Mould doesn't always announce itself. By the time you can see it clearly, the problem is often more extensive than what's visible on the surface. Knowing what to look — and smell — for early can mean the difference between a simple remediation and a major restoration project.

Visual Signs of Mould

The most obvious sign is visible growth, but it doesn't always look like the dramatic black patches you see in photos. Mould in homes can appear as:

Black, green, grey, or white spots on walls, ceilings, or grout lines
Fuzzy or powdery patches on drywall, wood framing, or insulation
Discolouration or staining that doesn't wipe off cleanly
Peeling or bubbling paint — often a sign of moisture behind the wall, which enables mould growth you can't yet see
Warped or buckled drywall or wood surfaces

Surface mould (the kind you can see) often represents only a fraction of the problem. Mould grows into porous materials — drywall, wood, insulation — and what you see on the surface can have extensive root systems (hyphae) penetrating several inches deep.

The Smell Test

Mould has a characteristic musty, earthy odour — sometimes described as damp soil or wet cardboard. If you notice this smell in a specific room, closet, or area of your basement, take it seriously even if you can't see anything. Active mould growth behind walls, under flooring, or inside HVAC systems often smells before it becomes visible.

Pay particular attention to:

Basements and crawl spaces, especially after heavy rain or flooding
Closets on exterior walls (condensation-prone)
Bathrooms without proper ventilation
Attics (ice damming and poor ventilation create ideal mould conditions)
Areas around HVAC systems, air handlers, and ductwork

Health Symptoms That Can Indicate Mould

Mould spores and mycotoxins (toxins produced by certain mould types) can cause a range of symptoms in occupants, particularly with prolonged exposure:

Persistent coughing or wheezing
Nasal congestion, sneezing, or runny nose
Eye irritation or redness
Skin rash or irritation
Headaches or fatigue that seem linked to time spent in the home
Worsening of asthma or allergy symptoms

A useful diagnostic clue: if symptoms improve when you leave the home and return when you come back, that's a strong indicator of an indoor air quality problem — and mould is a common culprit.

Where Mould Hides Most Often

In Canadian homes, particularly in Ontario with its humidity swings and cold winters, mould is most commonly found:

Basements

Especially in finished basements where drywall sits against concrete foundation walls. Condensation on the cold concrete migrates into the drywall, creating ongoing moisture that feeds mould growth invisibly for months or years.

Attics

Inadequate ventilation or a bathroom exhaust fan venting directly into the attic (instead of outside) introduces warm moist air into a cold space — perfect mould conditions. Attic mould can spread across roof sheathing quickly and extensively.

Behind bathroom walls

Grout cracks, failed caulking, and slow leaks behind shower surrounds and tub decks let water into wall cavities that never dry out. By the time the wall shows signs, significant framing and insulation may need to be replaced.

After any water event

Mould can begin growing on wet building materials within 24–48 hours. Any flooding, burst pipe, or significant leak that isn't fully dried within that window is a mould risk — even if you don't see growth immediately.

When Can You DIY? When Should You Call a Professional?

Health Canada guidance suggests that mould covering less than 1 square metre (about 10 sq ft) that is surface-level and the moisture source has been fixed can sometimes be handled by a homeowner. However, even "small" mould problems are often larger than they appear, and improper removal can spread spores throughout the home.

Call a professional when:

The affected area is larger than 1 sq metre
The mould is inside walls, under flooring, or in HVAC systems
There has been a major water event (flooding, sewage backup)
Occupants have respiratory conditions, allergies, or compromised immune systems
You've tried cleaning it and it comes back
You can smell it but can't find it

Professional remediation means the mould is physically removed (not just sprayed), the moisture source is addressed, and affected materials are properly disposed of. Post-remediation testing confirms the space is clear before walls are closed back up. This is how you solve a mould problem rather than delay it.

At Green Life Restoration Services, we use eco-certified, all-natural anti-microbial solutions — no harsh chemicals introduced into your living space. Contact us for a free assessment if you suspect mould in your home.

Signs You Have Mould in Your Home — And What to Do About It