Climate impact of energy retrofitting measures : An analysis of energy and carbon payback times for replacing windows

Sammanfattning: The European Commission has proposed a “renovation wave” to increase energy efficiency in buildings. Retrofitting can decrease operational energy usage but requires material production. Using the concepts of energy payback time and carbon payback time, this thesis aims to analyse under what circumstances operational energy savings from window replacement compensate for the climate impact of producing new windows. The literature review shows that carbon payback times for energy retrofitting measures are often reported to be merely a few years. In some cases, however, carbon payback time exceeds the service life of the added material. In general, diverging results can be attributed to both case specific circumstances and methodological choices. In this thesis, values for the main parameters determining carbon and energy payback time for window replacement are retrieved from environmental product declarations and scientific literature. The analysis shows that, only accounting for operational energy, energy payback times are within the expected service life of the researched windows for all energy saving scenarios and well within the service life for the midrange scenarios. Taking account of primary energy stretches the span of results. The analysis also shows that carbon payback time for window replacement varies by a factor of 38 for the midrange studied scenarios and a factor of almost 4 600 for the most extreme among the studied scenarios. Divergence stems from all investigated parameters: the embodied climate impact of window production, the amount of saved operational energy and the emission factor attributed to saved energy. In countries with mainly renewable operational energy, case studies may arrive at long carbon payback times for window replacement. The result can be altered if saved energy is considered to come from marginal production techniques, a methodological choice made in some case studies.   This thesis concludes that if energy and carbon payback time calculations are to be used for comparing retrofit alternatives, the research community needs to address methodological issues. Another conclusion is that the analysis of climate performance needs to include the interconnectedness of different societal sectors. It also needs to include more impact categories than energy and greenhouse gases, for example resource depletion.