The purpose of roof and deck insulation is to provide resistance to thermal energy. When insulation can inhibit the flow of thermal energy, the result will be a greater capacity to conserve heating and cooling energy. Heat flows from hot to cold due to conduction, convection and radiation. Heat is measured using the British Thermal Unit (BTU) where a BTU is the energy required to raise the temperature of one pound of water by one degree Fahrenheit. Therefore, a material with a higher R-value will provide greater thermal resistance. The roof of a building can be the structure's largest surface area, and consequently the area in which heat has the greatest opportunity to escape. With the correct type of insulation specified as part of the roof system, it will meet energy performance requirements and codes and provide many years of fundamental performance.