How insulation works
Heat naturally flows from hot to cold temperatures in three ways, conduction, convection and radiation. Only bulk insulation is effective in reducing heat transfer via these three ways, whereas reflective insulation is only effective at reducing heat transfer via radiation.
Conduction is the transmission of heat through a material, or from one material to another, through direct contact. It can take place in solids, liquids and gases. There are materials that are good conductors of heat and poor conductors of heat. If you placed a metal spoon in a hot drink the spoon would start to absorb the heat energy and get hot.
Therefore, when considering an insulant to slow down heat transfer via conduction, you should use a material that is a poor conductor of heat, like glasswool insulation.
Convection is heat transfer that occurs in gases and liquids. To reduce heat transfer by convection, an insulating material should contain small voids or air pockets within which air movement is minimised. Again glasswool is a good insulation to use in a building where you want to reduce heat transfer via convection due to it having lots of small air pockets to help minimise heat movement.
Radiation is the transmission of heat energy from a hot to cold surface, moving through air or a vacuum. When an object blocks the path of the heat energy, it will absorb it and turn it into heat. The fire example below illustrates how radiation works. When you sit in front of a fire you start to get warm as your body blocks the path of the energy and warms up.
Materials with a metallic finish are good for protection against radiant energy as they emit and absorb very little radiant energy and glasswool insulation is another good material providing a barrier to the radiant energy and then reducing heat transfer via convection and conduction.