Low temp leads to higher relative humidity and slower drying of surfaces, that's why it's increasing the risk for mold. There is a related problem when insulat

Low temp leads to higher relative humidity and slower drying of surfaces, that's why it's increasing the risk for mold. There is a related problem when insulating your house more. If it's badly insulated, walls are cold and any dampness happens on the inside surface of the wall. You turn up your heater and ventilation and it evaporates. Now you add more insulation. Your walls on the inside are warmer. But the absolute humidity in the air is still the same, so it goes into the wall too. There is a temperature gradient inside the wall. When it's cold outside, there is a point in the wall where the relative humidity hits saturation, so the wall gets moist there. The better insulation, the deeper this is in the wall. No level of ventilation will save you here and if you don't do anything about it, you get mold inside the wall. Which you would not get without this extra insulation. The solution is vapour barriers. They need to be close to the inside of your walls to work. This is a tricky subject and part of the reason why adding insulation after the fact is hard.