My house is about forty five years old. It was built of bricks, with cavity wall insulation, and 15cm of rockwool in the loft. We have double glazing too.
But, I live in a very damp climate, and despite the fact that my house 'breathes', I live next to a burn, right next to a woodland, and yeah, it rains.
67% moisture is only driven out by heat, or as in our case, we use dehumidifiers....which give off heat as they pull in water
Breathing is a normal thing, we give out water vapour with every breath, if it can't get out of the house naturally, because it's wetter out than in, then we need to make it go out, iimmc ?
Old houses with open fires managed it fine, mostly. Damp corners, etc., created that characteristic smell that I still associate with the past.
I know a neighbour who was told to cut down the amount of breathing they did in the bedroom, just to try to stop the mould growing

right enough that was forty years ago, and I won't tell you what she told the 'inspector' in reply about what she thought of what he did in his bedroom
M