The most common problem in small residential buildings is the presence of moisture in the basement. A significant number of cases of hidden defects is connected to this problem.


Humidity in the basement causes the appearance of mold and fungi that could be harmful. In some cases, the mold forms a thick, black layer on the foundation walls.

détématerial deterioration

Also, the presence of mold and moisture in the basement can cause deterioration of building materials, especially those in direct contact with the moisture source. For example, it could be the wooden structure of the floor that rests on the concrete slab on wet ground or the rotting of the ends of the wooden beam (embedded in the foundation walls). Thus, moisture rises in the concrete, by capillarity, and attacks the wooden structure that touches the concrete.


Moisture, mildew and rotten wood in the basement usually give off foul odors in the building. Note that mold spores, flying in the air, can affect the health of occupants, especially those with asthma or chronic respiratory problems.

Very often, new homeowners, who are struggling with basement moisture problems, think this situation is a hidden vice. This is because it is generally difficult to detect this problem during the pre-purchase inspection, as this inspection is visual in nature and the majority of the basements are fully developed.

Moisture sources

There are many sources of humidity in the basement of a building and can be caused by deficiencies related to normal wear and tear of materials or poor workmanship in the installation of certain construction elements or even inappropriate human activities, lack of regular maintenance or necessary non-replacement of building components.

- causes the most common for moisture problems in the basement are:

  1. Cracks in the foundation walls- these cracks can sink in the foundation walls. They are not visible inside a finished basement, because they are hidden behind the finish and it is generally difficult to know that these cracks flow without an expert can open the interior walls and perform a watering test;
  1. Absence of curbs in front of basement windows - when the threshold clearance of a basement window is not adequate in relation to the ground level, then the water can flow during heavy rain, inwards, behind the finish;
  1. Negative slope of the ground - the negative slope of the ground directs the water towards the building and causes infiltrations of water, towards the interior, by the cracks and the bottom of the windows. This solicits the drainage of the foundations and causes infiltration of water at the junction to enter the slab on the ground and the sole;
  1. Descents of the gutters - the downspouts of the gutters, which drain near the foundation walls, also significantly solicit the drainage of foundations and promote water infiltration and excess moisture in the basement;
  1. Inadequate clearance between the foundation walls and the ground - If the clearance between the junction of the foundation walls and the ground is inadequate, there is a risk that the water seeps in, towards the interior of the basement, by this junction. This is more particularly in the case where the sealing system, at the base of the outer facing, is poorly done;
  1. Land higher than the structure of the basement ceiling- in some of the buildings examined, the exterior layout (terraces, balconies, etc.) had been made in such a way that the wooden structure of the basement ceiling [resting on the foundation walls] was lower than the exterior ground . For this, the soil moisture attacks this structure and causes its deterioration and then significant water infiltration occur inside the basement;
  1. Deficiencies in masonry- at the base of the masonry facing, there is a sealing system and chantepleures. The waterproofing system prevents water from seeping into the interior of the building through the top of the basement foundation walls. As for the chantepleures, they direct the water - which can penetrate behind the bricks - towards the outside of the building. In the event of deficiencies of this waterproofing system and / or the absence of weeping holes at the base of the exterior facing, the water enters through cracks and shears (in the masonry) or through hollowed mortar joints or again by cracked or missing joints. Then the water trickles down to the base of the siding and seeps inward causing moisture in the basement;
  1. French drain: sometimes the French drain is badly installed at the origin (inverted slope which prevents the drain to evacuate the water in the retention pit, drain installed in zigzag, drain crushed or clogged, drain installed on the sole or lower than the footing, lack of backfill around the drain, backfill with clay soil with very fine particles, etc.). It is also possible that the French drain has not been replaced at the end of its useful life (between 25 and 30 years), and that it is not functional. As a result (poorly installed or aged and non-functional drain), water accumulates around the building and eventually penetrates, inside the basement, by the junction between the foundation walls and the slab on the ground or by possible cracks in the slab on the ground.
  1. Human activities - finally, human activities can generate a lot of moisture that will not be evacuated to the outside, such as the presence of a bathroom in the basement, while there is no window or fan connected outside. In addition, there may be a lot of goods of an organic nature that may have been deposited near the exterior walls of the basement. This environment is ideal for causing mold formation on the surfaces of goods and building materials.

It should be noted that porous organic materials such as textiles, paper, cardboard and leather tend to absorb suspended moisture particles. When such materials are stored under conditions of high relative humidity, for example in a wet basement, they can mold quickly.

In the case of a wet basement, installing a dehumidifier can reduce the humidity, but that does not solve the problem. For this reason, it is necessary, at first, to identify the source of the problem with an expert and then to solve the problem of humidity in order to have a dry and healthy basement. The resolution of this problem will also prevent the formation of mold and rotting of the wood structure and finish as well as unpleasant odors or discomfort and associated health problems.

Humidity and mold in the basement

Humidity and mold in the basement