The purpose of home insulation—that is, thermal insulation—is to slow the flow of heat from one place to another. In most houses, that means trying to keep heat from flowing out of the house during the winter and trying to keep heat from flowing into the house during the summer.
Heat flows naturally from a hotter region to a colder region because of statistics. The molecules in a hotter region are jittering about more vigorously than those in a colder region and when the two are brought into contact, it is statistically most likely that the jittering will even out between the two regions. The thermal energy that causes this jittering will gradually flow from the hot region to the cold region so that molecules in the hot region will jitter less vigorously and those in the cold region will jitter more vigorously. In time, the jittering will become equal and thus the temperatures of the two regions will become equal.
Thermal insulation acts to slow this flow of thermal energy from hot to cold. Anything that interferes with that heat flow is helpful. Since there are three general mechanisms for heat flow, insulation typically impedes all three.
The first classic heat transfer mechanism is thermal conduction, whereby thermal energy is transferred through a material by the bumping and shoving of adjacent atoms as they jitter and by the movements of mobile electrons as they bounce around in electrically conducting materials. The best insulators make it difficult for conduction to work, usually by having relatively little solid material connecting the hot and cold region and by keeping electrons from moving so that they can’t help. Thin fibers of electrically insulating material make good thermal insulators, because they make it difficult for the atomic bumping to transfer heat and because they don’t let electrons carry heat.
The second classic heat transfer mechanism is thermal convection, whereby hot liquids or gases move about and carry thermal energy with them. Good insulators block convection, either by having no gases or liquids at all, or by making it hard for those gases or liquids to move about. Finely divided materials slow airflow and stop air from carrying heat via convection. Empty space, or vacuum, also stops convection because there is nothing to convect!
The third classic heat transfer mechanism is radiation, or more specifically: electromagnetic radiation. Surfaces are always emitting and receiving electromagnetic waves, which we know as infrared, visible, or ultraviolet light. The hotter a surface is, the brighter and more visible its light is. You can often feel this radiation when you stand near a hot radiator, stove, or fire. Since a hot region sends more thermal radiation at a cold region than that cold region returns to the hot one, radiation tends to transfer heat from hot to cold. Good insulation blocks this flow of radiative heat by getting in its way or by reflecting it. Shiny surfaces or opaque insulation tends to slow heat flow via radiation.
Answered by Louis A. Bloomfield of the University of Virginia