Hi - first time poster here. Appreciate any help or advice people can give We've moved to a new house last week. It is semi-detached, built in 1910. It is brick with a white paint/coating of some sort. Over the course of the week we've painted most of the walls upstairs and down. In the front room (which has a bay window and fireplace) there are two dark patches that have appeared on the walls. The first came around 24 hrs after painting. It is slightly colder to the touch and although the wall isn't wet, the paint feels slightly chalky compared to the dry paint on the rest of the wall. The patch is on an external wall, and on the other side there is a plant growing up the outside of the building. The dark patch is around halfway up the wall and there is a pattern to it that roughly seems to match the structure of the branches of the plant. The second patch appeared around 48 hrs after painting. It is lower down the wall, around an inch above the skirting board. However, this is on an internal wall, with the other side of the wall being under the stairs. Poking around under the stairs I've since found that under the wall between floorboard and skirting board there was a load of carpet underlay that had been shoved in. When this was removed a clear draft blew through, so I'm assuming it was shoved in as a slapdash way to prevent the draft. Both patches are roughly similar in size - around 30-40 cm wide. The floorboards and skirting boards are dry beneath both. Based on what I've read online, I think the problem may be condensation forming on the walls and then not having adequate opportunity to evaporate due to lack of ventilation (caused by the plant and underlay squashed under the wall) in the affected areas. Would other posters agree that is likely, or could there be another cause. We also noticed when painting upstairs that the corners of walls on the outside of the house took forever to try or didn't dry on their own. Again, I think this indicates moisture settling in cold spots and preventing paint from drying. It has rained a lot but we've not seen any evidence of water coming down the corners of the house from the guttering. The wet corners haven't re-appeared since drying. The house was unoccupied for several months before we moved and there aren't ventilation grills above the windows, except the bathroom. I've tried opening windows and keeping the temperature steady since the dark patches appeared. No further patches have emerged in the 4-5 days since and the existing dark patches haven't changed. They may have decreased slightly but hard to really be sure. I suppose my questions are: Is my theory about the condensation and lack of ventilation plausible and of not what is likely to be causing this issue? Could there be a more serious problem (e.g rising damp)? Should we get an expert in? How should we address the dark patches that have appeared on the walls? Any advice is welcome. Thanks in advance to anyone able to help.