Vapor permeability.
Technical notions on the values used to measure vapor diffusion.
The diffusion of water vapor, the principles that
regulate it.
The diffusion of water vapor is nothing but the movement of molecules through porous materials (i.e. wood, insulators, reinforced screed, etc.) due to the pressure differential or vapor pressure. Water vapor will tend to always migrate from high pressure areas to low pressure areas. That is to say that, as for the transmission of heat, it will migrate from hot to cold (this is because the hot air is able to absorb more water than cold air).
In winter, with the passage from the inside to the outside of a structure, condensation can occur in dispersing structures (perimeter walls, roof slabs, fixtures, etc.) and can take two forms, either superficial or interstitial.
Condensation can give rise to the development of mold and therefore to the development of unhealthy environments, while damaging some layers of the structure, especially in those points which are moisture-sensitive.
What mentioned above has important repercussions on the choice of materials and the definition of the stratigraphy, in fact what may work in some areas may not be the right solution in others.
Water vapor diffusion, how to regulate it.
Vapor diffusion is typically thought of as a negative phenomenon - one that needs to be completely stopped. However, the opposite is true, vapor diffusion is a positive mechanism that can be used to a designer’s benefit, and is a very important drying mechanism for an enclosure assembly.
For example in buildings located in cold climatic zones, semi-transpiring membranes are often used, the usage of which ensures that the release of water vapor is moderated. This interior vapor retarder limits the diffusion of moisture through the wall assembly toward the exterior. Outward vapor diffusion drying can still occur from within the wall cavity to the exterior through the sheathing, membrane, and cladding.
Vapor permeability and diffusion, the key parameters.
Among the many parameters used to assess the transpiration of materials are vapor permeability δ (delta), permeance π (pi) and water vapor diffusion resistance WVR.
Vapor permeability δ [kg/(msPa)] can be considered as equivalent to thermal conductivity λ, but for vapor transmission instead of heat transmission. It is defined as the amount of vapor that can pass through a meter of material thickness per second, with a given pressure differential measured in Pascals between the two opposite surfaces of the material.
Permeance π indicates the degree of water vapor permeability of a non-homogeneous material of a given thickness and is thus derived from the permeability δ by the formula π = δ/d, where d indicates the thickness in meters of the material.
Vapor diffusion resistance WVR represents the material's ability to resist the diffusion of vapor into its interior, with WVR = d/δ [(m²sPa)/kg]. In Europe, this resistance is commonly expressed with the water vapor diffusion resistance factor µ, which is obtained from the ratio of the vapor permeability of air to that of the material.
Taking SUPERCEL BUILDING as an example, with a value of µ = 50, it follows that the layer of material resists water vapor diffusion 50 times more than an equivalent thickness of air.
