Heat transfer occurs when in a figure there is a temperature difference. Energy transfers from the point at higher temperature to that with lower temperature. Schematically, heat transfer is based on three basic mechanisms:

- conduction

- convection

- radiation

Thermal trasmittance U (conforming to UNI EN ISO 6946) is defined as the heat flow passing between a unitary surface subjected to a temperature difference of 1°C, and it is connected to the characteristics of the material composing the structure and to the conditions of thermal exchange. In formula:

                                             U = 1/R_T          

Where R_T = R_Si + R₁ + R₂ + ..... + R_n + R_Se is the summary of resistances of various layers,

with:   R_Si          Internal surface resistance

          R_1,....R_n Thermal resistances of every layer

          R_se       External surface resistance

                                              R = d/λ

where:   d          thickness of the material layer in the frame

             λ          thermal conductivity

For calculation of thermal trasmittance of windows and door frames reference Standards are UNI EN ISO 10077-1 /2 (see section"Calculation").