Applies to glass, coated glass and materials opaque in the far infrared wavelengths. Gives the fundamental rules for calculating the thermal transmittance in the glazing central area (edge effects are not included). The rules are intended to enable the heat loss through glazing in a building to be estimated from the 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 (e.g. condensation on glazing surfaces, seasonal heat loss through glazing) can be calculated using the same procedure.