This paper summarizes the recently concluded PhD study by the first author. The study addressed uncertainties in building performance evaluations and their potential impact on design decisions. Design evolution involves a chain of design decisions. Each decision is supported by input supplied by the various domain experts to the design team at large. The research has focused on the domain expertise of the building physics expert, responsible for those inputs that sustain rational decisions with respect to energy use, thermal comfort, HVAC system sizing etc. Prime attention has been given to thermal comfort.