This International Standard applies to glass, coated glass and materials opaque in the far infrared wave-lengths. It gives the fundamental rules for calculating the thermal transmittance, u 1), in the glazing central area . The combined edge effects due to the thermal bridge of a glazing unit spacer and of the window frame are not included. These rules intended to enable the heat loss though glazing in a building to be estimated from the glazing U values and, together with heat losses through the opaque elements of the building, are used to determine the capacity of the heating or cooling plant. In addition, U values for other purposes can be calculated using the same procedure, in particular for predicting: a) conduction gains in summer; _________ 1) in some countries the symbol k is used