Water vapor and liquid water behave differently. The idea of waterproof/breathable fabric is that it blocks liquid water (rain), but transmits water vapor (evaporated sweat). Transport will be in the direction of high humidity (inside the jacket) to low humidity (outside the jacket). There are two problems here, first if it is could enough for sweat to condense on the inside of your jacket, the material will block it, and second if the atmospheric humidity is high the driving force is reduced. On a cold dry day, the first problem is your main concern, on a warmer wet day, it's the second problem.

Paul