U value is basically a measure of heart loss through a structural element. It is calculated on the rate at which heat transfers through 1 square meter of structure and is stated in W/m2K. U Values are used by construction industry to compare the thermal performance of different materials and products.
A double glazed Warmcore door can reach 1.4 U value, and a triple glazed door at 1.0 u value. Traditional aluminum doors are currently achieving between 1.94 and 1.76 u value.