cavity wall insulation
Cavity wall insulation is the process of injecting insulation into the gap between a property's inner and outer walls. It greatly reduces heat loss through the walls. As a rough guide, pre-1930s homes have solid walls with no cavity.
Filling an existing wall cavity with insulation is generally the cheapest and least disruptive way of insulating a wall. The thickness of insulation is limited by the width of the cavity and the cavity needs to be at least 50 millimeters for the process to be successful.