This paper presents part of the results of a large-scale, long-term experimental research conducted at the Faculty of Civil Engineering and Architecture Osijek. Among other research goals, this research aims at further development and improvement of a relatively new method used for the measurement of ther- mal transmittance of walls (U-value) in literature, often called temperature-based method (TBM). This research also partially overlaps with other researches carried out at the Faculty of Economics in Osijek, where the main research goals were development of machine learning and neural network models for predicting energy consumption in buildings, which will reduce the energy performance gap between design and actual energy needs. Building thermal performance as a whole can be quantified by the heat loss coefficient (HLC) or the total heat loss (THL). Experimental research presented in this paper was conducted by using a built test chamber in a laboratory, and the research lasted for 40 days. This is an innovative element of this research, since the test chamber is built inside a laboratory where external weather conditions are simulated by omitting the negative influence of wind, precipitation, and solar radiation on the experimental results. The actual heating energy consumption by the test chamber was recorded daily for 40 days during the winter season, together with internal and external temperatures, relative humidity (RH), U-values of walls, and wind speed. Chamber airtightness was measured at the beginning of the experiment. These measurements made it possible to perform the Co-heating test. This test is used to calculate the total heat loss of a building, both fabric and ventilation loss. Parallel with the Co-heating test, the design heating energy need of the test chamber was determined by calculating the heat loss coefficient and the total heat loss. Actual and design values of heat loss coefficient and total heat loss were used to characterize the energy performance gap. Energy performance gap in this study was found to be between −40% and 13%. Research results indicate the variables affecting the actual and design values of heat losses significantly. Presented results provide guidance for more accurate determination of actual energy consumption in buildings, and therefore help in the reduction of the energy performance gap.
actual energy needs, Co-heating test, design energy needs, energy performance gap, heat loss coefficient, temperature-based method (TBM), ventilation and transmission heat loss
 Lu, T., Lü, X. & Viljanen, M., A new method for modeling energy performance in buildings. Energy Procedia, 75, pp. 1825–1831, 2015. https://doi.org/10.1016/j.egy- pro.2015.07.154
 Hsu, D., How much information disclosure of building energy performance is necessary? Energy Policy, 64, pp. 263–272, 2014. https://doi.org/10.1016/j.enpol.2013.08.094
 Čulo, K. & Krstić, H., Cost benefit analysis of energy efficient family houses. In Sec- ond International Conference on Harminisation Between Architecture and Nature, Eco- Architecture, Algarve, Portugal, 2008.
 Borković, Ž.H., et al., Pilot projekt povecanja EE u zgradarstvu, Energetski institut Hrvoje Požar; MZOPU, 2003.
 Jack, R., et al., First evidence for the reliability of building co-heating tests. Building Research & Information, 46(4), pp. 383–401, 2018. https://doi.org/10.1080/09613218. 2017.1299523
 Bauwens, G. & Roels, S., Co-heating test: A state-of-the-art. Energy and Buildings, 82, pp. 163–172, 2014. https://doi.org/10.1016/j.enbuild.2014.04.039
 Directive (EU) 2018/844 of the European Parliament and of the Council of 30 May 2018 amending Directive 2010/31/EU on the energy performance of buildings and Directive 2012/27/EU on energy efficiency, Official Journal of the European Union, 2018.
 Branco, G., et al., Predicted versus observed heat consumption of a low energy mul- tifamily complex in Switzerland based on long-term experimental data. Energy and Buildings, 36(6), pp. 543–555, 2004. https://doi.org/10.1016/j.enbuild.2004.01.028
 Burman, E., Mumovic, D. & Kimpian, J., Towards measurement and verification of energy performance under the framework of the European directive for energy perfor- mance of buildings. Energy, 77, pp. 153–163, 2014.
 Majcen, D., Itard, L. & Visscher, H., Actual and theoretical gas consumption in Dutch dwellings: What causes the differences? Energy Policy, 61, pp. 460–471, 2013.
 de Wilde, P., The gap between predicted and measured energy performance of build- ings: A framework for investigation. Automation in Construction, 41, pp. 40–49, 2014. https://doi.org/10.1016/j.autcon.2014.02.009
 Sunikka-Blank, M. & Galvin, R., Introducing the prebound effect: The gap between performance and actual energy consumption. Building Research & Information, 40(3), pp. 260–273, 2012. https://doi.org/10.1080/09613218.2012.690952
 Norford, L.K., et al., Two-to-one discrepancy between measured and predicted per- formance of a ‘low-energy’ office building: Insights from a reconciliation based on the DOE-2 model. Energy and Buildings, 21(2), pp. 121–131, 1994. https://doi. org/10.1016/0378-7788(94)90005-1
 Majcen, D., Itard, L.C.M. & Visscher, H., Theoretical vs. actual energy consumption of labelled dwellings in the Netherlands: Discrepancies and policy implications. Energy Policy, 54, pp. 125–136, 2013. https://doi.org/10.1016/j.enpol.2012.11.008
 Demanuele, C., Tweddell, T. & Davies, M., Bridging the gap between predicted and actual energy performance in schools. In World renewable energy congress XI, UAE Abu Dhabi, 2010.
 International Organization for Standardization. Thermal insulation – Building ele- ments – In-situ measurement of thermal resistance and thermal transmittance – Part 1: Heat flow meter method (ISO 9869-1:2014).
 Standard UNI 10351, Materiali da costruzione. Conduttività termica e permeabilità al vapore [Construction materials: Thermal conductivity and vapour permeability]. 1994.
 Albatici, R., Tonelli, A.M. & Chiogna, M., A comprehensive experimental approach for the validation of quantitative infrared thermography in the evaluation of build- ing thermal transmittance. Applied Energy, 141(0), pp. 218–228, 2015. https://doi. org/10.1016/j.apenergy.2014.12.035
 Lucchi, E., Thermal transmittance of historical brick masonries: A comparison among standard data, analytical calculation procedures, and in situ heat flow meter mea- surements. Energy and Buildings, 134, pp. 171–184, 2017. https://doi.org/10.1016/j. enbuild.2016.10.045
 Desogus, G., Mura, S. & Ricciu, R., Comparing different approaches to in situ mea- surement of building components thermal resistance. Energy and Buildings, 43(10), pp. 2613–2620, 2011.
 Asdrubali, F., et al., Evaluating in situ thermal transmittance of green buildings mason- ries—A case study. Case Studies in Construction Materials, 1(0), pp. 53–59, 2014.
 Evangelisti, L., et al., In situ thermal transmittance measurements for investigating dif- ferences between wall models and actual building performance. Sustainability, 7(8), pp. 10388, 2015. https://doi.org/10.3390/su70810388
 Gaspar, K., Casals, M. & Gangolells, M., A comparison of standardized calculation methods for in situ measurements of façades U-value. Energy and Buildings, 130, pp. 592–599, 2016. https://doi.org/10.1016/j.enbuild.2016.08.072
 Evangelisti, L., Guattari, C. & Asdrubali, F. Influence of heating systems on thermal transmittance evaluations: Simulations, experimental measurements and data post-pro- cessing. Energy and Buildings, 168, pp. 180–190, 2018.
 Teni, M., Krstić, H. & Kosiński, P., Review and comparison of current experimental approaches for in-situ measurements of building walls thermal transmittance. Energy and Buildings, 203, pp. 109417, 2019. https://doi.org/10.1016/j.enbuild.2019.109417
 Energy performance of buildings – Method for calculation of the design heat load – Part 3: Domestic hot water systems heat load and characterisation of needs, Module M8-2, M8-3 (EN 12831-3:2017).
 Maeiro, J.R.M., Analysis of the thermal performance of opaque building envelope components using in situ measurements, Faculdade de Engenharia da Universidade do Porto, 2016.
 Johnston, D., Wingfield, J. & Miles-Shenton, D., Measuring the fabric performance of UK dwellings. In Proceedings of the Association of Researchers in Construction Man- agement (ARCOM) Twenty-Sixth Annual Conference, Leeds, 2010.
 Farmer, D., Johnston D. & Miles-Shenton, D. Obtaining the heat loss coefficient of a dwelling using its heating system (integrated coheating). Energy and Buildings, 117, pp. 1–10, 2016.
 Tehnički propis o racionalnoj uporabi energije i toplinskoj zaštiti u zgradama (Transla- tion: Technical Regulation on the Rational Use of Energy and Thermal Insulation in Buildings). „Narodne novine“ broj 128/15, 70/18, 73/18, 86/18; Available from: http:// narodne-novine.nn.hr/clanci/sluzbeni/2014_08_97_1938.html
 Butler, D. & Dengel, A., Review of co-heating test methodologies, M. Keynes, Editor, NHBC Foundation, 2013.