Cavity Insulation - Cavity insulation is used to prevent heat escaping through the walls, they are normally boards/sheets of expanded polystyrene.
It requires careful construction to prevent rain penetration at perpends (see 'Brick Joint-Perp Joint') or coldbridges (where pipes run through the cavity and can cause condensation when a cold pipe meets hot air in cavity i.e. in the summer or when a hot pipe meets cold air i.e. in the winter).
Below is a 3D image that shows Cavity Insulation. You can move the 3D object around with your mouse.
