The importance of reducing adventitious infiltration in order to save energy is highlighted by the relevant 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 afresh using a theoretical model of adventitious infiltration for a conjoined dwelling. The model is used to predict the mean infiltration rate during the heating season for an apartment and a terraced house located in 14 different UK cities for two extreme assumptions of party wall permeability. The first assumption is that party walls are permeable – this results in a predicted leakage-infiltration ratio that is significantly greater than that used by building codes to evaluate the energy and environmental performance of dwellings. The second assumption is that party walls are impermeable – this results in a predicted leakage-infiltration ratio close to that used by building codes. 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. These findings have significant energy and health implications and should be of great interest to the policy makers of any country with a large number of conjoined dwellings.