I agree on low humidity being bad, but your remark on insulation does not seem correct. I'm only familiar with cold climates, so perhaps I'm overlooking something, but mold needs moisture to grow, so it will appear where water condenses out of warm air on a cold surface. You can typically see this in windows.

So you need to prevent cold surfaces. The best way to do that is improving the insulation. The reason you can see water drops in the windows is precisely because they are inadequately insulated (e.g. only double paned).

Now, it's true that if you make an airtight home, then you need some kind of air exchange. And if it's really cold outside, then even if it's wet, once that air is warmed up, it ends up at a low relative humidity. In that case you need a humidifier. I guess depending on the outside air, you'd probably need both to stay in a comfortable range.