Our cavity wall insulation sits between the inner skin and the outer skin of your double brick cavities. This creates a thermal break between the two brick skins, reducing heat transfer and protecting your home from external temperature changes. Not dissimilar to a bean bag bead, Cavity Shield is made predominately from expanded polystyrene. No different to how a thermos keeps your coffee warm, or an esky keeps your beers cold, polystyrene is one of the most widely used insulators worldwide.