Well exactly because of that. If your house has plenty of air circulation (that's what "people's houses weren't far off from the outside environment" means) the

Well exactly because of that. If your house has plenty of air circulation (that's what "people's houses weren't far off from the outside environment" means) then things might get damp, but they will also dry out naturally too. Now you get a huge plastic bag and wrap it around a building. It will keep out rain yes, but also trap in moisture internally and the damp parts will get mouldy. Obviously nobody will do something as silly as that, but with modern materials it become much easier to accidentaly approximate the plastic-bag-wrap experience.