Examples of double fa?ade modelling are however very rare and modelling guidelines are virtually non-existent. A major issue with building energy software also concerns building physics simplifications becoming problematic in the new double-fa?ade domain. Where as previously building skins provided fairly strict physical boundaries to the outside, double facades have a much closer and complicated interaction with external environment ¨C so the level of resolution initially provided may now not be sufficient. In this study an examination of the potential limits of building energy software (ESP-r) is undertaken in conjunction with an extensive sensitivity analysis of double fa?ade configurations to help answer this question. The study also aims to provide helpful modelling guidelines in doing so.

It is apparent in the study that modelling double-skin facades is a complex task involving the adaptive control of major energy flowpaths during the facades changing operation modes, such as convection, ventilation and insolation distribution. Indeed a wide range of operation regimes is evident. The characteristics of double-skin cavity itself significantly impact on the environment behind the internal fa?ade and results are highly sensitive to the way it is modelled. To obtain an accurate prediction of the fa?ade performance, it is very important to ensure appropriate treatment of: solar insolation, cavity convection regimes, surface view-factors, blind spatial position, airflow resistances, vertical temperature gradient and cavity divisions with fictitious divisions.

A complete representation of airflow windows is attainable via a separate zone representing the window cavity, controllable across all modes of operation. Double-skin facades utilising roller blind shading are modelled with two-zones representing the cavity. Complete control of this type of fa?ade is attainable via property substitution and the use of fictitious constructions. Although these fictitious surfaces introduce errors they are tolerable in the main ¨C particularly in a design situation where the ability to represent significantly different operation modes is important. Facades spanning significant heights can be further divided vertically with the use of fictitious surfaces in order to expose significant vertical temperature gradients. Double-skin facades utilising Venetian blinds are modelled with two zones again but with a dividing surface constructed in a saw -tooth pattern. Such a depiction is found to offer the best representation for the complex processes associate with the Venetian blind system in ESP-r.