It's not super intuitive at first, but its cold climates were this is actually a big issue. Water vapor "wants" to equalize from a more humid to less humid environment. So when it's cold out - the humidity can drop rather low. Then you take a shower and create water vapor - which flows into the walls, through the insulation, and condenses against the colder exterior. Open a wall in any stick frame house in the rocky mountain region and mildew will be present. I am building a highly insulated house in a cold climate area and have blasted all the wood with copious amount of a silver based coating to inhibit mold growth.