Mold vs Mildew: What's the Difference?

Mold vs mildew comes down to this: mildew is a kind of mold. In everyday use, mildew means flat, powdery, grey or white growth that sits on a damp surface and wipes off, while mold is used for fuzzy or slimy growth in darker colours that gets into the material. There is no strict scientific line between them, and both mean the same thing about your house, which is that something is staying damp.

Mold vs mildew: the plain difference

Both are fungi. Both grow from spores that are always in the air, and both need moisture and something organic to feed on. Scientists do not sort household growth into two neat groups called mold and mildew. The word mildew has a precise meaning only in plant science, where powdery mildew and downy mildew are specific diseases of crops and garden plants. The white coating on squash leaves or lilacs in late summer is that kind of mildew, and it does not move into your house.

Indoors, mildew is a household word. Cleaning product labels, paint cans and your grandmother all use it for the early, shallow stage of mold growth on a surface. So when someone asks whether they have mold or mildew, what they are really asking is whether the problem is on the surface or in the material. That question does matter.

What mildew usually looks like

• Flat against the surface, with no real thickness
• Powdery or dusty in texture
• White, grey or pale yellow at first, sometimes darkening with age
• Found on shower grout and caulking, window sills, shower curtains, damp fabric, paper and leather
• Comes off with a wipe and ordinary cleaner, leaving a sound surface behind

What people usually mean by mold

• Raised growth that looks fuzzy, velvety or slimy
• Green, black, brown, blue-green, and sometimes white or orange
• Irregular patches that spread outward, often with staining around them
• Found on drywall, wood framing, subfloor, attic sheathing, carpet backing and stored cardboard
• Leaves a stain, a soft spot or damaged material after wiping, or grows back quickly

Colour and texture are rough guides only. A white fuzzy growth on basement joists is still mold, and a dark flat stain on a tub surround may wipe off in seconds. You cannot identify a species by looking, and for household purposes you do not need to.

Is mildew less of a health concern than mold?

Health Canada does not make a distinction. Its guidance is that mold growing indoors should be removed regardless of type, since any of it can irritate the eyes, nose and throat and aggravate asthma and allergies in people who are sensitive. In practice, a bit of mildew on shower grout is a very small amount of growth and a routine cleaning job. The amount of growth and how long people are around it are what change the picture, and the name does not. Anyone with ongoing symptoms should see a doctor.

How to tell a surface problem from a deeper one

This is the check that decides what you do next. Wearing gloves, try the following.

1. Wipe it. Use a damp cloth with a little dish detergent. If the growth comes off completely and the surface underneath looks and feels normal, it was on the surface.
2. Press it. Push on the spot with a finger or the handle of a screwdriver. Drywall that is soft or crumbly, or wood that dents easily, has been wet for a while.
3. Look at the material. Tile, glass, metal and glossy paint do not let growth in. Bare drywall paper, raw wood, fabric and cardboard do.
4. Look at the location. A cold window sill or a shower corner points to condensation and surface growth. The base of a wall, a ceiling under a bathroom, or the back of a closet on an outside wall points to water coming from inside or behind.
5. Give it two weeks. Surface growth that was cleaned and kept dry stays gone. Growth that returns in the same shape is being fed from behind.

Cleaning mildew, and when it is more than mildew

For surface growth, scrub with water and dish detergent, rinse, and dry the area. Then change the conditions. Run the bath fan during showers and for 20 to 30 minutes after. Squeegee or wipe down the shower walls. Wash or replace the shower curtain. In winter, wipe condensation off window sills and keep indoor humidity around 30% so less of it forms. In a London summer, a basement dehumidifier set to about 50% does more than any cleaner.

Treat it as more than mildew if the patch is larger than about a square metre, if there are several patches, if the material is soft or stained through, if there is a musty smell with no visible source, or if it follows a leak or flood. At that point the useful work is finding out how wet the material is and where the water comes from. A moisture meter answers that quickly. If the wall reads dry and the growth wiped off, you had mildew and you are done. If it reads wet, the name on the stain is the least important part of the problem.

Common questions