Mould in a Quebec Home: Detect, Declare, Act

A black stain at the bottom of a basement wall almost always triggers the same reaction: get the air tested, get a number, find out whether it is serious. That reflex leads to a dead end, because in Quebec no number delivers that verdict. Here is what actually decides, and three situations where the recourses and the clocks are not the same: the owner in their own home, the buyer after the sale, the tenant facing their landlord.

Why no mould threshold exists in Quebec

The Institut national de santé publique du Québec (INSPQ) is categorical: there is at present no exposure guideline value for mould inside non-industrial buildings. This is not a regulatory lag. It is simply not possible, with current knowledge, to quantify the relationship between exposure to airborne mould and health effects.

Health Canada aims straight at the laboratory report: in the absence of guideline values, the results of tests for the presence of mould in indoor air cannot be used to assess risk. The department has even disowned its own 1995 values, which do not represent a safe level.

A Quebec standard does exist, and it contains no number. The Bureau de normalisation du Québec published standard BNQ 3009-600 on May 1, 2020. The INSPQ presents it as the first Quebec standard on mould in dwellings: it covers identifying the causes, assessing the contamination, carrying out the work and quality control. It is a voluntary document, not a regulation, and it sets no concentration threshold.

So be wary of a report announcing that a standard has been exceeded. What the public bodies recommend is to correct the situation wherever there is excessive humidity and persistent contamination, visible or hidden behind materials, whatever the species.

What feeds it, and the 48 hour window

Mould does not grow on a dry material. The INSPQ names the triggers: high relative humidity, water damage, flooding, infiltration, sewer backups, and porous materials soaked with water and exposed to humidity for more than 24 to 48 hours. The SCHL (Canada Mortgage and Housing Corporation) sets the same limit after a leak: you have less than 48 hours before mould starts to form. Water damage dried within 24 hours therefore does not have the same sequel as water damage dried in two weeks. The springtime origin of that damage is covered in our piece on spring melt and basement water infiltration.

Indoor humidity level. The SCHL and the INSPQ aim at the same range, 30 to 50%. The SCHL adds a winter detail few owners know: when it is -10 °C outside, the indoor level should be 30%, and it can drop to 25% in extremely cold regions. A hygrometer costs $10 to $60 according to the SCHL. Failing that, four signs are enough: condensation on the windows, damp stains on walls and ceilings, mould in the bathroom, a musty smell.

The scale the SCHL uses instead of a number

Since concentration settles nothing, it is the surface affected that settles it. The SCHL manual sorts situations into three tiers.

Three details from the same source contradict widespread habits. Do not use bleach: the SCHL recommends warm water and unscented dish soap, which lets you smell whether the odour persists after cleaning. Do not paint over it: paint does not destroy mould and does not stop it from growing. And cleaning alone stays temporary, since the mould will come back if the source of humidity is not fixed. Minimum equipment, even for a small surface: safety goggles, an N95 mask or equivalent, rubber gloves.

Mould in a home for sale: box D7.3

The mandatory Déclarations du vendeur sur l’immeuble form, DV 00001 version V24 of June 2022, devotes its section D7 to indoor air quality. Three questions there touch the subject directly.

• D7.1: has regular and significant condensation ever formed in winter, on windows, skylights, walls or the ceiling?
• D7.2: has there ever been a foul odour, including from humidity?
• D7.3: have there ever been traces of mould, rot or fungus, and where?

Two other boxes catch the cause rather than the effect: D4.1 on water infiltration, and D5.2 on problems that have affected the basement or the crawl space, which lists rot. All of them bear on what has already happened, not on what remains. The form is taken apart in our article on the seller’s declaration in Quebec.

Hiding a known problem is not erased by a clause. Article 1733 of the Code civil is explicit: a seller may not exclude or limit their liability unless they have disclosed the defects of which they were aware or could not have been unaware, except where the buyer buys at their own risk from a non-professional seller. If an episode of humidity comes back to you before listing, the seller’s guide lists the documents to gather.

The buyer who finds mould after the sale

Here the clock counts for more than the stain. Article 1739 of the Code civil requires the buyer to give written notice of the defect within a reasonable time from its discovery, and it adds a sentence cut for mould: where the defect appears gradually, the time begins to run on the day the buyer could have suspected its seriousness and extent.

Éducaloi places that period between six months and a year depending on the circumstances, longer if the defect shows up seasonally. Two steps condition what follows: letting the seller observe the defect before having it repaired, except in an emergency, and respecting the three year prescription of article 2925, which runs from the discovery and not from the date of purchase. The four conditions of a latent defect are detailed in our article on latent defects in Quebec.

The case of an odour with no stain. An inspector who writes “musty smell in the basement, source not located” is not giving you a diagnosis. He is giving you a sign, and he is dating it. Have the source of humidity looked for while the promise to purchase still protects you, then keep the report: it is what will establish what you could see, and when.

Tenant and lessor: article 1910 and what the TAL actually decides

Article 1910 of the Code civil obliges the lessor to deliver a dwelling in a good state of habitability and to maintain it in that state throughout the lease. A stipulation whereby the lessee acknowledges that the dwelling is in good condition is without effect. Article 1913 covers serious cases: a dwelling is unfit for habitation where its condition constitutes a serious risk to the health or safety of the occupants.

The Tribunal administratif du logement (Quebec’s rental board) classes mould among situations of unsanitary conditions and immediately draws the nuance: a dwelling in a poor state of habitability is not automatically unfit for habitation. The tenant may ask for resiliation of the lease, an order compelling the lessor to carry out work, a rent reduction or damages. Punitive damages are possible where the dwelling became unfit through the lessor’s negligence, for applications filed since February 21, 2024. On obvious documentary evidence, inspection reports, evacuation notices or photographs, the Tribunal summons the parties within ten working days.

Two decisions summarized by the Tribunal show why nobody can promise you a result. In Lumande c. Raamco International Properties Canadian Ltd., of June 25, 2019, the tenants obtained $10,060 in rent reduction and $8,000 in moral damages, with this reasoning: proof of the contamination must rest on objective criteria, the contaminant and the extent of the contamination have to be identified. In Henrichon c. Hoang, of November 3, 2023, a microbiologist established that the mould did not make the premises unfit, and the lease was resiliated at the tenants' fault. Same family of facts, opposite results.

What insurance covers, and what it excludes

Here is the most counterintuitive part. In its brochure on insurance and water damage, the Bureau d’assurance du Canada lists as “not covered” damage caused by humidity, condensation, rot, mould and gradual deterioration. Its example is precise: a roof that has reached the end of its useful life and that you are slow to replace, hence infiltration that causes humidity and mould on the ceiling and the walls.

The word that decides is in the wording of the endorsements. Ground water and sewer backup covers sudden and accidental infiltration of ground or surface water. Above ground water covers sudden and accidental infiltration of rain or snow. Sudden and accidental: slow degradation fits into neither one. Mould that follows a covered loss can therefore be indemnified along with it, while the same mould born of a leak tolerated for two winters is not.

The same organization recommends, for that matter, having a roof more than 20 years old inspected by a professional. Have your own situation confirmed by your insurer, and ask for the answer in writing.

Frequently asked questions

Will an air test prove my house is contaminated? No. Health Canada writes that in the absence of guideline values, those results cannot be used to assess risk. A sample documents a file, it does not deliver a verdict.

I cleaned the stain and it came back. Why? Because cleaning does not fix the source. The SCHL says it plainly: as long as the humidity problem persists, the mould comes back.

The seller checked no on D7.3 and I found mould. Do I automatically have a recourse? No, but you have a dated and signed document. What follows depends on proving that the problem existed before the sale and that the seller knew of it or could not have been unaware of it. Give written notice without delay and consult a lawyer before undertaking any work.

My landlord is doing nothing. Can I stop paying the rent? Not on your own initiative. The route provided runs through the Tribunal administratif du logement, which can order work or grant a rent reduction. The rent exemption of article 1915 applies to a dwelling that has become unfit and requires notice to the lessor before leaving or within the 10 days that follow.

To go further, read the pre-purchase inspection item by item and what makes a home hard to insure.

An odour in the basement making you hesitate before an offer or before listing? Write to me. I will tell you which verifications to write into the promise to purchase, and which documents to gather before filling out the seller’s declaration.

Georges Matar

Residential Real Estate Broker · RE/MAX DU CARTIER INC.

Contact Georges