Damp in most houses arises from two sources and has different methods of transmission and treatment. Very few houses have a damp-proof course in their walls or under the floor, so they suffer from rising damp this manifests itself in the walls of the ground floor or lower. Many properties have little proofing against rain penetration through the walls. In 2009 we had a lot of rain and the water was coming through, then running down the inside (note - this was not condensation,) in addition, this also makes the walls very cold so any airborne dampness in the house condenses on the cold surface.
How to deal with these forms of dampness.:
1. Cover it up: When we bought our house, the walls on the ground floor had some horrible plastic cladding up to a metre from the floor. We have seen other houses that have tiles half way up the wall which can be quite attractive but they are only to keep the damp in the wall and stop it coming to the surface where it forms moulds which are both unsightly and unhealthy for the occupants. We removed the ugly cladding, cut back the affected plaster and painted the wall thoroughly with PVA (poly vinyl acetate = Unibond which is difficult to get in Spain so we used white glue (same stuff) diluted 2:1 with water to make it easier to apply and painted the walls up to a metre with this solution, left it to dry, repainted and again left it to dry; Replastered the wall using ready-mixed filler, then painted with plastic emulsion - end of problem. The damp is still in the wall especially the lower parts but the PVA acts as a barrier and prevents it coming to the surface.
2. Keep it out: Outside walls on the lower and upper floors can suffer from rainwater penetration which is made worse by the fact that the paint on the outside is frequently "Cal" (whitewash.) What is required is for the outer surface to be hacked back and resurfaced with "Capa-fina" which forms a water-proof barrier on the outside of the wall. The Capa-fina is then over painted with plastic emulsion (pintura plastica) which will last at least seven years unless you are in a very exposed location. Allow the wall to dry out; paint the inside of the wall where the dampness was with PVA diluted as before (this is to prevent any salts in the wall drawing any moisture in the wall to the surface, then paint with pintura plastica. These should stop you having a damp house unless, you are in the habit of allowing steam from the kitchen/laundry free access to other rooms.