Clearly, you don’t want to be digging up your concrete floor; it would be much better to use rigid insulation on top of the concrete. The most obvious downside of this will be that the floor will be raised up by the thickness of the insulation layer. If, however, you do decide to go this route, you need to think carefully about skirting boards, doors, and very importantly, staircases – having the bottom step of a stair at a different rise from the others can be a trip hazard. Once levels issues are thought through, you need to choose the type of insulation. In its basic form, we ‘re essentially talking about rigid polystyrene, but there are much more advanced, high-density expanded foam boards available. A green alternative would be a wood-fibre board, such as Pavatex. Also, it is very important that you prevent condensation from forming, so it’s advisable to lay a vapour barrier.