Laying Rockwool In Loft - Remove Old Polystyrene Or Keep?

[p7777m]

My current loft insulation is 40mm solid polystyrene fitted between the joists.
I'm about to lay Rockwool - about 270mm - and am wondering whether to lay on top of the polysturene or discard polystyrene.
If I discard the polystyrene I remove a fire risk.
If I lay on top I'll get better insulation. But how much better? - what thickness of Rockwall is equivalent to 40mm polystyrene?

 

[ajrobb]

I hope the polystyrene hasn't been in contact with any wiring! It makes PVC electrical insulation brittle.

Probably best to get rid of the polystyrene just for that reason and get the wiring checked. Mineral wool and expanded polystyrene have the same conductivity. Once you get above 150mm there is very little heat lost compared to the rest of the building. The current recommendation is at least 250mm. The difference between 150mm and 250mm is 0.1W/°C/m², which is 2W/m² for a 20°C temperature drop or about the same as a single light bulb for a typical semi.