The short answer
Mildew sits on the surface, is powdery and gray-white, and wipes off. Mold penetrates the material, is fuzzy or slimy, and comes back after you clean it. The visual difference matters less than the cause: mildew means humidity, mold means something behind the surface is genuinely wet.
People ring us and ask "is it black mold?" more than any other question. The honest answer is that it usually does not matter, and the anxiety attached to the phrase is out of proportion to how the decision actually gets made.
What "black mold" means, and does not
Stachybotrys chartarum is the species that gets the headlines. It is dark, often slimy in appearance, and it grows on chronically wet cellulose — persistently wet drywall, wet cardboard, saturated wood. It is genuinely worth removing.
It is also far less common indoors than Aspergillus, Penicillium and Cladosporium, all of which are equally worth removing and none of which are always black. Color is not a reliable identifier: plenty of dark growth is not Stachybotrys, and plenty of Stachybotrys is not obviously black.
Critically, remediation protocol does not change by species. Area affected and material type drive it. A hygienist can identify what it is, but the answer very rarely alters what gets done.
Mildew versus mold, practically
| Surface mildew | Established mold growth | |
|---|---|---|
| Where | Tile grout, painted ceilings, window frames, shower silicone | Drywall, wood, framing, insulation, subfloor |
| Appearance | Thin, flat, powdery or spotty | Fuzzy, raised, sometimes slimy; spreads in patches |
| Cause | Surface condensation and poor ventilation | A moisture source inside or behind the assembly |
| Cleaning | Wipes off; returns if ventilation is not improved | Cleaning the surface does nothing; hyphae are inside the material |
| Smell | Often none | Persistent musty odor |
| Response | Clean it, fix the ventilation | Find the moisture source; contained removal |
The test that actually decides it
Forget species. Ask three questions.
- What is it growing on? Non-porous — tile, glass, sealed surfaces — means you can clean it. Porous — drywall, wood, insulation, carpet — means the growth is inside the material and cleaning the surface achieves nothing.
- How much is there? Under roughly 10 square feet on a non-porous surface is a homeowner job. Between 10 and 100 square feet on porous material needs professional contained removal. Over 100 square feet needs engineered containment and clearance testing.
- Does it come back after you clean it? If yes, there is a moisture source you have not found, and cleaning it again is not the answer.
When to call regardless of what it looks like
- Anyone in the household is immunocompromised, asthmatic, pregnant, elderly or very young. The area threshold does not apply.
- It keeps coming back. There is an unfound moisture source.
- There is a persistent musty smell with nothing visible. Growth in an unseen assembly, and the smell is a reliable indicator.
- It followed a water loss. Assume the assembly behind the finish is involved.
- It is in or near the HVAC system. Ducting distributes spores through the whole building.
- It is in the crawl space or attic. Confined-space work, and air moves from those spaces into the living area.
What you can genuinely handle yourself
Surface mildew on bathroom grout, silicone, a painted ceiling or a window frame, under about ten square feet. Clean it with a household product, ventilate properly, and then fix the reason it grew: run the extractor fan during and for twenty minutes after every shower, check that the fan actually vents outside rather than into the attic, and open a window if you have no fan.
If it returns within a few weeks, the ventilation is not fixed or there is a moisture source behind the finish.
What proper remediation involves
Find and fix the moisture source, build sealed containment with negative air pressure, remove colonized porous material inside that containment, HEPA vacuum and damp wipe everything, treat and dry the remaining structure, and verify — with an independent hygienist on anything substantial, because a firm that both writes and passes its own protocol has an obvious conflict of interest.
See mold remediation in Lafayette. This is a common conversation in the wooded, foggy canyon pockets like Sleepy Hollow, where ambient humidity keeps surfaces close to growth conditions year round.
Frequently asked
How do I tell black mold from mildew?
By where it is growing rather than what color it is. Surface mildew grows on non-porous surfaces — tile grout, silicone, painted ceilings, window frames — from condensation and poor ventilation, wipes off, and returns if the ventilation is not fixed. Established mold grows on porous material such as drywall, wood or insulation, is fuzzy or raised, comes with a musty smell, and indicates a moisture source inside the assembly.
Does it matter what species of mold it is?
Rarely. Stachybotrys chartarum gets the headlines but is less common indoors than Aspergillus, Penicillium and Cladosporium, all of which are equally worth removing. Remediation protocol is driven by the area affected and the material type, not by species — so identification is interesting but very seldom changes what gets done.
How much mold can I clean myself?
Under roughly ten square feet on a non-porous surface such as tile, grout, silicone or a painted ceiling — clean it and fix the ventilation. Between ten and one hundred square feet on porous material needs professional contained removal, and over one hundred square feet needs engineered containment and clearance testing. If anyone in the household is immunocompromised, asthmatic, pregnant, elderly or very young, call regardless of area.
Will bleach kill mold?
On non-porous surfaces like tile and glass, household bleach handles surface mildew fine. On drywall or wood it does not work — the chlorine stays on the surface while the water carries deeper into the material, feeding the growth you were trying to kill. There is also no spray or coating that makes removal unnecessary on colonized porous material, whatever the packaging claims.