It seems to me if what I'm looking at is a window, they obviously have the window slightly open, the condensation appears to be on what looks like a roman blind, the cold air (Which admittedly falls) but does create cold spots. is directly above the window, (Steam rises and condences).
You appear to have a cold wall, which directly lets cold air onto the ceiling.
Celotex will create a warm wall, but if the window remains open there will always be a cold spot. It may (And I'll back down for more expert opinions) be an idea to put in an extractor fan. As its single skin its easier than cavity.
It looks like condensation to me. Excess steam in the bathroom!