Final answer:
An air gap is usually large enough to prevent convection and take advantage of air's low conductivity.
Step-by-step explanation:
The size of an air gap depends on the purpose for which it is used. In general, an air gap is usually large enough to prevent convection and take advantage of air's low conductivity. For example, the space between the inside and outside walls of a house is about 9 cm, which allows for effective convection. Similarly, the gap between the two panes of a double-paned window is about 1 cm, which prevents convection and reduces heat loss or gain.