"The outer or hard shell should keep out wind and water, but allow moisture to escape outwards. This is done by using waterproof breathable fabrics of which there are many types. These generally perform best in dry, cold conditions, when warm moist air is more easily transferred out through the fabric. This driving effect is enhanced by a water-repellent coating on the outside that causes water to bead and roll off. A saturated outer fabric means a reduced rate of breathability. Unfortunately this is exactly the type of problem liable to be encountered in poor winter conditions. On cold days, wearing thick insulation means that less body heat reaches the outer layers, so decreasing the driving effect pushing moisture out through the shell."
In fact, when the outer fabric is cold and saturated, membrane garments have virtually zero breathability. This situation doesn't change until the outer becomes warm and dry. This is why animals have evolved to use highly breathable feathers and fur outside of their skins, for optimum performance across a wide range of conditions, even after saturation.
"Winter Skills", written by Andy Cunningham and Allen Fyffe, is the official handbook of the Mountaineering Instructor Certificate and Winter Mountain Leader schemes.