That happens when the cold walls are cooling down warm air, dropping the moisture out of that air. If you have a warm house and open all the windows in winter, you'll get condensation on the walls. If the house was cold in the first place, e.g. the walls and air are in a cold thermal equilibrium, you won't get condensation on the walls.

Bathrooms and kitchens are a special case, which can be addressed with ventilation. One of the upsides of keeping your home cool in the winter is it's much cheaper to circulate in new air from outside. You get to have fresh air all winter without paying to heat all that air up. I believe fresh air throughout the year has many health benefits.

It really depends. The air that you exhale is hot and humid. As soon as it touches the exterior walls, it will condensate. I'm talking about a house that is not insulated, but also does not leak air out (new windows with good seals). If it leaks some air out, then the condensation is very limited as outside cold air is dry in winter.

It might be like you said only if you don't use that room, or you have a big house so maybe you don't use most rooms and then you don't fill them with humidity from your own breath. Walls are cold but also dry.

Opening the windows in the winter is how you let all that humidity out.

> Opening the windows in the winter is how you let all that humidity out.

Absolutely, being able to open your windows is a big advantage of keeping your thermostat low.

When the inside of your home is warmer than the outside when there is cool weather outside, you don't have humidity, mold, and condensation problems.

You can, and also in the inverse situation; it depends on how the walls are built.