Vapour Conversions

Vapour Permeance (µg/N·s) to Vapour Resistance (MNs/g)

Vapour permeance is a measure of a material's ability to allow water vapor to pass through it. It quantifies the ease with which vapour flows through a unit area of the material under a unit pressure difference. The higher the vapour permeance, the more vapor can pass through, implying the material is more permeable.

For vapour barriers or vapour permeable membranes, vapour permeance is tested ested in accordance with the ASTM-E96 Procedure B – Water Method at 23°C 50% relative humidity. It needs to be processed through a series of conversions to become usable in hygrothermal modelling.

Vapour Resistance is a measure of how effectively a material resists the passage of water vapor through it. It is the inverse of vapour permeance, indicating how difficult it is for vapour to move through a material under a certain pressure gradient. The higher the vapour resistance, the less vapor can pass through, indicating that the material is more resistant to moisture transport.

The formula for converting vapour permeance (P) to vapor resistance (R) is:

$$R= 1/P$$

Vapour Resistance (MNs/g) to Vapour Resistivity (MNs/g·m)

Vapour resistivity is a material property that quantifies how resistant a given thickness of material is to water vapor flow, normalized per unit of thickness (m). It indicates how difficult it is for vapour to pass through each meter of a material and is an intrinsic characteristic, independent of the material's actual thickness.

The formula for converting vapour resistance (R) to vapour resistivity (ρ), with t being the thickness of the material in meters, is:

$$ρ= R/t ​$$

Vapor Resistivity (MNs/g·m) to µ-value or Water Vapour Diffusion Resistance Factor

The µ-value (also known as the water vapour diffusion resistance factor) is a dimensionless value that indicates how resistant a material is to the passage of water vapour compared to still air. Essentially, it tells us how much more difficult it is for water vapour to pass through a material compared to a layer of still air of the same thickness.

The formula for converting vapour Resistivity (ρ) to µ-value (μ), with δa​ being vapour permeability of still air, given as 0.2 g/m⋅MNs, is:

$$μ= ρ/δ a ​$$

Note: The µ-value is a critical factor in determining the moisture performance of building materials, especially when designing the building envelope. It must be determined for hygrothermal modelling via HAMT in EnergyPlus.