Raised timber joist floors can be insulated with rigid foam or glass mineral wool which is suspended on netting between timbers. New tongue and groove floorboards are then laid and sealed around the perimeter/skirting line which will significantly reduce any air movement. This is a project worthy of doing when changing any floor coverings. Solid floors can also be upgraded. Under floor heating is an option for any floor construction with systems available which have little effect on raising floor levels.