Question:What is Cavity Wall Insulation?
Although the cavity wall reduces heat losses by conduction, the main method of heat transfer through air (and all other fluids) is actually convection. So there will still be a heat loss due to convection in a cavity walled house.
Convection currents transfer the heat from the inside wall of your house to the outside wall, and even to the outside, since walls always have deliberate ventilation holes and sometimes unintentional cracks.
To prevent convection a layer of insulating material is inserted between the inner and outer wall. This can be done in homes after they have been built by injecting a foam material that sets solid in the gap.
This is called cavity wall insulation.
The insulating material will contain a lot of trapped air to reduce its thermal conductivity.
Summary: Cavity walls reduce conduction heat losses.
Cavity wall insulation prevents convection heat losses.
Cavity wall insulation is a material that fills the gap.
It usually contains "Trapped Air" to improve its insulating properties.
Even though solid materials are better conductors than air, removing convection, even if it increases conduction slightly reduces heat losses substantially.
Air becomes a much better insulator if it can be stopped moving, because then convection cannot happen.