Reducing adventitious infiltration in order to save energy is important and is highlighted by the building standards of many countries. This operational infiltration is often inferred via the measurement of the air leakage rate at a pressure differential of 50 Pascals. Some building codes, such as the UK’s Standard Assessment Procedure, assume a simple relationship between the air leakage rate and mean infiltration rate during the heating season, the so-called leakage-infiltration ratio, which is scaled to account for the physical and environmental properties of a dwelling. The scaling does not take account of the permeability of party walls in conjoined dwellings and so cannot be used to differentiate between the infiltration of unconditioned ambient air that requires heating, and conditioned air from an adjacent dwelling that does not. This article evaluates the leakage infiltration ratio in apartments, which share a large proportion of their envelope area with other dwellings. A stochastic approach is used that applies a theoretical model of adventitious infiltration to predict the distribution of the mean infiltration rate and total heat loss during heating hours for a sample of apartments of the English housing stock (a subset of the UK stock) for two extreme assumptions of party wall permeability. Knowledge of party wall permeability is not provided by a standard measurement of air leakage but is shown to be vital for making informed decisions on the implementation of energy efficiency measures. Accordingly, this paper provides probability distribution functions of operational infiltration in English apartments that can be used to help the policy makers of any country whose housing stock contains a large proportion of conjoined dwellings.