If you can't crack the windows open a few times a day for a few minutes, then you got bigger problems that require proper solutions. In this case a proper dehumidifier is much more energy-efficient than keeping the heaters on.

If you have a old house, with a basement and lots of stuff, than it is quite hard, that the ventilation reaches every corner.

And those corners will get moldy.

Oh and I don't have a house, nor a mold problem, but know enough people who do have, even though they do ventilate reasonable. Houses were build with the assumption, that they are heated.

Now you can build new houses that are better designed - but you cannot magically adjust all the existing houses.

>Houses were build with the assumption, that they are heated.

Yes and no.

It depends on the country and areas within it and it applies mainly to houses built in the last 60-70 years or so, maybe a few more years back in cities (for heating with radiators) before that time many, many houses were (poorly) heated by fireplaces and/or stoves.

Besides, the permeability of the building materials was traditionally much better, and windows/doors were leaky, guaranteeing a much better ventilation.

At least here in Italy the heating was a luxury until well after World War II, particularly in the country and in small vilages, anecdotally my family used to have a house in a small village in central Italy where the heating until the '70's was based on the devices that can be seen here (Italian):

https://www.muet.it/storie/tutte-le-storie/item/il-riscaldam...

Particularly the kitchen and dining room was heated by the "stufa economica" that was also used to cook meals, the sitting room had a fireplace that heated also (a little) the bedroom behind it, and there was another large bedroom heated by a "stufa in terracotta", having a "prete" for each bed was needed.

The house was anyway very, very cold, particularly at night/early morning.

"It depends on the country and areas within it"

Well, obviously it does, but I would think most houses in europe do have to be heated and were build in the last 60-70 years and those I was refering to.

And before that, yes it was something like 2 bugs cancel each other out. Leaky windows prevents mold.

(And in general only rich people could afford to heat the whole house)

Dehumidifiers are actually a pretty good way to heat your house if you have a humidity problem. You recover significant amounts of latent heat of vaporisation when running it.