Vapour-open
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-open and vapour-closed as a: ‘Qualitative description of how easily vapour can diffuse through a material. Vapour-open materials, such as mineral wool or wood fibre, have low μ-values (vapour diffusion resistance factors) and allow vapour diffusion to occur relatively easily. Vapour-closed materials, such as a PE-membrane or foil layer, have high a vapour diffusions resistance factor (μ-value), which greatly restricts vapour diffusion.’
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