The primary function of insulation is to prevent heat migration. This means keeping heat inside your home in winter and keeping it out during the summer. Heat can escape or enter your home via conduction, radiation, convection and air leaks.
1. Conduction is the transfer of thermal energy between neighboring molecules in a substance due to a temperature gradient. Metals are very good conductors. Anyone who has touched a hot saucepan knows that metals conduct heat quite efficiently. In your home, heat can be conducted through walls, floors, ceilings, windows, and doors.
2. Radiation is how heat jumps between two separate objects. This is how heat from the Sun crosses space to reach us here on Earth. The Sun heats your roof, which then radiates heat across your attic and heats your ceiling. Some of that heat then radiates to objects in your living spaces
3. Convection is the movement of heat in a gas or liquid. As air gets warmer it gets lighter (technically it becomes less dense) and rises. Hot-air balloons demonstrate this effect nicely the hot, low density air in the balloon provides a lifting force. In your home, warmer air carries heat upwards, leaving lower levels cool and upper levels overly warm.
4. Air leaks allow heat to hitch a ride on air which can sneak in or out of your home through gaps, cracks and crevices. A common culprit is gaps around door and window frames which may not fit snugly into their cutouts.