Go to Forum Home Building Refurbishment and Retrofit Vapour Permeability to Vapour Resistivity or Mu-value

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    • #31800

      Can any members with experience in hygrothermal simulation know how I go about converting the vapour permeability of a material (100 mg/m²/h) to an equivalent vapour resistivity or mu. I am attempting to compare the vapour resistance of a Multi-Pro magnesium silicate board with other products such as plasterboard and Fermacell. However, the magnesium board datasheet only gives the vapour permeability in mg/m²/h.

      Any guidance gratefully received as the manufacturer is not much help.

    • #38894
      Tom Foster

      It doesn't cover the conversion from permeability and IMHO is written to confuse. So many units to convert – I don't trust to getting it right, am also looking for a definitive set of conversions.

    • #38895
      Tom Foster


    • #38896

      Permeability and resistivity are just reciprocal measures. So not much to understand there.

      permeability = 1 / resistivity ; resistivity = 1 / permeability ; resistivity x permeability = 1

      As to all the different measures, well that's just life. People have created them all for better or worse. So if you need to convert one to another, you need the appropriate formula. Hence lots of formulae.

      As to definitive, well no. There might be particular numbers that have been arbitrarily defined for particular purposes. But the reality is that the permeability depends on at least the temperature and the humidity and the pressure, so it's all approximate unless you go into enormous detail. Here's an example:


      You're right that the builddesk note doesn't explain the reciprocal relationship between resistivity and permeability,but otherwise it seems reasonable to me. The note on p4 is very helpful: “Tip: these conversions become easier to understand if you look at the units of measurement involved in each case.”

      If you think it's confusing for the building industry, try the clothing industry:


    • #38897

      There might be particular numbers that have been arbitrarily defined for particular purposes.

      The answer to that appears to be ISO 10456:2007 – Building materials and products — Hygrothermal properties — Tabulated design values and procedures for determining declared and design thermal values

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