The work described in this document set out to improve the confidence with which thermal simulation modellers can treat the flow of energy through the doorways which connect the zones of a passive solar building. The study began by reviewing the requirements of the three models currently under consideration in the Passive Solar Programme, and by examining the theoretical treatment which can be used to obtain the input information required by those models. The models which require the user to specify the heat flow through doorways (SERI-RES and HTB2) do not model the pressure differences between zones, and thus only the buoyancy driven flow through doorways has been considered. Furthermore, as these models are generally used to produce predictions only down to an hourly timescale, the dynamic effects which occur on much shorter timescales when doors are opened and closed were also not considered. (author).