After a recent watch spanish holiday TV programme the guy had half his plaster in upstairs bedroom chopped off because it had "blown". Builder said that damp was being transferred from outside and separating the sand cement render to the bricks and proceeded after the chopping of old render to coat the walls with PVA which in itself not waterproof but is a good adhesive. Considering where this guy lives gets at least 300 days sun a year and bedroom was upstairs I doubted that it was damp but inside walls were powdery. Wondered what anyone thinks?