Damp on underground walls

[clairerid]

Hi,
We live in a 300+ year old townhouse in Spain and as a result of our house being built into a mountain our ground floor back walls are underground. I have recently noticed the books on the bookshelf on the back wall being damp and a bit mouldy and when I removed them I found that the paint is coming off the walls. There are no black marks but the walls feel cold and damp and there is white fluffy crystalised stuff on the wall. We do have paint flaking off our walls in various places and were told that this is to do with lime in the walls.
In our bathroom, again a room with one underground wall, there are black and dark green/grey spots on the walls which I have wiped off with bleach but they have come back. Is it just a case of ventilating these rooms more? Summer is here (27-30 degrees and it is dry heat) or should I use a dehumidifier or do I need to do something more drastic? I'm a bit clueless!
Thanks for your help,
Claire

[thedoctor]

If all this is happening only on the wall backing onto soil its pretty safe to assume damp is getting through the wall. See our project on underground waterproofing via the project links at the top of the page. The internal walls can be "tanked" which means applying a proprietory slurry to them to stop the water getting in. It is a pretty big job in so much as any plaster needs to come off the wall first. This is by no means a complete diagnosis but based on probability. First go for the ventilation option ( See our project on condensation ) and see how it goes.