A Nordtest method to measure the sound insulation of windows and other facade elements in the field is proposed. The method is based on the loudspeaker methods described in the international standard ISO 140/V. It gives supplementary specifications to this standard. In addition it puts some restrictions on possible parameter variations in order to get measured sound reduction index values which can be compared with those of traditional laboratory measurements. Measurements which have been carried out in the laboratory and in the field support the method.