hi guys i have to raise the floor in my extension to the same height as the kitchen so the level is the same when the wall is removed .At the minute there is a 125mm drop and i was wondering how best to raise this i could either have a 75mm polystyrene board on top of a membrane then 50mm of screed or a 50mm polysyrene and 75 mm screed which would you recommend me do .ps it will be going on top of an already 7 " concrete slab