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Observations and numerical studies have allowed an understanding the fire development inside apartments and over the façade during the Grenfell Tower fire. During these analyses, the highlighted need was for a deeper investigation into the failure of windows during the fire. This detailed and complex model for window failure validates the thermal breakage criteria used in fire simulations of the whole Grenfell Tower fire. However, the present investigation is dedicated to a single pathway for the influx of fire and toxic smoke. Other pathways from the façade to the apartments, such as the melting of elements surrounding the window frame, are not addressed here. To analyse window behaviour during fire, Finite Element Method thermomechanical modelling is used. First, a heat transfer analysis is performed for windows subjected to the external flames from the identified façade fire scenario. The thermal loads evaluated from the fire are applied to the window structures to estimate their failure times. Two window casement configurations, closed and tilted-in, are investigated numerically. Then, a thermomechanical analysis of the window is addressed for each casement configuration. The modelled failure times are compared with those used in the general fire development model and from the observations. The good correlation that is observed, regardless of casement configuration, justifies the use of a simplified criterion for window failure in CFD fire models.