Vapour diffusion
Assessing risks in insulation retrofits using hygrothermal software tools, Heat and moisture transport in internally insulated stone walls, by Joseph Little, Calina Ferraro and Beñat Arregi, published by Historic Environment Scotland in 2015, defines vapour diffusion as: ‘The movement of vapour molecules in capillaries within a component, in an attempt to equally distribute themselves and reach equilibrium. It always occurs in the direction of high vapour pressure to low vapour pressure.’
It defines vapour diffusion resistance factor (μ-value) as a: ‘Material property describing the rate of vapour diffusion in a material as compared to still air: By definition, still air has a μ-value of 1; all other materials have a μ-value greater than 1.’
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