It is widely agreed that dynamics of building stocks are relatively poorly known even if it is recognized to be an important research topic. Better understanding of building stock dynamics and future development is crucial, e.g., for sustainable management of the built environment as various analyses require long-term projections of building stock development. Recognizing the uncertainty in relation to long-term modeling, we propose a transparent calculation-based QuantiSTOCK model for modeling building stock development. Our approach not only provides a tangible tool for understanding development when selected assumptions are valid but also, most importantly, allows for studying the sensitivity of results to alternative developments of the key variables. Therefore, this relatively simple modeling approach provides fruitful grounds for understanding the impact of different key variables, which is needed to facilitate meaningful debate on different housing, land use, and environment-related policies. The QuantiSTOCK model may be extended in numerous ways and lays the groundwork for modeling the future developments of building stocks. The presented model may be used in a wide range of analyses ranging from assessing housing demand at the regional level to providing input for defining sustainable pathways towards climate targets. Due to the availability of high-quality data, the Finnish building stock provided a great test arena for the model development.

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The Energy Performance of Buildings Directive together with the Energy Efficiency Directive and Renewable Energy Sources Directive define the frame and target state for energy performance of the existing building stock. This should be very energy efficient and decarbonised by 2050. The Finnish target is more ambitious, to achieve the target state already in 2035. In this paper, we discuss and concretise the role of HVAC in overcoming the challenge. Buildings in the Nordic countries are already very energy efficient. Structural improvements of energy efficiency are relatively expensive and have limited potential for energy saving. The best cost-benefit ratio can be obtained by combining HVAC with dynamic building automation systems. Also the EPBD calls for improvement of building automation systems and related measurements in new as well as existing buildings. The performance of buildings can be verified and deviations can be detected by monitoring-based commissioning during their life cycle. This means that special attention must be paid to the instrumentation level and an improved online reporting system for stakeholders. As a conclusion, we see that HVAC systems are in a key role in decarbonisation of existing buildings and a strategic sector on the way to a carbon-neutral society.

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EU aims to reach carbon neutrality by 2050. Besides energy consumption reduction, also greenhouse gas emissions have to be cut starting from the production of materials and construction work through the use phase to the end of the use of the building. Existing buildings are estimated to provide high potential for reducing global warming. This paper focus to research question, how reasonable are energy efficiency improvements of existing buildings, as the materials used in the process produce CO2 emissions and increase costs compared with conventional maintenance. This issue is a part of the Sustainable Development Goal 13 Climate Action, which integrates climate change measures into national policies, strategies and planning and a part of the Goal 11 Sustainable cities and communities, which tries to increase the number of cities and human settlements adopting and implementing integrated policies and plans towards inclusion resource efficiency mitigation and adaption to climate change. The carbon footprint of an existing renovated building constitutes mainly from energy consumption emissions. In life cycle costs, the deciding factor is investment. If the building was heated by zero-emission ground source heat, structural renovations would not be worth doing. On the other hand, structural improvement of energy efficiency is recommendable if a building is connected to district heating. Strong reasons, either endogenous or exogenous, must exist for replacing an existing building with a new. They cannot be justified with the carbon footprint or lifecycle costs. These results apply to countries, where the energy efficiency of existing buildings is reasonably good.