Mold in Your Home? Here's What to Do (2026) | GetAHomePro
Mold in Your Home? Here's What to Do (Step-by-Step)
·12 min read
L
Lisa NguyenGeneral Contractor & Renovation Specialist
Published March 21, 2026
Key Takeaway
Found mold? Step-by-step guide covering health risks, DIY vs pro, costs ($800-$6K+), Ontario tenant rights, and prevention tips.
Mold in Your Home? Here's What to Do (Step-by-Step)
You notice a dark patch behind the toilet. A musty smell in the basement that wasn't there last month. Discolouration creeping along the grout in your shower. Your first instinct is right: don't ignore it.
Mold in the house is a health issue, a structural issue, and — if you're a tenant in Ontario — a legal issue. But before you panic or reach for a bottle of bleach, you need to know what you're actually dealing with. Not all mold is the same. Not all of it requires a professional. And treating it wrong can make things significantly worse.
This guide walks you through everything: types of mold, health risks, when to DIY vs. call a remediation company, what professionals actually do, cost ranges, Ontario-specific tenant rights, and how to keep it from coming back.
What Kind of Mold Are You Dealing With?
The word "mold" covers thousands of fungal species, but in a residential home, you're most likely dealing with one of a handful of common types.
Common Household Mold (Usually Lower Risk)
Cladosporium is one of the most common indoor molds — olive-green or brown, found on fabrics, carpets, wood surfaces, and HVAC systems. It's an allergen but rarely dangerous for healthy adults.
Penicillium grows fast on water-damaged materials and insulation. It spreads easily and has a distinctive musty smell. Also allergenic, but manageable with prompt removal.
Aspergillus is extremely common indoors and comes in many colours. Most strains are harmless, but some can cause infections in people with compromised immune systems.
Black Mold (Stachybotrys chartarum) — The One Everyone Fears
"Black mold" has become a catch-all term, but true black mold — Stachybotrys chartarum — is a specific species that requires sustained moisture (think flooding, chronic leaks, or long-term water damage) to grow. It's slimy, dark greenish-black, and typically found behind drywall, under flooring, or on ceiling tiles after serious water events.
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Here's the important nuance: black mold is potentially more harmful than common household molds, but the colour alone doesn't tell you the whole story. Any mold in large quantities, in hidden spaces, or in a home with vulnerable occupants is a serious concern — regardless of species.
If you're unsure what type of mold you have, that uncertainty itself is a reason to call a professional.
Health Risks: Who Is Most Vulnerable?
Mold affects people differently. For healthy adults, a small patch of mold in a well-ventilated area might cause minor irritation. For others, the same exposure can trigger serious health events.
Common Symptoms of Mold Exposure
Persistent coughing, wheezing, or shortness of breath
Nasal and sinus congestion
Itchy, watery, or red eyes
Skin irritation or rashes
Headaches and fatigue
Aggravated asthma symptoms
Who's Most at Risk
Children — developing lungs are more susceptible to airborne spores.
Elderly individuals — weakened immune systems reduce their ability to fight off mold-related respiratory infections.
People with asthma or allergies — mold is a well-documented asthma trigger. Even low-level exposure can cause flare-ups.
Immunocompromised individuals — those undergoing chemotherapy, organ transplant recipients, or anyone with HIV/AIDS face the highest risk of severe complications, including fungal lung infections.
Pregnant women — emerging research links mold exposure during pregnancy to respiratory issues in newborns.
If anyone in your home falls into these categories, your threshold for calling a professional should be lower — don't wait to see if it gets worse.
The DIY Threshold: When Can You Handle It Yourself?
Here's the honest answer: a small amount of mold on a non-porous surface can often be handled by a careful homeowner. Anything beyond that warrants a professional.
When DIY Is Reasonable
✅ Area is under 10 square feet (roughly 3 ft × 3 ft)
✅ Surface is non-porous — tile, glass, metal, sealed countertops
✅ The moisture source has been fully identified and fixed
✅ No one in the home is immunocompromised, asthmatic, or very young/old
✅ The mold is visible and accessible — not behind walls or under flooring
How to do it safely:
Wear an N95 respirator (not just a dust mask), nitrile gloves, and eye protection
Ventilate the area — open windows and use a fan exhausting outward
Spray with undiluted white vinegar or a commercial mold cleaner (not bleach on porous surfaces — bleach doesn't penetrate and the mold will return)
Scrub thoroughly, then wipe clean with a damp cloth
Dry the area completely — dehumidifier if necessary
Bag and dispose of all cleaning materials in sealed plastic bags
When to Call a Professional — No Exceptions
🚫 Area is over 10 square feet
🚫 Mold is on or inside porous materials — drywall, insulation, wood studs, carpet, ceiling tiles
🚫 You can smell mold but can't find it (it's behind a wall or under a floor)
🚫 Mold has returned after previous DIY treatment
🚫 The moisture source is unknown or ongoing
🚫 You or anyone in the home has symptoms connected to being in the house
🚫 You suspect black mold (Stachybotrys)
🚫 The mold is in your HVAC system
The 10 sq ft rule comes from the U.S. EPA and is widely adopted by Canadian remediation professionals. It exists because anything larger represents a significant spore load — disturbing it without proper containment spreads contamination to previously clean areas.
What Professional Mold Remediation Actually Looks Like
If you've called a mold remediation company, here's what the process should look like. A legitimate contractor won't just spray bleach and call it done.
Step 1: Assessment and Moisture Testing
The first visit involves a thorough inspection — visual examination, moisture meters to find hidden damp spots, and sometimes thermal imaging to detect moisture behind walls. This tells the remediator where the mold is and, critically, why it's there.
Step 2: Containment
Before anything is touched, the affected area is sealed off with heavy plastic sheeting and negative air pressure is established using specialized fans. This prevents spores from spreading to the rest of the home during removal. HVAC vents in the area are blocked.
Step 3: Air Filtration
HEPA air scrubbers run continuously throughout the job. These capture particles as small as 0.3 microns — mold spores are typically 1–30 microns. This is non-negotiable on a legitimate job.
Step 4: Removal
Non-porous surfaces: Cleaned with antimicrobial solutions
Porous materials (drywall, insulation, wood studs): Physically removed and bagged. You cannot clean mold out of porous material — the mycelium penetrates the surface. Cutting out and replacing is the only real fix.
Subfloor/structural wood: May be wire-brushed, sanded, treated with an encapsulant, or replaced depending on the extent of damage
Step 5: Post-Remediation Testing
Once the job is complete, a clearance test (air sampling or surface swab) is taken — ideally by a third party, not the same company that did the work. This confirms mold levels are back to normal baseline.
Step 6: Repairs and Rebuild
Remediation leaves holes. The mold company removes the bad material; a contractor handles the rebuild — new drywall, insulation, painting.
How Much Does Mold Remediation Cost in Ontario?
Costs vary significantly based on scope, location in the home, and severity of the infestation.
Scope
Typical Cost Range
Small bathroom/single-room job
$800 – $1,500
Medium infestation (crawlspace, partial basement)
$1,500 – $3,500
Large infestation (full basement, structural)
$3,500 – $6,000+
Severe whole-home contamination
$6,000 – $15,000+
These figures are rough Ontario market estimates. Factors that drive costs up:
Mold inside HVAC systems (full duct cleaning required)
Mold behind multiple walls
Discovery of additional damage during remediation
Older homes with asbestos in affected materials (requires separate abatement)
Get at least three quotes. Ask every company for their remediation protocol in writing. If a company won't outline their containment and HEPA filtration plan, walk away.
Ontario Tenant Rights: What Your Landlord Is Required to Do
If you're a tenant in Ontario and you've found mold, you have strong legal protections under the Residential Tenancies Act.
Landlords are legally required to maintain rental units in a good state of repair, fit for habitation, and compliant with health, safety, and housing standards. Mold — which violates Ontario's housing maintenance standards — triggers this obligation.
What You Should Do
Document everything. Photograph the mold with timestamps. Write down symptoms.
Notify your landlord in writing (email or letter with delivery confirmation). Give them a reasonable timeframe to respond — 24–48 hours for urgent health issues is standard.
If they don't respond or refuse: File a complaint with your local municipality's property standards department. Inspectors can issue orders requiring remediation.
Escalate to the Landlord and Tenant Board (LTB): You can file a T6 application (maintenance issues) seeking an order for remediation, a rent abatement, and/or compensation.
Do not withhold rent without a formal order — this can expose you to eviction proceedings even if you're in the right.
Ontario landlords who ignore mold complaints face fines, orders to remediate, and rent abatement awards. The law is on the tenant's side here.
Should You Test for Mold — or Just Remediate?
This is a question that comes up constantly, and the honest answer depends on your situation.
Test first if:
You smell mold but can't find it
You want to identify the specific species (relevant for sensitive occupants)
You need documentation for legal, insurance, or sale purposes
You want a baseline to confirm remediation was successful (post-clearance testing)
Skip testing and just remediate if:
You can see significant mold growth — you already know it's there
The cause (flood, leak, condensation) is obvious
Testing would delay action and you have vulnerable occupants
Testing adds $200–$600+ to your costs and takes days for lab results. If you're staring at a square foot of black growth on your drywall after a basement flood, you don't need confirmation — you need it gone.
The one case where testing is always worth it: post-remediation clearance testing. This independently verifies the job was done right. Request it as part of any remediation contract.
Preventing Mold: Keep It From Coming Back
The best mold remediation is the one you never need. Mold requires three things: a food source (organic material), a temperature above 0°C, and moisture. You can't remove the food source — it's your house. But you can control moisture.
High-Impact Prevention Habits
Control indoor humidity. Keep indoor relative humidity between 30–50%. A $30 hygrometer will tell you what you're working with. A dehumidifier in the basement during Ontario summers is often necessary.
Ventilate aggressively. Run bathroom fans for 20 minutes after every shower. Use the range hood when cooking. Crack a window in winter when the house gets stuffy — moisture builds up fast in sealed homes.
Fix leaks immediately. A slow drip under the sink takes 24–48 hours to start growing mold. Check under sinks, around toilets, and behind appliances on a regular schedule.
Insulate cold surfaces. Condensation on cold pipes, windows, and exterior walls is a chronic moisture source. Proper insulation eliminates it.
Direct water away from the foundation. Grading, eavestroughs, and downspout extensions keep water out of the basement — the most common site of serious mold in Ontario homes.
Don't carpet damp areas. Basement carpet over concrete is a mold farm waiting to happen. Use area rugs on raised flooring, or stick with sealed concrete.
FAQ
Q: Is all black mold dangerous?
Not all mold that appears black is Stachybotrys chartarum (true toxic black mold). Many common molds can look dark or black. That said, any significant mold growth — regardless of colour — should be taken seriously, especially in large quantities or when vulnerable occupants are present. When in doubt, have it tested.
Q: Can I use bleach to kill mold?
Bleach works on non-porous surfaces like tile and glass. On porous materials like drywall or wood, bleach doesn't penetrate deeply enough to kill the mold roots (mycelium) — it removes the surface discolouration temporarily, and the mold returns. For porous surfaces, physical removal is the only real solution.
Q: How long does mold remediation take?
A small single-room job can be completed in one day. A medium-sized basement remediation typically takes 2–4 days. Larger whole-home projects can run 1–2 weeks. Add time for post-remediation testing (lab results take 3–5 business days) and repairs/rebuild.
Q: Does home insurance cover mold remediation?
It depends on the cause. Sudden and accidental water damage (burst pipe, appliance failure) is usually covered. Gradual leaks or maintenance neglect typically are not. Review your policy and call your insurer before doing any work — documentation matters for claims. Many Ontario policies have limited or excluded mold coverage, so read the fine print.
Q: Can mold grow back after remediation?
Yes — if the underlying moisture problem isn't fixed. Remediation removes the existing mold, but mold spores are everywhere in the environment. Without addressing the source (leak, humidity, condensation), new growth will start within weeks. A reputable remediation company will identify and help you understand the moisture source before they begin the removal work. If they don't, that's a red flag.
The Bottom Line
Mold in the house is a problem that compounds — it doesn't stay small on its own, and it doesn't fix itself. Small spots on non-porous surfaces? Clean it, find the moisture source, fix it, and monitor. Anything larger, porous, hidden, or affecting vulnerable household members? Call a professional.
The risk of DIY-ing a large mold job isn't just that it won't work — it's that disturbing a significant mold colony without containment spreads it further. You can turn a basement wall problem into a whole-home problem in an afternoon.
Get it assessed. Get it done right. Then fix the moisture source so it doesn't come back.
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