Limitations of approaches in international standards

The proposed method for modelling the performance of glazings with or without shading devices is unsatisfactory. Significant deviations to real-world performance values have to be expected due to the following issues:

• global radiation propagating through the system is modelled as quasi-parallel, near-normal incidence only (ISO 9050, EN 410)
• in combination with a shading layer, ISO 15099 suggests considering an additional perfectly diffused radiation component but does not specify how glazings and coatings should be modelled accordingly.
• ISO 52022 suggests the consideration of a diffuse component for shading devices but feeds the calculation results back into the direct-normal radiation model.
• ISO 15099 and ISO 52022 provide calculation methods for shading devices only for the case of Venetian blinds.
• the Venetian blind model in both standards is oversimplified: perfectly flat, ideal diffusely reflecting, infinitely extended and infinitely thin slats are assumed.

Many issues may be caused by the fact that the initial purpose of the international standards was to provide a transparent, neutral and objective performance measure for windows and shading systems. The assumed boundary conditions, therefore, rather reflect a lab-like environment that does not reflect real-world conditions, as required for building performance simulation.