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.
