nail 50mm battern on the ends of sum 6by2 or 8by2 span from wall plate to wall plate or a load bearing wall, this will carry ur new floor, and also take the weight of the ceiling, as it wont be touching 50mm on ends, also u can stud up to perlings if u wont if there lower enouf, as for insulation u can put inbetween rafters as long as u cut out and put in vents in ur sofet board to stop condensation droping on ur plasterboard,the roll stuff is o,k but i found that ur best of battering it on as the plasterboard will look uneven hope this helps