To limit the probability that, where separate products are used for the vapour barrier and insulation, the location of the vapour barriers in the assembly is at a plane in the building envelope where the temperature is sufficiently low to cause the condensation of water vapour from interior spaces on interior surfaces or within assemblies.
This is to limit the probability of condensation, which could lead to deterioration, which could lead to compromised structural integrity of assemblies acting as environmental separators, which could lead to harm to persons.