Agreed, houses with insulation have a vapour barrier behind the plasterboard for this reason: counterintuitively, the warm internal air carries a lot more moisture than the cold air outside. The partial pressure of water vapour is higher inside the house than outside, so the dew point of the internal air is higher than the outside temperature (dew point depends on water content).
If there's a way for that moisture to diffuse through the wall from inside, through the insulation, to the cold structure (which would be colder than before due to the internal insulation), the conditions are right for condensation to form. Those conditions are: surface temperature ≤ dew point.
Condensation can be prevented by a combination of:
1. Reduce the rate of moisture diffusion from the interior (seal the rooms and use a vapour barrier)
2. Lower the dew point of the air in the "cold" structural areas by ventilating the cold space to outside, so the moisture can exit (this is why houses have air bricks, vents in the roof, etc.)
3. Prevent the building structure from being colder than the internal dew point (i.e. use external insulation)
If there's a way for that moisture to diffuse through the wall from inside, through the insulation, to the cold structure (which would be colder than before due to the internal insulation), the conditions are right for condensation to form. Those conditions are: surface temperature ≤ dew point.
Condensation can be prevented by a combination of:
1. Reduce the rate of moisture diffusion from the interior (seal the rooms and use a vapour barrier)
2. Lower the dew point of the air in the "cold" structural areas by ventilating the cold space to outside, so the moisture can exit (this is why houses have air bricks, vents in the roof, etc.)
3. Prevent the building structure from being colder than the internal dew point (i.e. use external insulation)