Aha, you have put your finger on a key issue, harbinger.
Getting rid of moisture from well-sealed, draught-free houses is a major issue. The crude way is trickle vents in windows or leaving them open a crack, so you have a draught you can control.
The clever way is Mechanical Ventilation and Heat Recovery (and host of similar names). This uses fans to extract warm moist air through a heat exchanger, to warm the incoming air. (Outdoors air is usually dryer than indoors air, as being colder it's naturally holding less moisture.)
You're still pumping your nice warm air out of the house, but you're recovering a fair bit of the heat from it. Not only do you solve your mould problems, but dryer air takes less energy to heat up from scratch, so iiuc MVHR is actually a net power-saver. But it's only worthwhile in a well-sealed home: if you live in the average colander you already have more ventilation than you need.
Btw, draught-proofing is a different issue from insulation.
Insulation = stop/slow heat travelling through solid objects like walls, windows.
Draught-proofing = stop air flow.
So a Ziploc bag is draught-proof, but not well-insulated.
And a crocheted jumper is good insulation, but not draught-proof.