Mould vs Mildew: What’s the Difference and Why It Matters

“Is it mould or mildew?” We get asked this at least twice a week. It matters more than you’d think, because the answer changes what you should do about it.

They’re related. Both are fungi. Both love moisture. But they behave differently, look different, and need different treatment. Getting it wrong means wasting time and money on the wrong fix.

Key Differences Between Mould and Mildew

Appearance. Mildew sits on the surface. It’s usually white, grey, or light brown, with a powdery or downy texture. Think of the greyish film you see on bathroom grout or damp fabric. Mould grows into the material. It’s typically black, green, or dark brown, and it looks fuzzy, slimy, or raised.

Depth. This is the big one. Mildew is a surface problem. Wipe it off and the surface underneath is usually fine. Mould penetrates. It sends root-like structures called hyphae deep into porous materials. That black patch on your wall isn’t just on the plaster. It’s in it.

Smell. Mildew smells musty but mild. Mould smells stronger, earthier, and more unpleasant. If a room has that heavy, damp, almost sweet-rotten smell, you’re probably dealing with mould, not mildew.

Speed. Mildew develops relatively quickly on damp surfaces but doesn’t spread aggressively into new territory. Mould is slower to establish but once it’s growing, it spreads relentlessly through connected materials.

Health impact. Both can cause allergic reactions and respiratory irritation. But mould, particularly species like Stachybotrys and Aspergillus, poses significantly greater health risks due to mycotoxin production. We’ve covered the health risks of black mould in detail separately.

How to Identify What You’re Dealing With

Here’s a quick test you can do at home.

Dab a small amount of household bleach on the affected area with a cloth. If it lightens within a couple of minutes and wipes off easily, it’s likely mildew. If it doesn’t shift or comes back within days, you’re looking at mould.

Common mildew locations:

• Bathroom tiles and grout
• Shower curtains
• Window sills with condensation
• Damp clothing left in a pile
• Leather goods in poorly ventilated wardrobes

Common mould locations:

• Behind furniture against external walls
• Inside built-in cupboards
• Around leaking pipes (often hidden)
• Under flooring after water damage
• In wall cavities with penetrating damp

One job we did in Redland last year showed the difference perfectly. The homeowner thought she had mildew on her bedroom wall. Grey, patches, looked surface-level. When we tested the moisture levels behind the paint, the readings were off the charts. Behind the wallpaper was a dense colony of Aspergillus that had been growing for months. What she saw on the surface was the tip of it.

Why the Distinction Matters for Treatment

Mildew is a cleaning job. Mould is a remediation job. Treating mould like mildew is a costly mistake.

For mildew: a decent anti-fungal spray, a scrub, and better ventilation will usually fix it. Keep surfaces dry, improve airflow, problem solved. Most people can handle this themselves.

For mould: you need to remove contaminated material, treat the area with professional-grade antimicrobials, and fix the moisture source. Spraying mould with a supermarket cleaner and wiping it off does almost nothing. The hyphae are still embedded in the material. The spores are still in the air. It’ll be back within weeks.

Honestly, we’d rather people called us for a quick check than spent months battling something they’ve misidentified. A ten-minute phone conversation can save you a lot of wasted effort.

When Mildew Becomes a Mould Problem

Mildew on its own is annoying but manageable. The problem is what mildew tells you.

Mildew needs moisture. If it’s growing, something is creating that moisture. Condensation, a small leak, poor ventilation. Left unchecked, those same conditions create mould. Not might. Will.

A bathroom with mildew on the grout is normal. A bathroom where mildew keeps appearing on the ceiling despite cleaning is telling you the ventilation isn’t working. And if that moisture problem isn’t fixed, mould will establish itself in the ceiling plaster, the wall cavities, the timber. Worth sorting early.

Here’s the progression we see repeatedly:

1. Mildew appears on surface. Homeowner cleans it off.
2. Mildew keeps coming back. Homeowner cleans it off again.
3. Dark patches start appearing on walls or ceilings that don’t wipe off.
4. Musty smell develops. Maybe some health symptoms.
5. Homeowner calls us. We find established mould behind surfaces.

Step 1 to step 5 takes about six to eighteen months. The fix at step 1 costs almost nothing. Better ventilation, a fan, opening windows. The fix at step 5 costs £500 to £1,500.

If you’re seeing persistent mildew despite regular cleaning, don’t just keep cleaning. Fix the moisture problem. Our guide on stopping condensation and preventing mould covers the practical steps.

And if you suspect what you’re looking at is actual mould, not mildew, get a professional opinion before it spreads. Our mould removal service starts with a proper assessment so you know exactly what you’re dealing with.

Call Bristol Cleaning Heroes on 07985 505061 for advice or a quote.

Bristol Cleaning Heroes | 290-294 Southmead Rd, Bristol BS10 5EN | £2M insured (AXA) | IICRC certified | 25 years’ experience