Black Mold vs. Mildew: How to Tell the Difference and When to Call a Pro

Mildew is a surface growth. It appears flat, powdery or downy, usually white, grey or light yellowish, and it stays on top of whatever it is on. Wipe it with a damp cloth and it largely comes off, leaving at most a light stain.

It shows up on bathroom grout, shower ceilings, window sills, and around poorly ventilated windows — surfaces that get wet regularly and dry slowly.

Mold is a colony growing into the material. It looks fuzzy, slimy or raised, often with visible texture and irregular spreading edges. Colours range across black, green, blue-green, brown, orange and white. Wiping it removes the surface and leaves the colony, because the hyphae are inside the material.

The test is genuinely that simple. Damp cloth, light pressure. If it comes away cleanly it is surface growth. If it smears, comes back, or the material underneath is stained and soft, it is growing in.

'Black mold' has entered the language as a synonym for danger, and it is not a useful category.

Stachybotrys chartarum — the species people mean — is real. It is greenish-black, slimy when wet, and it grows on continuously saturated cellulose materials like the paper facing on drywall. It needs sustained wetness rather than occasional dampness.

But plenty of other moulds are dark. Cladosporium is extremely common, often dark green or black, and generally far less concerning. Aspergillus and Penicillium turn up in most water-damaged buildings in Orange County and come in a range of colours.

Here is the practically important part: the remediation procedure is essentially the same regardless of species. Contain, filter, remove colonised material, clean, dry, fix the moisture source. Species identification affects the urgency conversation and sometimes medical advice — it rarely changes the work.

Be cautious of any company that leads with a frightening species name before they have found where the water is coming from.

Surface mildew on non-porous material in a small area is a homeowner job and you do not need us.

Bathroom grout, shower tile, glass, sealed surfaces — clean it, then deal with the ventilation that let it happen. A bathroom extractor fan that actually vents outside and runs for fifteen minutes after a shower prevents most of it.

Bleach is fine on hard non-porous surfaces. It is close to useless on drywall or wood, because bleach is mostly water: the water soaks into the porous material and feeds the colony while the chlorine stays on the surface.

If mildew keeps returning in the same spot after cleaning, that is a ventilation or moisture problem to solve rather than a cleaning schedule to maintain.

First: any growth on porous building materials — drywall, wood framing, insulation, ceiling tile, carpet. The visible growth is the smallest part of it, and disturbing it without containment spreads spores through the rest of the house.

Second: a musty smell with no visible growth. That means it is somewhere you cannot see — inside a wall, under flooring, in a crawl space or in attic insulation. Finding it requires thermal imaging and moisture meters rather than looking harder.

Third: growth following a water loss, particularly one that was not professionally dried. Where mold has appeared after a leak, the moisture source is very likely still active or the material never reached a dry standard.

As a rough threshold, anything larger than about ten square feet, anything in an HVAC system, and anything in a household with asthma, allergies or an immunocompromised member should be handled professionally.

Mold exposure causes real symptoms — congestion, cough, itchy eyes, throat irritation, worsened asthma — and the useful pattern is that they improve when you leave the building and return when you come back.

That pattern is worth taking seriously and worth mentioning to a doctor. What it is not is a way of identifying a species or a severity, and there is a lot of misinformation in this area.

Dead mold still causes allergic response, which is why surface killing without removal does not resolve symptoms. The material has to come out.

You can settle this in about ten seconds with a cloth and light pressure:

For the genuinely small, genuinely surface cases, doing it yourself is reasonable. Most of the harm we see from DIY cleaning comes from three specific mistakes rather than from the attempt itself.

If the affected area is larger than about ten square feet, is inside an HVAC system, or is on a material you cannot replace, stop and get a protocol written.