Loss in Doors and Windows
It is not easy to compute the coefficient of heat transmission (U-value) for a door because door construction varies considerably in size, thickness, and material. A fairly accurate rule-of-thumb method is to use the U-value for a comparable size single-pane glass window. If you wish to compute the coefficient of heat transmission for a door, the necessary data can be found in ASHRAE publications. The same source can be used for computing the coefficients of heat transmission of windows.