Condensation and vapor barrier

Condensation and vapor barrier

In several cases involving hidden defects , the vapor barrier membrane, in front of the foundation walls in the basement, was poorly installed or missing.

The building envelope components – which are thermally insulated – must include a vapor barrier (i.e., a material with low water vapor permeance). The vapor barrier's role is to minimize the amount of water vapor circulating by diffusion and to prevent moisture and condensation problems within the structure and on the surfaces of the building materials.

Vapor barrier installation

The vapor barrier should not be installed directly on the concrete foundation walls. This membrane must be installed in front of the insulation from the inside. A vapor barrier must also be installed in front of the insulation at the level of the wood framing [at the top of the foundation walls].

Improper installation of the vapor barrier membrane and/or the absence of the vapor barrier membrane causes condensation and the formation of mold on the concrete surface.

Vapor barriers have long been addressed in building codes and best practice documents. For example, the 1970 Canadian Residential Building Code clearly states that the vapor barrier must be installed on the warm side of the insulation.

Vapor barrier (CMHC 1984)

It is also known in the construction field that, according to best practices, the vapor barrier membrane must be installed on the interior side of the insulation.

According to a 1984 CMHC (Canada Mortgage and Housing Corporation) document, a vapor barrier is a barrier that limits the diffusion of water vapor through the walls, roofs, and floors of a building. Its primary role is to prevent condensation within the building envelope, which can damage building materials and reduce the effectiveness of insulation. The vapor barrier must be installed on the warm side of the insulation.