According to the UK Building Research Establishment, a house built to be reasonably airtight and to the current ROI building regulations (5 cubic meters/h/sqm at 50Pa) consumes 40 per cent less energy than a house built to the old building regulation standard of 10 cubic meters/h/sqm.
You’ve been told many times about the need to insulate your home to a high standard. This is indeed by far the most cost effective way to lower your fuel bills, not to mention comply with the energy requirements of the building regulations. But insulation and airtightness go hand in hand.
Indeed after the insulation is fitted, an airtight membrane/vapour control layer must be applied to the warm side of the insulation, otherwise all the hard work you put into insulating and protecting your home will literally leak at the seams.