Here is an interesting photograph of something I had only ever seen once before and at that on a different continent and over forty years ago. You will notice several things in this photograph. A) dark dots B) dark in the corners of the wall and ceiling C) darker fairly wide stripe on the ceiling Analysis A) the dark spots are nails and these conduct heat very well picking up dirt from warm air converting up from the room more than the slightly warmer parts of the ceiling and general damp, dirty air and condensing it under the nail heads leaving a black mark. I would say that clout nails had been used rather than plasterboard nails and an extra nail banged in in the corner, further the fixings are a little too far apart for my liking and using too long nails. B) we generally see more heat loss at the corners than elsewhere this is simply due to 3D effects. C) the dark stripe is again a result of damp and slightly dusty air condensing on the ceiling. Generally for a properly insulated ceiling I would expect more of this under the wooden joists as they are acting as thermal bridges than on the body of the ceiling leading to the conclusion that the ceiling lacks insulation or that it has been lifted and not replaced properly. Note that it is not so dark next the wall most likely as a result of that insulation being still in place. The whole building had a poor 100mm of insulation which was topped up to 400mm recently under my supervision.