For decades, fabricators, standards writers and code authorities have preferred prescriptive rules: here are the minimum characteristics your product must have in order to comply. That may be about to change as code writers look for ways to introduce energy efficiency standards into building codes. The energy efficiency of what you supply may not matter as long as the whole building achieves its efficiency targets. Ben Francis and Nick Schock of Red Pelican Building Science are experts in building energy modelling and they join Fenestration Review editor Patrick Flannery to discuss how these new paths to compliance might impact your approach to design and testing.