The cavities in a building facade can significantly increase the fire hazard, acting as pathways and accelerators for the vertical spread of flames and smoke, even in non-combustible facades. Ensuring fire safety during facade design requires a thorough understanding of how cavity geometry influences fire dynamics. However, established theories for this phenomenon are lacking. Therefore, in this study, we use the computational fluid dynamics code FireFOAM to develop step-by-step multiphysics simulations incorporating fluid mechanics, heat transfer, buoyancy, and combustion phenomena to investigate the non-linear behaviour in narrow vertical cavities. Four scenarios of increasing complexity are modelled and validated against experimental data from the literature. The simulations predict flow velocities and convective heat fluxes within 20% error and buoyancy-driven flow, radiative heat flux, and flame height predictions within 30% error across a range of cavity widths. The study also highlights the limitations of the models, offering insights for future refinement. The results demonstrate that computer simulations can reliably be used to study critical phenomena of cavity fires and, with future improvements, predict fire behaviour across various facade designs and conditions.