Mould and Serpula lacrymans

Mould is one of the reasons why remedies for hidden defects are sought. A potential buyer who finds out that what he thought was his dream home is contaminated by fungi may very well decide to look elsewhere.

Another form of fungus, called Serpula lacrymans, or dry rot, can develop in humid and poorly ventilated areas. Although it is not mould as such, the conditions in which it forms can lead to mould growth in the same area. Thus, it is possible that a building is contaminated by both dry rot and mould, in addition to other types of fungi.


Moulds are fungi that are invisible to the naked eye, but present in nature. They are carried into homes by drafts, humans and pets. In order to grow, mould needs water and nutrients, such as gypsum board or cardboard.

The main causes of mould in a building are:

  • an excess of moisture, lack of ventilation, or low temperature that causes water condensation on surfaces
  • the presence of water vapour in the air due to baths, showers and cooking
  • water infiltrations due to roof or plumbing leaks, foundation cracks or flooding

Other conditions are also conducive to fungal contamination, such as indoor cannabis production. The optimal temperature and humidity conditions required for this type of activity are also favourable to the spread of mould.

Health risks

  • Irritating effects
  • Irritation of eyes, nose and throat

Immune reactions

Allergic reactions caused by the inhalation of fungal spores have been recognized as a health problem by clinicians for decades.

Examples: allergic rhinitis, asthma, runny nose, sinus congestion, etc.

Toxic effects

The majority of toxic effects from the inhalation of mould have been associated with exposure in industrial or agricultural environments, i.e. places where they are highly concentrated and exposure is repeated or chronic. High exposure to mould-contaminated dust can cause organic dust toxic syndrome (ODTS). This condition is characterized by a sudden fever, flu-like symptoms and respiratory problems occurring within hours of a single high exposure. More recently, symptoms associated with ODTS have also been noted in occupants of apartments heavily contaminated by mould and in workers doing renovation work.

Other toxic effects caused by mould occur in cases of repeated exposure to environmental contamination, resulting in a high cumulative dose and occurring in the medium to long term.

Examples: Difficulty concentrating, chronic fatigue, irritability, headaches.

Infectious effects

Not many types of mould that proliferate on building materials or in ventilation systems cause infections. However, aspergillus is a very harmful pathogen that can occur in homes contaminated by mould. The presence of aspergillus could necessitate the evacuation of residents.

Clues that may indicate the presence of mould or excess moisture

Clues that may indicate the presence of mould or excess moisture include:

  • Presence of stains of varying colours, but often greenish or black:
    • on walls, ceilings or carpets
    • around windows
    • in closets, etc.
  • Presence of a characteristic musty, earthy smell
  • Presence of staining, warping, flaking or other signs of water infiltration on walls or ceilings, which may indicate the presence of a problem behind the materials



The broker’s role is not to diagnose a property’s problems, and he is not required to comment on the clues that might suggest the presence of mould.

However, he must share his observations by pointing out the presence of stains, for example, and recommend that his clients have the property professionally inspected, whether it is a residential immovable containing fewer than five dwellings, a multi-housing complex, or a commercial, industrial or institutional building.

Preventing mould contamination

The best way to prevent mould contamination is to reduce the humidity level. The optimum humidity level is between 30% and 50%. Measures to help maintain this level include:

  • ventilating showers and other sources of moisture directly to the outside
  • adjusting the humidity level using dehumidifiers and/or air conditioning units
  • insulating cold surfaces to prevent condensation from forming on the surface of pipes, windows, exterior walls, roofing and floors
  • properly maintaining the building and heating, ventilation and air conditioning equipment
  • cleaning any damage from spills or flooding within 48 hours

Fore more details: Indoor Air Quality - Moulds and Fungi 

Eliminating mould

It is important to identify and correct any cause of water accumulation or infiltration:

  • broke pipes
  • flooding
  • sewer backup

Affected surfaces must be cleaned without delay and damaged porous materials must be discarded.


Small surfaces

  • Clean stains with a clean cloth and an all-purpose cleaner
  • Dry thoroughly

 The use of bleach is not recommended.


Large (over 1 m2), numerous or difficult to clean surfaces, or if the mould reappears after cleaning:

  • Use the services of a specialized firm

Recommend that your clients:

  • make sure the firm has the expertise required
  • ask questions

Air samples can be difficult to interpret. In the vast majority of cases, they are not necessary to assess the problem.

Fore more details: Finding and eliminating mould from your home 

Reminder from the FARCIQ

Mould is one of the reasons why remedies for hidden defects are sought. Claims involving legal action for hidden defects are among the most common received by the FARCIQ.

To prevent disputes involving hidden defects, the broker must:

  • disclose any known factors that could unfavourably affect one of the parties to the transaction or the object of the transaction itself
  • recommend expert analyses when the situation warrants
  • document his records well and save written copies of the recommendations made to his clients

The broker must also stay within the limits of his knowledge, keep to the facts and avoid minimizing or amplifying them.

Last updated on: November 29, 2021
Reference number: 208807