DIY Doctor UK DIY Forums

Recently had my stairwell re-plastered and have a strange occurrence. When the air is damp, dark patches appear on the wall around 6ft up so i dont think its rising damp. Neighbour say its most likely something called "cold spots ". What are they? Can i emulsion over them? :(

Should be fine to paint over, try to get some heat in the area to make sure its completely dry tho.