Improving the insulation of any solid
Houses built generally before 1927 lack the necessary insulation to reduce heat loss. This is due to the walls being a solid wall, not a cavity.
Internal wall insulation reduces the heat loss by adding a layer of insulation to the inside of the wall. This, in turn, allows for older homes to be more efficient not only on the EPC but in the long term in terms of energy usage and energy costs.
Internal wall insulation effectively pays for itself over its lifetime especially if the insulation is covered by the ECO fund.