Boosting the energy efficiency of the EU building stock is critical the European Green Deal — and indeed to addressing climate change globally. The vast potential of energy efficiency is well documented: in fact, some refer to it being ‘2-Es that need 6-Ds’. The ‘desirable’ goal of a global transition that supports universal access to modern energy services while also achieving the emissions reduction will require disruptive action in four key areas: decoupling, decarbonisation, decentralisation and digitalisation.