Insulation wraps the house in a layer of material that slows the rate of heat loss to the outdoors. Still air does not conduct heat well and is a relatively good insulator. In large empty spaces such as wall cavities, heat can be lost across the air space by convection and radiation. Insulation divides the air space into many small pockets of still air, inhibiting heat transfer by convection. At the same time, the insulation material reduces radiation across the space.