I have a brown patch on the lower half of the internal wall in my upstairs bathroom next to the door frame that is spreading and has become sticky. It appears to also be affecting the wall in the living room downstairs (underneath it). I'm sure the wall cannot have pipes running through it as it is just a dividing wall (that was put up when the bathroom was extended to make room for a separate shower). We have lived here for 18 months. Noticed around 6 months ago. There is a gap between the door frame and the wall and we wondered if condensation from the shower was condensing in there and causing it. Flummoxed by the stickiness though...Would really appreciate some advice.
1. there must be water getting in there somehow, it may be a mains pipe feeding the loft tanks which gets condensation forming on it.
2. the stickiness may be because someone has used PVA on the wall either before or after plastering.