Definition of heat transfer coefficient of uPVC doors and windows The K value of architectural glass is defined as: under standard conditions, the heat transfer from one side of the unit area of the glass component to the other side of the air per unit time. The K value is mainly used to measure the heat transfer through the glass system. This value is a function of temperature. The calculation method is the heat passing through one square meter of glass per hour at a temperature difference of one degree Celsius. The unit system is w/m2.k, where w is the heating power, m2 is the glass area, and k is the temperature in Celsius.