Just as a temperature gradient exists within a structural element, a dewpoint gradient depending on the water vapour diffusion properties of the element exists too. If at any point in the structure the actual temperature is below the dew point then condensation will occur at that point.

Table A 3.1 gives some typical values of vapour resistance and thermal and vapour resistivities (thermal resistivity is the reciprocal of thermal conductivity – see Table A 2.3).

With more and more insulation being used, the designer must remember to consider both the thermal and vapour properties he or she is specifying. This is particularly true since some very good thermal insulants, for example glass fibre, are also very permeable to water vapour.

Let us now return in Fig. A 3.1 to the wall construction of Fig. A 2.1 and using the standard BRE procedure assess whether there is a risk of interstitial condensation.