The modelling of Rayleigh-Taylor instability during premixed combustion scenarios is presented. Experimental data obtained from experiments undertaken by FM Global using their large-scale vented deflagration chamber was used to develop the modelling approach. This development forms an expansion to the multi-phenomena turbulent burning velocity model that is under continuous development at HySAFER. Rayleigh-Taylor instability is introduced as an additional time-dependent, enhancing combustion, mechanism. It is demonstrated that prior to the addition of this mechanism the LES deflagration model under-predicted the pressure transients reported in the experiments, as the intensity of the external deflagration was not fully captured. It is confirmed that the instability plays a significant role throughout the coherent deflagration process due to the flame acceleration that occurs, towards the vent and also during combustion outside the chamber. The addition of the mechanism led to the model more closely replicating the form and the magnitude of the pressure peak associated with the external deflagration.