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.

Studien syftade till att ge en rekommendation angående hur fjärrvärmebolaget Stockholm Exergi bör utforma sin framtida strategi beträffande fasta oförädlade skogliga biobräanslen. Genom litteraturstudier och intervjuer utreddes dessa bränslens konkurrenskraft utifrån perspektiven klimatneutralitet, politiska direktiv och styrmedel, leveranssäkerhet samt lönsamhet. Resultatet visade bland annat att användningen av grenar och toppar kan medföra klimatnytta. Vidare framkom att implementeringen av EU:s nya förnybartdirektiv inte kommer att ha storskalig påverkan på Stockholm Exergis framtida användning av dessa bränslen. Gällande leveranssäkerhet och lönsamhet påvisades exempelvis en större framtida efterfrågan på skogliga restprodukter från andra sektorer. Ändock kunde slutsatsen dras att skogliga biobräanslen, under vissa förutsäattningar, har en viktig roll i Stockholm Exergis framtida fjärrvärmeproduktion. / The study aimed to give a recommendation regarding how the district heating company Stockholm Exergi should design their future strategy concerning unprocessed solid woody biofuels. Through literature studies and interviews, the competitiveness of the fuels has been assessed based on climate neutrality, political directives and instruments, security of supply as well as profitability. Among other things, the results showed that the use of tree branches and tops can imply positive climate effects. Furthermore, the implementation of EU's new renewable energy directive will only have a marginal impact on Stockholm Exergi's future use of woody biofuels. Regarding the security of supply and profitability,an increased future demand of forest residues in other sectors have been identified. However, the study concludes that, under certain circumstances, woody biofuels have an important role in Stockholm Exergi's future district heating production.