U Values measure how effective a material is an insulator. Also known more formally as ‘thermal transmittance,’ to calculate the U-Value of any type of insulation, you need to divide the rate of heat transfer through the structure by the difference in temperature across the area in question. The lower the U-Value, the better the material functions as an insulator.
How insulation works
To understand how insulation works, it is essential that you understand the way that heat flow works. Heat flow involves three basic mechanisms: conduction, convection and radiation.
Put simply, conduction is the way heat moves through materials, convection is the way that heat circulates through liquids and gases and radiation refers to the build-up of radiant heat which travels in a straight line, and heats anything solid in its path that absorbs its energy.