New approaches are needed to achieve the scale and standard of building retrofit required to meet climate targets. Transition experiments are innovation projects that take a societal challenge as their starting point; they can be both top-down (government led) and bottom-up (civil society led). However, such experiments often remain isolated events that have little impact on delivering systemic change. There is limited knowledge on why this is so and what can be done to increase the success of experiments. The paper therefore compares the top-down approach to piloting Local Heat and Energy Efficiency Strategies (LHEES) in Scotland with the bottom-up strategy used for the Social Innovation Labs for a Zero Energy Housing Stock (SMILE) in the Netherlands. The different approaches are compared using three mechanisms to characterise systemic change: deepening, broadening and scaling up. Using data from interviews with local authority and citizen actors, the paper shows that neither top-down nor bottom-up experiments are sufficient in themselves to foster the new norms, information-sharing or legislative mechanisms needed to reach climate targets. The paper specifies elements of top-down and bottom-up experiments which can usefully be incorporated for achieving systemic change in energy retrofitting. 'Policy relevance' Delivering building retrofit at scale is crucial to net zero greenhouse gas emissions targets. Policymakers can benefit from adopting long-term strategic approaches to retrofitting, incorporating leadership from local actors. Central government coordination is essential to providing a clear programme and timetable for local actors to coalesce around. In addition, localised projects need to be shared and supported through centrally coordinated repositories and knowledge exchange. Policymakers must develop complementary policies designed to improve support from both governmental and non-governmental actors. This will include planning and citizen engagement, managed at a local level; this is crucial for ...