The most expensive thing that most people buy is the house they live in. Roughly half the expense of building a house is the cost of the materials of whic it is made, and they are used in large quantities (family house : around 200 tonnes; large apartment block: around 20,000 tonnes). The materials are used in three ways: structurally to hold the building up as cladding to keep the weather out and as internal to insulate against heat and sound, and to decorate