The build

270mm is apparently the point of diminishing returns for mineral/glass loft insulation. I think EPS boards can provide similar efficiency at much lower thickness. Currently re-insulating the space above a bedroom and we're going for 300mm of mineral wool. Job made much harder by whoever demolished the original roof as they decided it was a good idea to leave all the rubble between the joists and put the fibreglass insulation on top. Now we're half way through dragging all the rubble out of a hole in the wall small enough that you have to slide through on your belly.

Don’t envy you that job. Been there. Had to do that for my grandparents when they had a new roof. The rubble left behind was horrendous.

Over 100kg of rubble removed, loads of dust to hoover up then finally we can lay the insulation tomorrow. Found a full box of old matches underneath the insulation and electric cables for lighting directly in contact with rubble...no fire risk there huh.