For calculations to predict the condensation risk in building structures it is necessary to assume suitable values of the thermal and vapour resistivities of the materials. For many materials, reference sources quote values of vapour resistivity, in particular, as a range rather than a single figure. This paper shows that the assumption of mean values may be inappropriate and that it is better to use extreme values from the ranges. Simple rules are stated to guide the choice of values from the range. An example is given, to demonstrate the application of the rules.
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