As simulation programs become more widely used for building performance assessment and building regulations compliance, there is a need to ensure that there are good quality empirical datasets which can be used to assess the predictive accuracy of these programs. This paper summarises a detailed experiment carried out on two identical full-scale buildings located at the Fraunhofer IBP test site at Holzkirchen in Germany and the associated modelling of the buildings. The work was undertaken as part of IEA ECB Annex 58 “Reliable building energy performance characterization based on full scale dynamic measurements”. The test sequence, applied to the side-by-side validation experiment conducted on the multi-roomed Twin Houses, consisted of periods of constant internal temperatures, a period of pseudo-random heat injections and a free-float period. All boundary and internal conditions were comprehensively monitored. Modelling teams were given details of the buildings and the boundary conditions, and over 20 teams submitted their predictions of the internal conditions which were subsequently compared with measurements. The paper focuses on a sensitivity study carried out to assess the overall prediction uncertainty resulting from the uncertainties in the input parameters, as well as identifying those inputs which had the most influence on predictions. An assessment of the measurement uncertainty is also included.