix one part household bleach with ten parts water — for example, 250 ml bleach in 2.5 litres of water. Pour it into a spray bottle.

Put on rubber gloves, eye protection, and an N95 respirator mask. Open windows and doors to create cross-ventilation. Spray the affected area generously and let it sit for 10 to 15 minutes. The bleach needs contact time to kill the spores. Scrub with a stiff-bristled brush — an old toothbrush works for small spots, a deck brush for larger areas. Rinse thoroughly with clean water and dry the wall with a cloth or allow it to air dry.

If you prefer not to use bleach, commercial mildew cleaners like Dettol Mould & Mildew Remover or Cillit Bang Mould & Mildew work similarly: spray, wait 10 to 15 minutes, scrub, and rinse. Follow the product instructions for contact time and dilution. These products are often less harsh on your lungs than bleach, though ventilation is still essential.

Do not mix bleach with ammonia, vinegar, or other cleaners — the combination produces toxic chlorine gas. Use one product at a time, rinse the wall thoroughly between products, and ventilate the room for at least 30 minutes after cleaning.

When to replace drywall instead of cleaning

If mould has penetrated the drywall — meaning the spots do not wipe away, the wall feels soft or spongy, or the affected area is larger than 10 square feet — cleaning will not solve the problem. The drywall itself is compromised and must be removed and replaced.

Mark out the affected area with a pencil. You will need to cut out the mouldy section, remove the insulation behind it if it is also damp, and install new drywall. This is a job for a carpenter or general contractor unless you have experience with drywall installation. The cost varies depending on the size of the area and your location, but expect to pay for labour, new drywall sheets, joint compound, tape, and paint.

Before replacing drywall, the moisture source must be fixed. Otherwise, mould will grow on the new drywall within weeks. A contractor should inspect the wall cavity to check for mould in the insulation or framing. If the framing is affected, it may need treatment or replacement as well.

Protecting yourself while cleaning: gear and ventilation

Mould and mildew spores are irritants. Inhaling them can trigger coughing, wheezing, and allergic reactions, especially in people with asthma or compromised immune systems. Protect yourself with the right equipment.

Wear an N95 or FFP2 respirator mask — not a dust mask, which does not filter spores. An N95 blocks at least 95 percent of airborne particles. Fit it snugly over your nose and mouth. Surgical masks do not provide adequate protection.

Wear rubber or nitrile gloves to avoid skin contact with bleach or mould spores. Wear eye protection — safety glasses or a face shield — because bleach splashes can damage your eyes. Long sleeves and trousers reduce skin exposure.

Open all windows and doors in the room and any adjacent spaces. If you have a window fan, place it in the window to pull air out of the room. Do not use a ceiling fan or portable fan inside the room — it will spread spores throughout the space. Keep the room ventilated for at least 30 minutes after you finish cleaning.

Preventing mildew and mould from returning

Once you have cleaned the wall and fixed the moisture source, keep conditions dry to prevent regrowth. In bathrooms, run the exhaust fan during showers and for 20 to 30 minutes afterward. Wipe down shower walls and the tub surround after bathing. Leave the bathroom door open when not in use to allow air to circulate.

In kitchens, run the exhaust fan while cooking and for 15 minutes after. Wipe down splashes and steam from walls and windows. may support your dryer vents to the outside, not into the attic or a crawlspace.

Keep indoor humidity between 30 and 50 percent. A dehumidifier helps in damp basements or during humid seasons. In winter, when outdoor air is dry, opening windows briefly on clear days can reduce indoor humidity without losing too much heat.

Check the cleaned wall monthly for the first few months. If mildew reappears, the moisture source was not fully addressed. Return to the steps above and look for a leak or ventilation problem you may have missed.

When to call a professional

Contact a mould remediation specialist or your local environmental health department if:

  • Mould covers an area larger than 10 square feet.
  • The mould returns within two weeks of cleaning, suggesting an unresolved moisture problem.
  • The wall feels soft, crumbly, or wet, indicating the mould has penetrated deep into the structure.
  • You cannot locate the moisture source after checking for leaks, condensation, and humidity.
  • You have asthma, a weakened immune system, or respiratory sensitivity and are concerned about exposure.

A professional can identify hidden leaks using moisture meters, assess whether framing or insulation is affected, and determine whether the mould is a health risk that requires containment during removal. They also know local regulations — some areas require professional remediation for mould above a certain size.

Frequently Asked Questions

Can I use vinegar or hydrogen peroxide instead of bleach?

Yes. White vinegar (5 percent acetic acid) and hydrogen peroxide (3 percent) both kill mildew spores. Spray either one on the affected area, wait 10 to 15 minutes, scrub, and rinse. They are less toxic than bleach and do not produce harmful fumes, but they are also slower-acting and less effective on heavy mildew. Vinegar leaves a smell that fades over hours.

Is black mould dangerous?

Black mould (often Stachybotrys) is not inherently more toxic than other moulds, despite its reputation. All moulds produce spores that can irritate the respiratory system, especially in people with asthma or allergies. If black mould covers a large area or you have respiratory concerns, have it assessed by a professional rather than cleaning it yourself.

How long does it take for mildew to come back after cleaning?

If the moisture source is fixed, mildew should not return. If it reappears within two to four weeks, the moisture problem is still present. Check for leaks you may have missed, may support exhaust fans are running, and consider a dehumidifier if humidity is high.

Can I paint over mildew to hide it?

No. Paint does not kill mildew spores and will peel or bubble as the mildew continues to grow underneath. Clean and dry the wall completely, fix the moisture source, and then paint. Use a mildew-resistant paint in bathrooms and kitchens if you want added protection.

What should I do if mildew is growing on the ceiling?

Ceiling mildew usually signals a roof leak or poor attic ventilation. Check the attic for water stains, wet insulation, or condensation on the underside of the roof. If you find a leak, it must be repaired before cleaning the ceiling. If the attic is straightforward poorly ventilated, improve airflow by adding soffit and ridge vents or a powered attic fan. Clean the ceiling mildew only after the moisture source is addressed.