On calculated and actual energy savings from thermal building renovations – Long term field evaluation of multifamily buildings Tomasz Cholewa a,⇑ , Constantinos A. Balaras b , Sandro Niz ˇetic ´ c , Alicja Siuta-Olcha a a Faculty of Environmental Engineering, Lublin University of Technology, Nadbystrzycka 40B, 20-618 Lublin, Poland b Institute for Environmental Research & Sustainable Development, National Observatory of Athens, I. Metaxa & Vas. Pavlou, GR-15236 Athens, Greece c LTEF-Laboratory for Thermodynamics and Energy Efficiency, Faculty of Electrical Engineering, Mechanical Engineering and Naval Architecture, University of Split, Rudjera Boskovica 32, 21000 Split, Croatia article info Article history: Received 20 February 2020 Revised 25 April 2020 Accepted 16 May 2020 Available online 20 May 2020 Keywords: Energy audit Building energy retrofit Thermal renovations Hydraulic balancing Energy savings Energy efficiency Energy performance gap Multi-family buildings abstract Thermal renovation of existing buildings is one of the most popular actions to decrease the energy con- sumption for heating and cooling. However, to the best of the authors’ knowledge, there are no long-term field studies that present the influence of hydraulic rebalancing of the heating system after the thermal renovation of the building’s envelope on the level of achieving the calculated energy savings. This work presents the results from a field study that collected data on actual operational energy over several heating seasons, from 11 similar multifamily buildings in Poland. All buildings were thermally renovated by insulating their envelope, while for some of them the works were completed by a hydraulic rebalance of their heating system. The modernization activities were implemented with a different sequence. For another group of buildings, the hydraulic balancing of the heating system was not per- formed after the envelope thermal renovation. This offered an excellent opportunity to compare the actual energy performance of the renovated buildings and quantify the achieved energy savings resulting from different practices. Energy audits were performed in all buildings in order to calculate the energy use of the buildings before and after the renovations. These estimates were then compared with the actual energy savings from the monitored energy use for all renovated buildings. Accordingly, the actual energy savings range between 8.8% and 74.8% of calculated energy savings, depending on the different renovations. The actual payback time for the analysed modernization actions was longer than the calcu- lated one, ranging between 3.1 and 104.8 heating seasons. Ó 2020 Elsevier B.V. All rights reserved. 1. Introduction It is well known that buildings consume around 30% of total final energy in the world [1] and around 40% of total final energy in the Europe Union [2]. About 80% of the energy used in the Euro- pean buildings is used for space heating [3]. These figures exem- plify the importance and justify the emphasis given by European policies and relevant directives on the building sector for improv- ing the energy performance of new and existing buildings. Resi- dential buildings dominate the European building stock, representing about 75% of the total floor area [4] and 27% of the total final energy consumption [2], with space heating at 67% being the most important end-use service in buildings [4]. In this direc- tion, several studies have focused on residential buildings, in an effort to assess different renovation strategies in order to improve their energy performance [5], and specific studies focusing on national buildings stocks in Hellenic [6] or Serbian buildings [7]. Among the most frequently employed strategies to decrease the heating loads and the energy consumption for space heating is the addition of thermal insulation on building envelopes. Several stud- ies have investigated this measure as a part of integrated design strategies applied in the renovation of exemplary buildings in Ser- bia [8], Switzerland [9] or Sweden [10]. Along these lines, it is very important to choose the cost-optimal insulation thickness [11] and consider proper insulation materials, for example in the form of aerogel-enhanced systems [12] or vacuum insulation panels [13]. The first step for renovating existing buildings is usually an energy audit in order to assess different modernization measures, analyze the anticipated energy savings and simple payback time, in order to select the best solution that will be implemented [14,15]. In this light, different methodologies have been proposed for building renovations that may use, for example, machine learn- ing methods [16], regression-based approaches [17], notch test https://doi.org/10.1016/j.enbuild.2020.110145 0378-7788/Ó 2020 Elsevier B.V. All rights reserved. ⇑ Corresponding author. E-mail address: t.cholewa@pollub.pl (T. Cholewa). Energy & Buildings 223 (2020) 110145 Contents lists available at ScienceDirect Energy & Buildings journal homepage: www.elsevier.com/locate/enb