Hi, I did up a bungalow and had all the walls re-skimmed. In the past 8 weeks I’ve notice efflorescence has appeared in a small area around the kitchen window. This appeared after a few days of heavy rain. The window in on a long south west facing wall and it’s only appearing in this one place. I had a look outside and noticed that where the builder had removed an old sky box he’d left a hole in the wall – about half of a brick. I asked the builder to fill it as I was concerned water was coming in here and collecting on the side of the window as this would support the markings of the efflorescence. He said it would be that and put silicone sealant around the window frame.
I waited a few weeks and removed the efflorescence and repainted the wall.
2 weeks later after a day and night of really bad driving rain it’s back, and worst. I’m convinced the hole above is where the water is coming in and travelling down. Could it come in this high (about 3 metres above the area) and then travel down to cause this problem? The efflorescence is so localised (it’s not appearing anywhere else on the wall) I’m convinced this is the problem as we didn’t have an issue whilst the sky tray was still in the wall nor before they did the re-skimming where the old plaster was!
Either way I think he should fill the hole up.
Thanks for any assistance you can provide.

Sometimes once the plaster has been affected by damp there is no alternative than to remove the plaster and have it replastered to stop the efflorescence, this is why you need to remove the plaster when installing an electro osmosis DPC, because when the plaster starts to dry out the salts form on the surface of the plaster.