Although there is a great deal of current debate about Building Regulations not changing fast enough to keep up with modern efficiency demands, the rules regarding external wall insulation are an exception. Gone are the days of simply picking an outer brick and an inner block and adding in some arbitrary cavity insulation; today, the process is a fine (and very calculated) art.
Cavity wall principles
Having two layers of brickwork with a space between them is known as cavity wall construction. This method was first introduced in the early 20th century as a way to provide better protection against penetrative damp and to help keep the inside of walls dry, so you can keep your funiture like your wooden shelf or best rated mattress dried without worrying them getting wet.
Dense concrete blocks started to be used for inner cavity layers after the Second World War, with lighter weight blocks (with air insulation) introduced in the 1960s and 1970s. A decade later and insulation started to be introduced into the cavities.