Depends, some climates buildings were moldy. But old buildings generally aren't even close to air tight. Under those conditions as long as they are heated even

Depends, some climates buildings were moldy. But old buildings generally aren't even close to air tight. Under those conditions as long as they are heated even a little the humidity is mostly lower than outside. Mean the amount of water in the air as a percent is the same inside and outside. But since it's hotter inside the house the relative humidity is lower. In temperate climates you don't get mold. It's actually modern air tight houses that have terrible problems with mold. Because they don't breath water vapor builds up inside.