LUISA F. CABEZA

Luisa F. Cabeza is a full professor at the University of Lleida, Spain, where she leads the Research Group on Energy and Artificial Intelligence (GREiA). She is a member of the European Technology and Innovation Platform on Renewable Heating and Cooling, and of the International Energy Agency ECES TCP. She is an expert in the European Commission Challenge 3 Committee. Today she coordinates the Buildings Chapter in Intergovernmental panel on climate change (IPCC) 6th Assessment Report, having previously participated in the Special Report on Renewable Energy Sources and Climate Change Mitigation (SRREN) and in the 5th Assessment Report.

In Europe, and worldwide, buildings and their related services are responsible for a large share of the total final energy consumption, therefore also for the environmental problems which ensue^[1],^[2]. Serrano et al. 2017^[3] showed that while the main driver for energy consumption in residential buildings in Europe is the specific energy consumption, this is decreasing due to various technological options and European policies^[4] . Other drivers, such as the residential floor area per person and the number in each household, are increasing. This suggests that efforts to reduce energy consumption in buildings should focus not just on the energy efficiency of household appliances^[5] (heating, ventilation and air conditioning (HVAC) systems and other appliances such as refrigerators) but also on embedded energy. According to Ürge-Vorsatz et al. 2013^[6] , when a building is constructed or retrofitted to a given energy efficiency level, it becomes extremely uneconomic to carry out a new energy retrofit until the next construction cycle. In buildings the lock-in effect is therefore high, and should always be kept in mind.

'To increase the energy efficiency of buildings, the first strategy is the renovation of their envelopes'

The heating and cooling sector in Europe is still highly based on fossil fuels (75 % of the fuel is non-renewable), although it is moving towards clean, low-carbon energy sources (renewable energy sources)^[7] . The heating and cooling sector spans buildings (45 %), industry (37 %) and services (18 %); the heating sector transition is therefore closely linked to the decarbonisation of buildings. This relationship is explored further in studies of specific cases in Switzerland^[8] , Germany^[9] , and Finland^[10]. Various strategies can be employed to achieve decarbonisation, as outlined below.

'After improving building envelopes, the next step in the decarbonisation of the building sector is the use of renewable energy to provide the required energy services'

The first strategy is the renovation of building stock by increasing the energy efficiency of the building itself (walls, roof, windows, etc.). This would reduce energy demand, which, as mentioned above, is key to achieving EU targets. When this renovation is designed, the materials selection should take into account the embedded energy and their whole life cycle^[11]; this is especially important in the case of insulation^[12]. Windows and doors should of course be designed to minimise infiltrations.

The next step in the decarbonisation of the building sector is the use of renewable energy to provide energy services. For heating, the renewable energies to use are solar thermal, geothermal, and biomass, but renewable electricity is also an option when using heat pumps to heat buildings^[13]. Moreover, district heating, especially if fed with renewable sources, is a good option. Again, the lifecycle of products should be taken into account, a point included in the circular economy strategy^[14].

Buildings can therefore contribute strongly to the transition of the heating sector in two ways. The first is the decarbonisation of building stock though renovation, specially upgrading the building envelope using materials with low embodied energy and ensuring less energy demand. The second is the integration of renewable energy sources in buildings to provide heating and cooling.

---