U-value measures how easily heat passes through a building element such as a roof, wall or floor. It is expressed in W/m²K. A lower U-value generally indicates better thermal performance.
R-value represents the thermal resistance of a material or construction. A higher R-value indicates greater resistance to heat flow.
Thermal conductivity, represented by the Greek letter λ (lambda), describes how readily heat passes through a material. It is expressed in W/mK. A lower lambda value generally indicates a material with better insulating properties.
Importantly, the U-value of a roof is determined by the performance of the complete construction, not simply the lambda value of the insulation product.