Penetrating damp is caused by water leaking through walls. This type of damp may expand across your walls or ceiling, but this will be horizontal movement. Penetrating damp is usually caused by structural problems in a building or from internal leaks, such as from damaged pipes or blocked downpipes.
Signs of penetrating damp are blotchy patches on internal walls, deterioration of plaster & paint, blistering, appearance of stains & salts. Also wet rot which is the rotting of skirting boards or timber.