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

"Is it black mold?" is the question we get asked most, and it's almost always the wrong question.

Here's the more useful framing.

	Mildew	Mold
Where it grows	On a surface	Into a porous material
Typical spot	Grout, shower tile, a window frame	Drywall, insulation, subfloor, cabinet base
What it means	A ventilation problem	A moisture problem inside the structure
Wipes off	Yes, permanently	The surface does; the growth in the material does not
The fix	Clean it and improve airflow	Contain, remove the affected material, dry the cause

Mildew: surface growth on a non-porous surface

What people call mildew is typically surface-level fungal growth on a non-porous material — shower tile, grout, a window sill, the rubber seal on a washing machine door.

Characteristics: flat and powdery, usually white, grey or a dusty black; sits on the surface rather than in the material; wipes off; comes back if the moisture and ventilation problem isn't addressed.

Cause: repeated wetting plus poor ventilation. A bathroom without an extractor fan, or with one that vents into the attic instead of outside.

Fix: clean it with an appropriate cleaner, then solve the ventilation. Run the fan during and for twenty minutes after every shower. Squeegee the shower walls. Leave the door open.

That is genuinely a DIY job.

Mold: growth in a porous material

What matters is growth in a material rather than on it — drywall, wood, carpet, insulation, particle board.

Characteristics: fuzzy or slimy rather than powdery; any colour including black, green, grey, white and orange; doesn't wipe off, because the root structure is inside the material; the material may be soft, stained or discoloured; frequently accompanied by a musty smell.

Cause: a moisture source. Always. A leak, a slab leak, condensation, a roof failure, a drainage problem.

Fix: remove the material, fix the moisture source. Not a DIY job beyond a very small area.

The colour question

Colour does not identify species, and species rarely changes what you do.

Several fungi are dark. Stachybotrys chartarum — the one popular usage calls "black mold" — is one of them, but so are various Cladosporium and Aspergillus species, and you cannot tell them apart by looking.

More importantly, the remediation protocol is the same regardless: contain the area, remove the contaminated porous material, HEPA clean, fix the moisture source, verify. Nobody changes the method based on the species.

Which is why we don't use species identification as a scare tactic, and why a company that leads with "you might have black mold" before doing any assessment deserves scepticism.

The health question, honestly

Reputable health authorities are consistent on this: indoor mold exposure can cause upper respiratory symptoms, coughing, wheezing, and asthma exacerbation in people with asthma. People with allergies, immunocompromise or chronic respiratory conditions can be more affected.

The claims you'll see online about specific "toxic black mold" syndromes go well beyond what the evidence supports.

The sensible position: mold indoors is a building problem that should be fixed, and some people react to it more than others. If members of your household have symptoms that improve when they leave the house, that's worth taking seriously — and worth mentioning to your doctor as well as to a remediation company.

How to tell which you have, practically

Where is it? Shower tile, grout, window sill, plastic seal → probably surface mildew. Drywall, wood, carpet, insulation, cabinet base → mold in the material.

Does it wipe off? Cleanly, leaving no stain → surface. Leaves a stain, or the material is soft → in the material.

Is the material sound? Press it. Soft, spongy or crumbling means the growth is structural.

Is there a smell? A persistent musty smell means active growth with volume behind it, not a surface film.

Is there a moisture source? This is the real question. Mildew on shower grout has an obvious source. Growth on a bedroom wall does not — and finding out why that wall is wet matters more than identifying what's growing on it.

When to call

Handle it yourself if: it's surface growth on non-porous material, under about ten square feet, with an obvious ventilation cause and no water damage history.

Call a professional if: it's on drywall, wood, carpet or insulation; it covers more than about ten square feet; there's a musty smell you can't source; it followed a water event; you can see it but not reach it (inside a wall, in an attic, in a crawl space); or anyone in the household has respiratory symptoms.

And don't scrub visible growth on porous material. Disturbing it distributes spores and expands the area that will need containment.

What proper remediation actually involves

The distinction matters most because it determines the scale of the response, so here is what the professional version looks like when growth is in the structure rather than on a surface.

Containment first. Plastic sheeting seals the work area from the rest of the building, and negative air pressure with HEPA filtration means air flows into the contained space rather than out of it. Without that step, opening a wall distributes spores through the whole house — which is how a contained problem in one room becomes a general one.

Removal, not treatment. Porous material with growth in it comes out: drywall, insulation, carpet pad, particleboard. Non-porous and semi-porous material — framing, concrete, solid wood — is cleaned rather than removed.

Drying the cause. Growth is a symptom of moisture, so the source is corrected and the assembly is dried to a verified standard. Remediation without that is a repeat visit.

Verification. The work area is cleaned, HEPA vacuumed and checked before rebuilding, ideally by someone independent of the company doing the work.

The reason surface mildew does not need any of this is that nothing has grown into the material. Wipe it off, fix the ventilation that caused it, and it is genuinely finished.

The bit that actually matters

Whatever is growing, it's growing because something is wet. Cleaning the growth without finding the moisture source is a delay, not a repair.

That source is frequently not obvious. In the slab-era tracts across southwest Rancho Cucamonga it's often an in-slab pinhole leak wetting the wall base from below. In older post-war housing like Montclair's, it can be a cast-iron drain failing under the slab. Both are invisible until someone goes looking with a meter.

Free inspection and moisture assessment: +1 (201) 277-9344.