Abstract The four different techniques which were used to test an advanced, four-pane glazing system and standard double-glazed unit are described. The results from each test are compared. Where agreement is not good, explanations are suggested. The advanced glazing system was found to have a U -value of 0.9 W/m 2 K and a shading coefficient of 0.48. The glazing simulation models WINDOW (Lawrence Berkeley Laboratories, Berkeley, CA, U.S.) and MULTB (Pilkington Glass, U.K.) were used to predict glazing performance. Simulation of the two glazing systems which were experimentally assessed allows comparison between models, and between predicted and measured performance. Agreement was within the error bands associated with each assessment.