I'm at the point now where the roof is going onto the extension. It's approximately 6x6m at the back of a 1920's semi. The concrete slab (sat on a DPM lapped into DPC, and insulated underneath) is approx 60-70mm below the timber floorboard level of the house which it will join. The final floor on the whole ground level will be engineered wood. I was going to just get the extension screeded up to the level, but am now rethinking options as 200mm slab + 60mm screed is a lot of (cold) thermal mass to heat up? 1) I've never installed UFH, would I be able to just lay a wet UFH system in the extension over the concrete slab, and then screed over it up to the FFL? Would I need any insulation or boards between the pipes and the concrete slab? Would quite like to give UFH a go... (Rest of the house will just be radiators off a gas combi) 2) If I'm not going to install UFH, as the concrete slab is relatively level, albeit not very even, is an alternative to just lay e.g. 40 or 50mm insulation boards with T&G flooring boards or OSB on top? Would they need fixing down into the concrete, or just left as completely floating? Thanks!