Mould vs Mildew: How to Tell the Difference and What to Do
Surface fungus or structural threat? A field guide to identifying what's growing in your bathroom or basement — and what to do next.
You step into your bathroom, glance at the ceiling, and notice a small, dark patch developing above the shower. Or perhaps you pull a storage box away from a basement wall and find a powdery white substance clinging to the concrete. Panic sets in. Is it toxic black mould, or just harmless mildew?
While both mould and mildew are types of fungi that thrive in moist environments, their impact on your home and your health are drastically different. Understanding how to identify which fungus you are dealing with will determine whether you can fix it with a weekend cleaning project or if you need to evacuate the room and call a professional remediation team.
Visual Identification: What Does It Look Like?
The easiest way to differentiate between the two is through visual inspection and examining the surface they are growing on.
Identifying Mildew
Mildew is essentially early-stage mould that remains on the surface. It is a surface fungus that does not penetrate deep into materials.
- Color: Usually starts white or gray and turns yellow, brown, or black as it ages.
- Texture: Powdery, fluffy, or flat.
- Location: Thrives on damp, non-porous surfaces like shower tiles, bathtub caulking, windowsills, and plant leaves.
- Damage: Causes cosmetic discoloration but does not rot or destroy the material it grows on.
Identifying Mould
Mould is much more aggressive. It extends microscopic root systems (hyphae) deep into porous materials to digest them.
- Color: Can be deep black, dark green, vibrant red, or blue.
- Texture: Often appears fuzzy, slimy, or fuzzy.
- Location: Feeds on organic, porous materials like drywall, wood framing, paper, ceiling tiles, and carpet.
- Damage: Actively breaks down and rots the structural integrity of the material, causing permanent damage.
| Characteristic | Mildew | Mould |
|---|---|---|
| Growth Pattern | Grows flat on the surface | Grows upward and roots deeply |
| Health Risks | Mild (sneezing, coughing) | Severe (asthma, toxicosis, fatigue) |
| Common Treatment | Household cleaners, vinegar | Professional extraction, antimicrobials |
Health Risks: When Fungi Become Dangerous
While both produce spores, the health implications vary wildly.
Mildew spores can trigger mild allergic reactions in sensitive individuals, such as a runny nose or itchy eyes. However, certain strains of mould (like Stachybotrys chartarum, or "black mould") produce mycotoxins. These toxic compounds travel through the air and, when inhaled over time, can cause severe respiratory distress, chronic migraines, neurological issues, and the development of asthma in children.
Warning: The Smell Test
Mould produces Microbial Volatile Organic Compounds (MVOCs) as it digests your home. This creates a distinct, heavy, earthy, or "rotting wood" smell. If your basement smells musty, you likely have hidden mould behind the walls, regardless of whether you can see it.
DIY Removal vs. Professional Remediation
The treatment approach is where the distinction between mould and mildew becomes critical.
How to Handle Mildew (DIY)
Because mildew sits on the surface of non-porous materials like bathroom tiles, you can safely clean it yourself. Use a commercial bathroom cleaner, white vinegar, or hydrogen peroxide. Scrub the surface, rinse thoroughly, and ensure the room is well-ventilated to dry.
How to Handle Mould (Professional Required)
Do not attempt to scrub mould off drywall or wood with bleach. Bleach is mostly water; the chlorine cannot penetrate the porous surface to kill the roots, meaning you are simply watering the mould and ensuring it returns aggressively. Furthermore, scrubbing dry mould blasts millions of toxic spores into your breathing air.
Professional remediation involves:
- Erecting heavy poly-sheeting containment barriers.
- Running HEPA-filtered negative air machines to catch airborne spores.
- Physically removing and disposing of the infested drywall and insulation safely.
- Applying commercial-grade, EPA-registered biocides to kill the root systems in the structural wood.
Prevention: Controlling the Moisture
Whether you are fighting mould or mildew, the ultimate solution is the same: moisture control. Neither fungus can survive without water.
In Eastern Ontario, where summers are humid and winters cause severe condensation on windows, you must manage indoor climate. Keep your home's relative humidity below 50%. Always run bathroom exhaust fans for 30 minutes after showering, ensure your dryer vents directly outside, and address any slow plumbing leaks or hidden water damage immediately.