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.
Ah, I see where you are coming from. The standard solution (for > 25 years) where I live is that you install a vapor barrier (plastic) to prevent the moisture from diffusing outwards. It needs to be on the interior side of the insulation in a cold climate.
Edit: Sorry, I can see that you're aware of these. Perhaps my point is just that all standard houses where I live in cold climate uses vapor barriers. Moisture control is part of the building code. Trying to solve the problem with overly dry air does not make sense to me - the whole point of a house is making a comfortable living space.
Have you investigated a combination of additional vapor permeable exterior insulation and impermeable primer for the interior walls? You may be able to minimize the amount of moisture staying in the walls and still hit your thermal goals.
