Properties in the UK that are built after the 1920s typically have cavity walls. Cavity walls are named for the gap in between their two wall layers. It is in this cavity, that insulation material can be injected.
While cavity walls are generally better insulated than solid walls, their thermal efficiency can still be improved drastically through Cavity Wall Insulation. The added insulation will help lower your energy bills and aid in combating mould and condensation.