There have been many different floor construction methods over the centuries, from earth floors to suspended timber, but its only relatively modern houses that have any insulation built in. Adding insulation to your floor can be anything from easy to impossible, depending on its construction and finish. Ventilated floors can feel cold due to draughts, but don’t just block off your airbricks without getting expert advice as you could find your floor joists rot!
Methods for insulating floors vary, but the general principle is to introduce a layer of insulation below the finished surface, and below any underfloor heating if you have it. Solid insulation materials like expanded polystyrene and phenolic foam can take the weight of a floor finish, whereas softer materials such as wool or Warmcell would need some form of structure around them. If adding insulation raises the floor level, you will need to think about ceiling heights and door openings.
In most cases it would be best to seek professional advice.