Plus, it is common practice for the Spanish to live in buildings with all the windows and doors shut all the time.

The body gives off masses of water! Add to this the evaporation from water in the property etc. and it's a recipe for disaster.

Ventilation is the key - if you need to leave the property secure whilst you are away, then leave as many cupboards and (internal) doors open as possible to let the air circulate.

Unfortunately, if it does rain and you have an older property, then damp may come in through the walls etc. - I can't see a way around this without leaving a de-humidifier or an air-conditioner on a timer.

Newer properties don't seem to suffer as badly.

It's just the same in UK - a property will start to smell musty if doors and windows are left closed for too long.

Big bowl of salt in each room should do it if low levels of humidity. Saves on electric as well

Agree, it's reckoned we exhale up to 2 pints of water a day, so for a family of four that's a gallon day. That's a lot of liquid that has to go somewhere, wherever you live. Houses everywhere without damp courses and insulation will suffer, and using gas estufas, bombonas just makes the problem worse. We have had a decorator working in our house painting new doors etc and his next job is in a very old hotel, trying to stop damp coming through the walls. As he said, it's an ongoing job, they line the walls, repaper but they'll never cure it as the hotel is at the moment.