In this work we created a numerical model of the ISO 13785-2 test setup for testing the fire behaviour of building façade systems, and used the model to simulate the thermal environment on the façade. The model, created using Fire Dynamics Simulator-software, was first validated using the experimental data by Yoshioka et al.(2012). Next, the sensitivity of the façade heat fluxes on the geometrical and model parameters was studied, revealing for instance that the size of the combustion chamber window will influence the thermal exposure high above the window. Finally, the model was used to estimate the thickness of non-combustible insulation layer that is needed to protect combustible materials from melting or decomposition.