An environmental control system for a vehicle, in particular for predicting internal misting of a vehicle windscreen, the system comprising a windscreen temperature sensor assembly 28, an ambient humidity sensor assembly 32 and a second temperature sensor assembly 30 for sensing the temperature of the humidity sensor assembly 32. The windscreen temperature sensor assembly 28 is mounted on an interior rearview mirror support bracket 10 for engagement with the interior surface 18 of the windscreen, and the second temperature and humidity sensor assemblies are mounted within a rearview mirror housing 12 mounted to the mirror support bracket 10.