In the corner of an upstairs room the ceiling for the last five years has been effervesing (looks like foam). The area is approximately a 12 inch radius quarter circle. The ceiling was relathed (metal)and plastered about fifteen years. A few years ago I has a plaster look at it and he did not know what was causing it. He skim coated it but it has reappeared. I have checked the attic for keaks and kave not found any.