An 'active façade' responds to (and anticipates on) changing indoor and outdoor conditions. To assess the potential of such a façade for minimising energy consumption and improving indoor comfort, a simplified building simulation model has been developed, that combines the simplicity and understanding of easy to use programs with the flexibility and possibilities of advanced programs. To validate the program, it is applied to two ECN research facilities. The model appears to be able to adequately describe the thermal behaviour of both. In addition, its performance in this respect is comparable to that of the much more sophisticated building simulation model TRNSYS. With the model, different strategies for sun shading, night cooling etc. can easily be assessed. The energy balance provided is a powerful aid in focussing the effort to improve the thermal performance of a dwelling. The model also allows insights into which part of the thermal mass determines indoor temperature variations.