Quantifying the Trade-off between Upfront and Operational Carbon in High-Performance Building Envelopes

In recent years, the reduction of Greenhouse Gas (GHG) emissions across the entire lifecycle of buildings has become critical. Design is now required to aim for both the reduction of Operational Carbon (OC), which is generated during a building's operation due to energy saving, and the suppression of Upfront Carbon (UC), which arises from material procurement, manufacturing, and construction. This study quantitatively evaluated the balance between the increase in UC associated with a high-performance building envelope and the reduction in OC, using a medium-sized office building as the subject. We examined the adaptability of various envelope specifications according to Japan's climate characteristics and orientation conditions. The highperformance envelopes studied were Low-E triple glazing and a high-performance Double Skin Façade (DS), which were compared to Low-E double glazing, a common specification in Japan. The analysis covered the 8 climate zones of Japan's energy-saving regional classification. The OC reduction effect was evaluated using the Carbon Payback Time (CPBT) against the increase in UC. Furthermore, the effect by orientation was verified from the perspective of heating and cooling loads, and the orientations with a higher priority for highperformance upgrades were analysed. It was confirmed that Low-E triple glazing offers a high OC reduction effect relative to the increase in UC emissions, and payback is possible within the assumed lifespan (30 years) set for this verification in all climate conditions. Specifically, in cold regions, the reduction is large, and payback can be expected in about 5 years. The DS also showed a significant reduction effect in cold regions, with a payback expected within 20 years. On the other hand, in warm regions, the tendency shown was that the materials used, such as the use of recycled materials, need to be considered to achieve payback within the lifespan. The orientation analysis indicated a high reduction effect in all regions for the East and West sides, which are susceptible to the influence of solar radiation due to the low solar altitude, suggesting they have a high priority for high-performance upgrades.