Black Mould vs Mildew: How to Tell the Difference
Visual identification guide to distinguish black mould from mildew on walls.
Mildew is flat, powdery, and light-colored (grey or white). It wipes off easily and lives on surfaces. Black mould is raised, slimy or fuzzy, dark green to black, and penetrates into porous materials like plaster and grout. Mildew is cosmetic. Black mould indicates a moisture problem that needs fixing.
Last updated: October 2026 ·Practical identification for UK homeowners
Visual Differences
Most people use "mould" and "mildew" interchangeably. They are both fungi but behave differently.
Mildew Appearance
- Color: White, grey, or pale yellow
- Texture: Flat, powdery, or downy
- Location: On surfaces (tiles, shower curtains, window frames)
- Removal: Wipes off easily with a damp cloth
- Growth pattern: Spreads in patches, stays shallow
Black Mould Appearance
- Color: Dark green, brown, or black
- Texture: Slimy when wet, fuzzy or velvety when dry
- Location: Inside porous materials (plaster, grout, wood)
- Removal: Requires scrubbing with cleaner, may stain permanently
- Growth pattern: Penetrates deep, leaves dark staining even after cleaning
Smell Test
Both have distinctive odors:
- Mildew: Light musty smell, similar to damp towels
- Black mould: Strong earthy smell, like rotting wood or wet soil
If you smell mould but cannot see it, check behind furniture, inside cupboards, under carpets, and around window frames. Mould often grows hidden before it becomes visible.
Where Each Grows
Common Mildew Locations
- Bathroom tiles and grout
- Shower curtains and seals
- Window frames (especially uPVC)
- Fabric blinds
- Kitchen splashbacks
Mildew thrives in humid environments but stays on non-porous surfaces where it can be easily wiped away.
Common Black Mould Locations
- Plaster walls with damp penetration
- Behind wallpaper
- Ceiling corners (condensation)
- Around leaking windows
- Timber skirting boards
- Inside cupboards on external walls
Black mould needs sustained moisture. It indicates a damp problem, not just poor cleaning.
Health Implications
Mildew
Low risk. Mildew can trigger mild allergic reactions in sensitive people (sneezing, itchy eyes) but rarely causes serious health problems. Regular cleaning keeps it under control.
Black Mould
Moderate to high risk, depending on exposure. Black mould (especially Stachybotrys chartarum) releases mycotoxins. Symptoms include:
- Persistent cough and wheezing
- Nasal congestion and sinus pain
- Eye irritation and redness
- Skin rashes
- Headaches and fatigue (with prolonged exposure)
People with asthma, weakened immune systems, or young children are at higher risk. If you have respiratory symptoms that improve when you leave the house, mould may be the cause.
What Causes Each?
Both mildew and black mould need moisture. The difference is the moisture level and duration.
- Mildew: Grows in humid conditions (bathrooms, poorly ventilated rooms). Surface dampness is enough.
- Black mould: Needs sustained dampness inside materials. Caused by condensation, leaks, penetrating damp, or rising damp.
Mildew is a ventilation problem. Black mould is a damp problem.
Treatment Differences
Mildew
Easy to treat at home:
- Wipe with white vinegar or diluted bleach
- Dry the surface thoroughly
- Improve ventilation (open windows, use extractor fans)
Mildew does not usually return if you keep the area dry.
Black Mould
Requires a two-step approach:
- Remove the mould: Scrub with mould cleaner (small areas only). Large areas need professional removal.
- Fix the damp source: Identify why the wall is damp (survey), then treat the cause (damp proofing, leak repair, ventilation).
Black mould returns within weeks if the underlying damp is not fixed. Cleaning alone is a waste of time.
When to Call a Professional
You can clean mildew yourself. Call a damp specialist if:
- You have black mould covering more than 1m²
- Mould keeps returning after cleaning
- You can smell mould but cannot see it
- Mould is growing on plasterboard, timber, or inside walls
A professional survey uses moisture meters to find the source. Treatment costs depend on the cause but fixing it properly stops mould for good.
Frequently Asked Questions
How can I tell if it's mould or mildew?
Mildew is flat, powdery, grey or white, and wipes off easily. Black mould is raised, slimy or fuzzy, dark green to black, and penetrates into surfaces. Mildew smells musty. Black mould smells earthy and damp.
Is mildew as dangerous as black mould?
No. Mildew is a surface fungus with minimal health risk. It causes cosmetic staining but rarely triggers respiratory symptoms. Black mould releases mycotoxins that can irritate lungs, eyes, and skin, especially in people with asthma or weakened immunity.
Can mildew turn into black mould?
No. They are different species. Mildew stays on surfaces. Black mould grows into materials. But both thrive in damp conditions, so mildew in a bathroom often signals poor ventilation that could allow black mould to grow elsewhere.
Does dehumidifier stop mildew and mould?
A dehumidifier reduces airborne moisture and helps with mildew. It will not stop black mould caused by structural damp (rising damp, penetrating damp, leaks) because the moisture comes from inside the wall, not the air.
Mould caused by damp?
If you have black mould on walls, we can identify the source with a professional moisture survey. Free quotes, fast response in Ipswich and Suffolk. Contact us for same-week appointments.
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Last updated: October 2026