Black mould vs mildew and other mould: what is the difference?

What people mean by black mould

In everyday British use, black mould means the dark grey to black spotting that appears on bathroom sealant, window reveals, the corners of ceilings and behind furniture against cold outside walls. Several different moulds can look like this. Laboratories identify species by growing samples and looking at them under a microscope, not by colour.

The term toxic black mould is often linked online to one particular species. You cannot tell from a glance whether that species is present, and home test kits give limited information. For a home, the practical message is the same whatever the species: deal with the growth and the moisture behind it promptly.

What mildew means

Mildew is an older, informal word for mould that grows flat on a surface, often white, grey or pale green, and sometimes powdery. You see it on damp fabrics, shower curtains, leather in a damp wardrobe and the surface of paint in a steamy room. Because it sits on the surface, it often wipes off more easily than growth that has worked into sealant or porous plaster.

In gardening, mildew also refers to specific plant diseases, which is part of why the word is confusing. Indoors, treat mildew as early-stage or surface mould.

White fuzz that is not mould

White crystals on a masonry wall are often salts carried out of the brick as water evaporates, known as efflorescence. They brush off as powder and do not grow. This points to water moving through the wall, which is a different problem from surface condensation. Our guide on signs of damp walls explains how to tell these patterns apart.

Why the fix is the same

Mould needs a damp surface to grow. In most UK homes that dampness comes from condensation, where warm moist indoor air meets a cold wall or window and drops below its dew point. Leaks and penetrating or rising damp are the other main causes.

Cleaning the growth only helps for a while if the surface stays wet. The order that works is: find and fix the moisture source, clean the growth, then keep humidity down. A dehumidifier set to around 50 to 60 % helps in rooms that stay damp, and better ventilation and steady heating help keep surfaces above the dew point.

See dehumidifiers for condensation →

Cleaning small areas safely

For small patches on hard surfaces, follow the instructions on a mould remover product, or the method in our guide on how to remove black mould. Wear gloves and eye protection, ventilate the room and never mix cleaning products, especially bleach with other cleaners. Bag and bin heavily affected soft items you cannot wash.

If growth covers a large area, keeps coming back after cleaning, or is linked to a leak or structural damp, get it looked at by a qualified damp surveyor or, if you rent, report it to your landlord. Our guide to damp and mould in a rented home covers the steps.

Note:Never mix bleach with other cleaning products. Some combinations release harmful gases.

Frequently asked questions

Is black mould different from normal mould? Black mould is a common name for dark mould growth, which can be several different species. You cannot identify the species by colour, and the way to deal with it is the same as other household mould.

What does mildew look like? Usually a flat, white or grey powdery film on fabrics, painted surfaces or shower curtains. It tends to wipe off more easily than mould that has grown into sealant.

Can I test which mould I have? Home kits exist but give limited information. Laboratories identify species from samples. For most homes the action is the same whatever the type: fix the moisture and clean the growth.

Why does mould come back after cleaning? Because the surface is still getting wet. Bring humidity down and stop condensation or leaks, or it will regrow.