Houses are a lot tighter than they were in the past which makes it harder for them to "dry out" unless you have some kind of air circulation system. My house certainly doesn't and it wouldn't be easy to retrofit.
Apartments in our block have air-tight windows and doors. But also there are passive ventilation input in every room, which allow flow of some fresh air through closed windows. Central ventilation is constantly taking air from the apartment so we have no issues with excess moisture. Some people even put a humidifier during cold months to get moisture level up to 50%.
In cold weather, I think 35-40% RH is a good target, but where lower is okay and higher is not. There’s too much chance INO for that warm, humid air to migrate to the cold outer wall and drive condensation.
I don't know about that. Most new houses have air conditions for the hot humid days and a heater for the cold days. Most fully below grade basements have dehumidifiers in wet climates. I've never had issues with moisture except in cases of some system failure, like a sump pump.
