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Mould is one of the most emotionally charged issues in residential tenancies — and one of the most legally consequential for landlords. In Ottawa, our extreme humidity extremes (muggy summers, tightly-sealed winter homes) create ideal conditions for moisture problems. This guide explains your legal duty, how to respond to complaints, and how to prevent mould before it becomes a $20,000 problem.
Under Ontario's Residential Tenancies Act, landlords must keep rental units in a good state of repair and fit for habitation. Persistent mould that affects habitability can constitute a breach of that duty — and tenants can file an application at the Landlord and Tenant Board (LTB) if you fail to act.
Responsibility often turns on causation. The LTB generally distinguishes between:
| Cause | Typical responsibility |
|---|---|
| Roof, plumbing, or foundation leak | Landlord — structural defect |
| Poor ventilation or inadequate exhaust | Landlord — building systems |
| Tenant keeps unit too humid / blocks vents | Shared or tenant — behavioural |
| Tenant fails to report a leak | Tenant may bear some responsibility |
Important nuance: Even where tenant behaviour contributes, landlords usually still cannot simply ignore the problem. The duty to maintain is ongoing, and the LTB expects a landlord to investigate and take reasonable steps regardless of fault.
Ottawa's climate is uniquely unforgiving for moisture. Our humid summers mean high indoor humidity, while our long, cold winters create condensation on cold surfaces. Here are the most frequent sources we see.
Warm, moist indoor air hits cold exterior walls, windows, and corners — especially in poorly insulated units or older buildings. This is the single most common cause of mould in Ottawa homes.
Inadequate or blocked exhaust fans trap steam in bathrooms and kitchens, feeding mould growth on ceilings, grout, and window frames.
Slow leaks under sinks, behind appliances, or in the ceiling above create invisible mould colonies long before visible staining appears.
Poor grading, clogged weeping tiles, or foundation cracks let water in, particularly during spring snowmelt and heavy summer rain.
Furniture pushed against exterior walls, closed closet doors, and full storage areas prevent air movement and trap dampness.
How you respond in the first 72 hours usually determines whether a complaint becomes a cooperative resolution or an LTB application. Follow this sequence.
Mould remediation costs in Ottawa vary widely depending on the extent and location. Here are typical ranges for common scenarios.
| Scenario | Typical cost | Who typically pays |
|---|---|---|
| Surface mildew (tenant cleaning) | $0 – $100 | Tenant |
| Small area remediation (bathroom ceiling) | $500 – $1,500 | Landlord (building cause) |
| Moderate drywall removal & rebuild | $2,000 – $6,000 | Landlord |
| Extensive remediation (multiple rooms, HVAC) | $8,000 – $25,000+ | Landlord — may be insured |
| Secondary source fix (grading, weeping tile) | $3,000 – $15,000 | Landlord |
Check your insurance. Many landlord policies cover sudden water damage but exclude long-term seepage and gradual mould growth. Review your policy carefully — and if remediation stems from an insured event, file promptly with full documentation.
Rent abatement risk: If mould makes part of the unit unusable and you fail to address it promptly, the LTB can order a retroactive rent abatement — sometimes 25% to 100% of rent for the affected period. Ignoring the problem is far more expensive than fixing it.
Prevention is dramatically cheaper than remediation. Work through this checklist for every Ottawa rental unit, ideally before winter.
Ottawa Prime Properties conducts seasonal moisture inspections, coordinates certified remediation, and documents everything — protecting both your property and your legal position across Ottawa, Kanata, Orléans, and Barrhaven.
Get a Free Moisture Assessment