Mould vs Mildew: What Homeowners Need to Know

Is that growth in your bathroom mould or mildew? While these terms are often used interchangeably, mould and mildew are different types of fungal growth with distinct appearances, behaviors, and appropriate treatments. Understanding the difference helps homeowners respond appropriately.

As IICRC-certified mould specialists serving Central Coast and Newcastle, we’re often asked to explain this distinction. In this guide, we’ll break down the key differences and help you identify what you’re dealing with.

The Basic Difference

Both mould and mildew are types of fungi that grow in damp conditions, but:

Mildew

• A specific type of surface-level fungus
• Grows on flat, moist surfaces
• Easier to remove and less destructive
• Generally considered a nuisance rather than a serious problem

Mould

• A broader category including hundreds of species
• Penetrates porous materials
• Can cause structural damage
• Requires more thorough remediation

Think of it this way: all mildew is a type of mould, but not all mould is mildew. Mildew is the less concerning cousin in the mould family.

How to Tell Them Apart: Visual Identification

Mildew Appearance

• Color: White, gray, or light yellow
• Texture: Flat, powdery, or downy
• Pattern: Spreads in thin, even patches
• Surface: Stays on top of materials

Mould Appearance

• Color: Black, green, red, blue, or brown
• Texture: Fuzzy, slimy, or raised
• Pattern: Appears in irregular patches or clusters
• Penetration: Grows into porous materials

Common Locations

Mildew typically grows on:

• Shower tiles and grout
• Bathroom walls
• Window glass
• Fabric shower curtains
• Paper and leather goods

Mould commonly grows on:

• Drywall and plasterboard
• Wood framing and flooring
• Insulation materials
• Behind wallpaper
• In carpet and padding
• Subfloor areas

Growth Conditions: What Each Needs

While both require moisture, their specific needs differ:

Mildew Growth Conditions

• Surface moisture (condensation, steam)
• Warm temperatures
• High humidity (above 60%)
• Poor air circulation

Mildew thrives in bathrooms, particularly in Central Coast and Newcastle homes where coastal humidity keeps surfaces damp.

Mould Growth Conditions

• Persistent moisture in materials
• Water damage or leaks
• Chronic dampness
• Organic materials to feed on

Mould indicates more significant moisture problems, such as leaks, poor ventilation, or water intrusion.

Health Considerations

Both mould and mildew can affect indoor air quality, but to different degrees:

Mildew Health Effects

• Generally less concerning
• May cause mild respiratory irritation
• Can trigger allergies in sensitive individuals
• Produces musty odors

Mould Health Effects

• Broader range of potential effects
• Can trigger or worsen respiratory conditions
• May produce mycotoxins (depending on species)
• Creates stronger musty odors

According to Restoration Industry Association guidelines, both should be addressed promptly, though mould requires more comprehensive remediation.

Treatment Approaches: DIY vs Professional

Mildew Removal (Often DIY-Appropriate)

For small areas of mildew on non-porous surfaces:

1. Scrub with water and detergent
2. Rinse thoroughly
3. Dry the area completely
4. Improve ventilation to prevent return

Common DIY Solutions:

• Vinegar (spray, leave for an hour, scrub, rinse)
• Baking soda paste (apply, scrub, rinse)
• Commercial mildew removers

Important: Never mix cleaning products, especially bleach and ammonia, which creates dangerous fumes.

Mould Removal (Often Requires Professionals)

Mould remediation is more complex because mould penetrates materials:

1. Identify and fix the moisture source
2. Contain the affected area
3. Remove contaminated porous materials
4. Clean remaining surfaces properly
5. Dry thoroughly
6. Verify successful remediation

The IICRC S520 Standard provides detailed protocols for professional mould remediation, including containment procedures, air filtration, and proper disposal of contaminated materials.

When DIY Is Appropriate

You can likely handle the problem yourself if:

• The affected area is less than 1 square meter
• Growth is on non-porous surfaces (tiles, glass, metal)
• It appears to be surface-level mildew
• There’s no underlying moisture problem
• No one in your household has respiratory sensitivities

When to Call Professionals

Professional mould assessment is recommended when:

• The affected area exceeds 1 square meter
• Growth is on porous materials (drywall, wood, insulation)
• You can smell mould but can’t locate it
• The growth returns after cleaning
• There’s been significant water damage
• Mould is in HVAC systems or subfloor areas
• Anyone in your home has respiratory conditions

Common Scenarios in NSW Homes

Scenario 1: Bathroom Shower Mildew

What it is: White or gray patches on tile, grout, or shower curtain
Cause: Steam and condensation after showers
Solution: Regular cleaning, improve ventilation, reduce humidity
DIY-appropriate: Yes

Scenario 2: Ceiling Mould After Leak

What it is: Dark spots on ceiling plasterboard
Cause: Roof leak or plumbing issue
Solution: Fix leak, assess extent of contamination, possibly replace materials
DIY-appropriate: No – requires professional assessment

Scenario 3: Window Frame Mildew

What it is: Powdery white/gray growth on window frames or sills
Cause: Condensation from temperature differences
Solution: Clean affected areas, improve ventilation, address condensation
DIY-appropriate: Yes for surface growth; professional if wood is rotting

Scenario 4: Subfloor Mould

What it is: Dark, fuzzy growth on floor joists or underside of boards
Cause: Poor ventilation, moisture from ground, plumbing leaks
Solution: Professional subfloor remediation and ventilation improvement
DIY-appropriate: No – requires specialized equipment and expertise

Prevention Strategies for Both

The good news is that preventing both mould and mildew involves similar strategies:

1. Control Moisture

• Fix leaks promptly
• Repair dripping taps immediately
• Maintain gutters and downpipes
• Ensure proper drainage around foundation

2. Improve Ventilation

• Use exhaust fans when cooking and showering
• Run fans for 15-20 minutes after use
• Open windows regularly for cross-ventilation
• Don’t block air vents or grilles

3. Reduce Indoor Humidity

• Keep humidity below 60%
• Use dehumidifiers in damp areas
• Dry wet areas within 24 hours
• Hang wet towels and clothes outside when possible

4. Regular Maintenance

• Inspect vulnerable areas monthly
• Clean bathroom surfaces regularly
• Check for leaks around sinks, toilets, showers
• Monitor areas with previous mould/mildew growth

Special Considerations for Coastal NSW

Central Coast and Newcastle homeowners face unique challenges:

High Ambient Humidity

Coastal humidity makes both mould and mildew more common. Dehumidifiers can help maintain indoor humidity below 60%.

Salt Air Corrosion

Salt air gradually corrodes building materials, creating small leaks that lead to mould growth. Regular maintenance and inspections are essential.

Older Homes

Many coastal properties have inadequate ventilation by modern standards, particularly in subfloor areas. Consider upgrading ventilation systems if mould or mildew is recurring.

The Smell Test

Both produce musty odors, but with differences:

Mildew Smell

• Slightly musty, but less intense
• Often described as “damp” or “stale”
• Disappears when the area is cleaned and dried

Mould Smell

• Stronger, more pungent musty odor
• Persists even after cleaning visible growth
• May indicate hidden mould behind walls or in subfloors

If you can smell mould but can’t see it, professional assessment can locate hidden growth using moisture meters and thermal imaging.

Long-term Damage Comparison

Mildew

• Minimal structural impact
• May stain surfaces if left untreated
• Primarily a cosmetic and air quality issue

Mould

• Can cause significant structural damage
• Rots wood and degrades building materials
• May require removing and replacing affected materials
• Can spread through HVAC systems

This is why early identification and appropriate response are so important.

Testing: Is It Necessary?

For Mildew

Testing is rarely necessary. If it looks like surface-level mildew on non-porous surfaces, you can clean it and monitor for return.

For Mould

Testing may be recommended when:

• You need to confirm mould presence in hidden areas
• You want to identify specific species
• Determining if remediation was successful
• Legal or insurance purposes require documentation
• Health concerns warrant specific identification

Professional mould assessors can perform air sampling, surface sampling, and bulk sampling to identify what’s present and how extensively.

Cost Considerations

Mildew Removal Costs

• DIY: $10-50 for cleaning supplies
• Professional cleaning: Typically unnecessary unless extensive

Mould Remediation Costs

• Varies significantly based on extent and location
• Small areas: May be manageable with proper DIY approach
• Large areas or structural involvement: Requires professional remediation
• Includes fixing moisture source, containment, removal, disposal

Addressing small mould problems early prevents them from becoming expensive major issues.

Conclusion

Understanding the difference between mould and mildew helps homeowners respond appropriately. Mildew is generally a surface problem that can be addressed with regular cleaning and improved ventilation. Mould indicates more significant moisture issues and often requires professional remediation.

Regardless of which you’re dealing with, the key is addressing moisture problems promptly. Left unchecked, today’s minor mildew can become tomorrow’s major mould problem.

If you’re unsure whether you’re dealing with mildew or mould—or if you’ve addressed the problem but it keeps returning—contact MouldSafe for professional assessment. Our IICRC-certified team can identify what’s growing in your Central Coast or Newcastle home and provide appropriate solutions.

---

Not sure if it’s mould or mildew? Book a professional assessment with MouldSafe. We service Sydney, Central Coast, and Newcastle.