Heavyweight cavity walls are commonly used as party wall between row houses and between dwellings in apartment buildings in Belgium. The double wall junctions complicate the prediction of flanking transmission. In this paper, empirical rules for the vibration reduction indices of rigid double wall T- and X-junctions are proposed. They are determined from Monte Carlo simulations carried out with a statistical model that was previously validated by laboratory measurements. In general, a double wall junction can be replaced by an equivalent single wall junction in flanking transmission models. The EN ISO 12354 approximation of a double wall T-junction by an equivalent single wall X-junction proves justified. For double wall X-junctions, the thickness of the equivalent.