Mold vs Mildew: Telling Them Apart

Both are fungi. The distinction is not taxonomic, it is practical: one sits on top of a surface and one has grown into it, and that determines whether wiping it off actually solves anything.

The field test

Mildew is flat. It looks powdery or downy, usually white, grey or a dull yellow, and it stays on the surface. Shower grout, the underside of a bathmat, the rubber gasket on a front-loading washer, a window sill after a humid week. Wipe it and it comes off, and the material underneath looks normal.

Mold is raised. Fuzzy, sometimes slimy, and often green, blue, brown or black. It sends hyphae into whatever it is growing on, so on drywall, wood, insulation or a ceiling tile, removing the visible layer removes the top of the organism, not the organism.

The practical version: wipe a small area with a damp cloth. If the surface underneath is clean and undamaged, you likely had mildew and you have dealt with it. If there is staining left behind, or the material is soft, warped or crumbling, you are past cleaning.

What colour does not tell you

Black does not mean toxic. “Black mold” in common use refers to Stachybotrys chartarum, but plenty of harmless species are dark, and Stachybotrys is not reliably black. Nobody can identify a species by looking at it, including us.

More usefully: from a response standpoint, the species does not change what you do. Find the water, stop the water, remove the material if it is porous and affected. That sequence is identical regardless of what grew.

Why Central Texas gets both

Austin’s problem is not persistent damp — it is cycles. Summer runs 100°F-plus from June through September and everything is sealed and air-conditioned. Bathrooms without a working exhaust fan hold humidity for hours. Then a spring or autumn storm week pushes outdoor dew points up and interior surfaces sit warm and damp.

The reliable trouble spots:

• Shower grout and silicone, especially where hard-water scale gives it texture to hold onto
• Under kitchen and bathroom sinks, where a slow supply-line drip goes unnoticed
• Window frames in older Central Austin housing stock, from condensation
• The gasket of a front-loading washer
• Around AC condensate lines and drain pans, which clog and overflow
• Behind furniture on an exterior wall in a poorly insulated room

Water intrusion after a storm is the other one, and it does not stay a cleaning issue for long. Fungal growth on wet building materials can begin within a day or two.

What actually removes mildew

For non-porous surfaces — tile, glass, sealed grout, fibreglass, plastic — detergent and physical scrubbing works. The mechanical action matters more than the chemistry.

If you want something with disinfectant action, hydrogen peroxide is a reasonable choice and leaves nothing behind. Where scale and mildew are both present, which is common on Austin shower glass, deal with the limescale first with an acidic descaler and its dwell time, because the mildew is holding onto the mineral deposit.

Do not mix products. Bleach and an acidic descaler in the same session, on the same wet surface, releases chlorine gas. Rinse thoroughly and ventilate between steps if you are using both.

Where bleach is the wrong tool

On porous material — drywall, unsealed grout, wood, ceiling tile — bleach is mostly water, and water is what fed the problem. The surface stain lightens and the growth in the material does not care. It looks solved, which is the worst outcome, because the underlying moisture keeps going.

Where cleaning stops

Stop and call a remediation contractor when:

• The affected area is larger than about 10 square feet, which is the threshold in EPA guidance for homeowners
• It is on or behind drywall, insulation or subfloor
• It came back within weeks of being cleaned, which means the moisture source is still active
• There is a smell in a room with nothing visible
• It followed a plumbing leak, roof leak or flooding
• Someone in the house is immunocompromised or has significant respiratory illness — that is a conversation with their doctor first

Health questions about mould exposure belong with a physician. We can tell you what is on a surface and remove it; nobody at a cleaning company should be telling you what it is doing to you.

Prevention, which is cheaper than all of it

Run the bathroom fan during a shower and for twenty minutes after. Keep indoor relative humidity between 30 and 50%. Squeegee shower glass, which also slows the limescale. Fix drips the week you notice them. Leave the washer door open between loads.

We handle mildew on hard surfaces as part of a normal bathroom clean, and we will tell you plainly when what we are looking at needs a remediation contractor instead of us.