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Traditionally, the emissions embodied in construction materials have not been considered important; however, they are becoming crucial due to the short time-frame in which the emissions should be reduced. Moreover, evaluating the environmental burden of construction materials has proven problematic and the reliability of the reported impact estimates is questionable. More reliable information from the construction sector is thus urgently needed to back and guide decision-making. Currently, the building sector environmental impact assessments predominantly employ commercial software with environmental impact databases and report results without knowledge about the impact of the software/database choice on the results. In this study, estimates for the embodied environmental impacts of residential construction from the two most widely used life cycle assessment (LCA) database-software combinations, ecoinvent with SimaPro software and GaBi, are compared to recognize the uniformities and inconsistencies. The impacts caused by two residential buildings of different types, a concrete-element multi-story residential building and a detached wooden house, both located in Finland, were assessed, including all building systems with a high level of detail. Based on the ReCiPe Midpoint method, fifteen impact categories were estimated and compared. The results confirm that the tool choice significantly affects the outcome. For the whole building, the difference is significant, around 15%, even in the most widely assessed category of Climate Change, and yields results that lean in different directions for the two cases. In the others, the estimates are entirely different, 40% or more in the majority of the categories and up to several thousand percentages of difference. The main conclusion is that extensive work is still urgently needed to improve the reliability of LCA tools in the building sector in order to provide reliable and trustworthy information for policy-making.

While buildings are often credited as accounting for some 40% of the global greenhouse gas (GHG) emissions, the construction phase is typically assumed to account for only around one tenth of the overall emissions. However, the relative importance of construction phase emissions is quickly increasing as the energy efficiency of buildings increases. In addition, the significance of construction may actually be much higher when the temporal perspective of the emissions is taken into account. The construction phase carbon spike, i.e. high GHG emissions in a short time associated with the beginning of the building's life cycle, may be high enough to question whether new construction, no matter how energy efficient the buildings are, can contribute to reaching the greenhouse gas mitigation goals of the near future. Furthermore, the construction of energy efficient buildings causes more GHG emissions than the construction of conventional buildings. On the other hand, renovating the current building stock together with making energy efficiency improvements might lead to a smaller construction phase carbon spike and still to the same reduced energy consumption in the use phase as the new energy efficient buildings. The study uses a new residential development project in Northern Europe to assess the overall life cycle GHG emissions of a new residential area and to evaluate the influence of including the temporal allocation of the life cycle GHG emissions in the assessment. In the study, buildings with different energy efficiency levels are compared with a similar hypothetical area of buildings of the average existing building stock, as well as with a renovation of an area with average buildings from the 1960s. The GHG emissions are modeled with a hybrid life cycle assessment. The study suggests that the carbon payback time of constructing new residential areas is several decades long even when using very energy efficient buildings compared to utilizing the current building stock. Thus, while increasing the overall energy efficiency is important in the long term, the construction of new energy efficient buildings cannot be used as a means to achieve the short term and medium term climate change mitigation goals as cities and governments often suggest. Furthermore, given the magnitude of the carbon spike from construction and its implications, the climate change mitigation strategies should set reduction targets for the construction phase emissions alongside the ones for the use phase, which currently receives almost all of the attention from policy-makers.