This is not the way to fit 25mm ISOBOARD. This was fitted by builder who had no clue how to install ISOBOARD. The board was fitted directly onto the rafters which was approxmatily 1.2 ms apart which is far to far apart for the board to be supported. He should have fitted batons 700mm or less apart from centers and then installed the board using the clips, glue and screws
You will also notice he tried to butt the board against each other then polyfilled the joint, all looked very nice untill summer arrived. You CANNOT butt joint board together, one must first fit extra batons at the joint, fit a H section slide the boards into the H then secure the board with screws and washers. You may ask why did the owner accept the job in the first place? When you fit Isoboard like the builder did the sagging and shrinking of the board will only start once the hot weather sets in. So if this job was done in winter(which it was) all will be hunckdory untill summer comes along.