What Happens If Mould Is Not Treated?

Untreated mould does not stay in one place. It keeps growing while the moisture is still there, and the repair gets bigger every month.

Quick answer: If mould is not treated, it keeps spreading while the moisture source is still there. It damages building materials over time, can affect the people living in the home, and turns a small, simple job into containment, removal and rebuilding. Fixing the moisture and cleaning the visible growth early is almost always the smaller job.

Quick check: how far has it gone?

What happens over time if mould is not treated?

Mould follows a pattern that is predictable once the moisture is present. The sooner it is treated, the less of the home is involved. The diagram below shows the usual progression. It is a diagram, not a measured chart.

1. Days 0 to 2: the moisture is present and mould can start on damp materials. Growth is often limited to one area.
2. Weeks: the growth moves onto nearby surfaces and into the air. Stains and odour return after cleaning if the moisture is still there.
3. Months: the growth reaches insulation, wall cavities, subfloors and framing. The job now involves containment, removal and rebuilding.

Each stage costs more to fix than the one before, and each one adds more risk for the people in the home.

Why does untreated mould keep spreading?

Mould needs moisture, a food source and time. Most homes provide the food (drywall paper, wood, dust), so the moisture decides how far it goes. If a leak, condensation or poor ventilation is still running, the mould keeps growing. That is why cleaning the surface alone does not solve the problem.

The common advice from building and health guidance is to correct the moisture source first, then remove the visible growth. University of Toronto guidance and the Swindon council guidance both say the same thing: treat the cause, not only the stain.

What counts as a moisture source?

• A slow leak in a pipe, a roof junction or a window frame.
• Condensation on cold surfaces, especially in basements and corners.
• Poor ventilation in bathrooms, kitchens and laundry areas.
• Water that was not fully dried after a flood or a backup.

Our water damage restoration work starts with finding and stopping the source. Then the drying, then the cleaning. See our restoration services for how this is scoped.

Does untreated mould damage the structure?

Yes, over time. Mould feeds on the materials it grows on, and it can weaken them. Sources and industry guidance commonly list drywall, ceiling tiles, wood, insulation and carpet as materials that can be damaged. Soft materials that are heavily infested, such as carpet and upholstery, often cannot be saved and have to be replaced.

The structural problem is usually found late. Growth inside a wall cavity, under flooring or in an attic can go unseen until staining, sagging, soft spots or a strong smell appear. By then the repair often includes the framing and the insulation, not only the finishes.

Materials that usually cannot be saved

• Carpet and underlay that stayed wet.
• Upholstered furniture and mattresses that were damp.
• Insulation that was soaked or heavily infested.
• Drywall that is soft, crumbling or deeply stained.

What does untreated mould do to people?

Mould spores, fragments and the compounds some moulds release can affect people. University of Toronto guidance lists eye irritation, a runny nose, sneezing, fever, skin rashes, and aggravated asthma among the possible symptoms. It notes that people with compromised immune systems, people with allergies, children and older people may be more affected.

The Swindon council guidance adds that long-term exposure can cause coughing, sneezing, itchy eyes and nasal congestion, and that living with black mould can affect mental wellbeing. Its message is the same as the other sources: the health risk goes down once the mould is treated.

People at higher risk include infants and children, seniors, pregnant people, and people with asthma or other respiratory conditions. If one of them lives in the home, the work should be planned with that in mind: containment, protection and the removal of the people from the work area.

Myths that make untreated mould worse

What to do, in order

1. Stop the moisture. Find and fix the source, or call for it to be found.
2. Document the area. Take dated photos of every affected surface before cleaning.
3. Protect the people. Keep higher-risk people out of the work area.
4. Contain the work. Seal the area and protect the floors and rooms next to it.
5. Remove what cannot be saved. Carpet, soaked insulation and deeply damaged drywall.
6. Clean and dry. Clean the surfaces that can be saved and dry the area fully.
7. Check at handover. Inspect the area after drying and keep the written record.

Health Canada guidance lists six cleanup steps that match this order. The minimum protective equipment for people cleaning mould includes safety glasses or goggles, a disposable N95 respirator, and gloves.

Photo checklist

Save these in your phone before the work starts. They make the scope clearer and protect you at handover.

• Every affected surface, from a distance and close up
• The source of the moisture (leak, window, roof junction)
• Any stain, odour source or damp area behind a removed panel
• The room layout and the access route
• The ventilation and the bathroom or kitchen exhaust
• The area again after cleaning and drying

How much does it cost to treat mould?

We do not publish a price for this, because the cost depends on the size of the area, the moisture source, whether materials must be removed, how hidden the growth is, and the containment needed. A written scope lists each of those, so the price is built on the work, not a guess.

When to call a professional

• The growth covers more than a small area, or it is behind walls, under flooring or in the ceiling.
• It came back after you cleaned it.
• There is a persistent musty smell with no visible source.
• Someone in the home is at higher risk, or has symptoms.
• The moisture source is not clear, or it has been running for some time.

A professional starts with an inspection and a written scope. We confirm the moisture source, the extent of the growth and the containment, and we put the plan in writing before any work begins. See mould remediation and our inspection services, including air quality testing.

Frequently asked questions

Sources

• University of Toronto, Environmental Health and Safety, mould guidance: ehs.utoronto.ca/mould. Used for the 24 to 48 hour timing and the health symptoms.
• Health Canada, mould guidance: 48-hour growth timing, humidity of 30 to 50 per cent, bleach advice, six cleanup steps, protective equipment, and at-risk groups. Link to be confirmed by the owner before publishing.
• CAUT fact sheet on mould: bulletin-archives.caut.ca. Used for the 48-hour water response, the under 70 per cent humidity guidance, and the 25 to 50 per cent best range.
• Occupational Health and Safety Council of Ontario, mould recognition, assessment and control: pshsa.ca.
• Construction Safety Association of Ontario article on mould (1999, dated): fix the moisture before remediation, and follow up 3 to 6 months after. Dated source, used for practice only.
• Swindon Council, damp and mould guidance (UK): used for the general health and mental wellbeing points only.

Insurance and who to trust

Keep dated photos, the inspection report and the written scope. Ask your insurer for their written position before work starts. Ask any contractor for the scope in writing, the names of the people on site, and a clear record of what will be done and checked.

Related services and guides

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