cavity wall insulation reduces heat loss from a house because...?

Answer 1

...it provides a barrier that has very low thermal conductivity, lower than the air it replaces. So, the thermal energy does not conduct through it very easily, keeping the air in the house warm, even if the air outside is cold.

Answer 2

cavity wall reduced heat loss because it is a non conductor of energy(that's what an insulator is, silly!) and the air cavities(another insulator) improve the insulating properties of the wall. what that means is that it takes heat longer to escape from the walls.if your walls were made of copper you would lose heat very quickly as copper is a very good conductor of energy.