Mold vs mildew: what is the difference?

Two words for the same kind of thing

Scientifically, there is no clean line between the two. Mold and mildew are both fungi that grow where there is moisture and something to feed on. In everyday use:

• Mildew usually means thin, flat growth on a surface. It is typically grey, white or light brown, sometimes black in showers, with a powdery or downy look. Gardeners use the word for particular plant diseases, which is where it comes from.
• Mold usually means thicker, fuzzy or slimy growth, often green, black or blue-green, that gets into the material it is growing on.

Cleaning product labels, home inspectors and insurers use the words loosely. Health Canada’s guidance does not distinguish them. It treats all indoor mold growth the same way: it should be removed and the moisture fixed, whatever the type or colour.

A more useful distinction

Instead of asking which it is, ask three questions.

1. Is it on the surface or in the material?

Growth on tile, glass, metal, glazed surfaces and sound paint sits on top and wipes off. Growth in drywall, carpet, ceiling tiles, insulation and unfinished wood has roots in the material.

2. How big is the area?

Health Canada describes a small area as three or fewer patches with a total area under one square metre, a medium area as more than three patches or patches between one and three square metres, and a large area as a single patch over three square metres. Small areas can generally be cleaned by a householder with basic precautions. It advises professional help for large areas, for mold that returns after cleaning and for mold hidden inside walls or ceilings.

3. Why is it damp?

Condensation from showers and cooking is one kind of problem. A leak, a flood or water in the walls is another, and usually means there is more growth than you can see.

What people usually mean by mildew

• Black or pink marks on shower caulk and grout
• Grey spots on a bathroom ceiling
• Black dots on window frames and sills in winter
• A powdery film on stored leather, books or fabric in a damp basement
• Grey marks on outdoor cushions, awnings and canvas
• A musty smell in towels or a front-loading washing machine

All of these come from surfaces staying damp. They are common, mostly cosmetic, and manageable with cleaning and better drying.

What people usually mean by mold

• Fuzzy or blotchy growth on drywall after a leak
• Black or green patches spreading on a basement wall
• Growth on the underside of a roof deck in an attic
• Growth behind baseboards, under carpet or inside cabinets after water damage
• Anything that returns soon after cleaning

These involve building materials that have been wet, and the fix involves finding the water and often removing material.

Telling them apart at home

• Wipe test. Surface growth on a hard, non-porous material comes off with detergent and leaves a clean surface. If the stain is in the material, or the surface is soft, crumbling or swollen, it has gone deeper.
• Location. On tile, glass or metal: surface. On drywall, wood or fabric: possibly deeper.
• Return. If it is back within days or weeks, moisture is still present.
• Smell. A strong musty smell with little visible growth suggests more is hidden.

A drop of bleach lightening a dark spot is sometimes offered as a test. It shows only that the spot is organic growth and not dirt. It does not tell you how deep it goes.

Cleaning surface growth

1. Wear rubber gloves, eye protection and a mask rated for fine particles.
2. Scrub with water and dish detergent. Health Canada advises that bleach is not necessary for cleaning up mold.
3. Rinse or wipe with a clean damp cloth.
4. Dry the surface completely.
5. Wash or discard the cloths.

Never mix bleach with ammonia or with other cleaners. The fumes are dangerous.

For fabrics, wash in hot water where the fabric allows, and dry thoroughly, in the sun if you can. For leather and books, brush or wipe outdoors and let them dry in moving air.

When cleaning is not enough

• The material is porous and the growth is in it. Moldy drywall, ceiling tile, carpet, underpad and insulation are removed, not cleaned.
• The total area is more than about one square metre, or there are more than three patches.
• It followed a flood, sewage backup or a leak inside a wall.
• It keeps coming back.
• Anyone in the home has asthma, serious allergies or a weakened immune system.

Health

Either word describes something that can affect people. Health Canada links damp and moldy homes with eye, nose and throat irritation, coughing, wheezing and worsening of asthma, and notes that some people are more sensitive than others. There is no established safe level of indoor mold, which is why the advice is to remove it whatever it is called.

Prevention is the same for both

• Run the bathroom fan during and after showers, and the range hood when cooking
• Wipe down shower walls, and hang towels to dry
• Keep indoor humidity moderate, and use a dehumidifier in a damp basement
• Leave space for air behind furniture on outside walls
• Fix leaks promptly and dry wet materials within a day or two
• Vent the clothes dryer outdoors
• Do not store paper, cardboard and fabric against basement walls or on the floor

What about products labelled for mildew?

Mildew-resistant paints, caulks and shower products contain additives that slow growth on their own surface. They help in bathrooms. They do not solve a ventilation problem or a leak, and painting over existing growth without cleaning and drying first does not work.

More: mold removal and remediation.