With energy bills rising it’s tempting to tightly shut all the doors and windows and try to seal off your home from the cold outside. But if you don’t ventilate properly you could be opening yourself up to some severe problems. This isn’t just about getting some nice fresh air into stuffy rooms – regular air exchange prevents excess humidity from settling on the walls, where it can form health-threatening mould. “A four-person household releases 6 to 12 litres of water into the room air every day,” says Arian Freytag, a consumer adice expert from eastern Germany. This humid air needs to be rep…