Blistering paint mystery

[Michael Rosen]

I have a small patch of blistered paint from where the room was repainted last year. The previous paint ( which was on for about a decade was also blistered in exactly the same way. Can anyone guess what the issue may be and a remedy? This is an interior wall and the plaster is old. Behind the wall is a void which is under the main house floor.( This room is a side room which you have to step down into)

 

[Wayners]

Some kind of bridging behind that spot which is causing damp to blister under emulsion.
Probably lime plaster as old so don't coat with vinyl emulsion as will seal wall. I suspect that the emulsion is a vinyl?

 

[Michael Rosen]

Thank you. Yes it was a Dulux vinyl emulsion paint. What would be the best way to repair this?