Abstract
Since households are one of the most energy-intensive sectors in Europe, retrofit of dwellings is promoted to increase energy efficiency. Recent research, however, shows that the energy performance after retrofit does not always meet the target values, which can be caused by amongst other things, a deviating building envelope performance. This paper compares the theoretical and measured building envelope performance for a real-life case study in post-retrofit state, in order to illustrate the limitations of calculation methods and characterization models. First, the performance is evaluated on building scale by verifying the correspondence between the default theoretical heat loss coefficient (HLC) and the measured HLC, which was determined by following the guidelines formulated within IEA EBC Annex 58 and Annex 71. In order to illustrate the limitations of the standard calculation method in real-life conditions, the theoretical variability of the HLC is evaluated, generated by variating infiltration heat losses and heat exchange with neighboring dwellings. Second, the performance is investigated on a component scale by assessing the theoretical and measured thermal resistances, identified from heat flux tests. Additionally, nonhomogeneous assembled components and air leaks are simulated to verify probable causes for the locally varying measured values and to illustrate the limitations of calculations and characterization methods. The results illustrate the limitations of the calculation methods by the assessment of the strong variability of the theoretical HLC, depending on assumptions regarding infiltration and heat exchange with neighboring dwellings. In addition, component simulations indicated that deficiencies on a component scale could be caused by a nonhomogeneous assembly and air cavity flows of the component. Moreover, a detailed assessment of an unreliable thermal resistance illustrates the limitations of the used characterization method. Finally, a contrast was found between the quite good performance on building scale (15% deviation between the theoretical and measured HLC) and poor performance on a component scale (only one out of nine monitored components met their theoretical target values), which illustrates the complexity of the building envelope performance.
Highlights
In order to adhere to the European climate targets [1] set to reduce greenhouse gas emissions and increase energy efficiency and the share of renewables, the total energy use needs to be decreased.Since households in Europe are responsible for circa 25% of the total energy use in Europe [2], mandatory criteria prescribing a minimum insulation quality, energy efficiency, and a minimum share of renewable energy [3,4] were imposed on the energy performance of buildings
The sensitivity analysis of the theoretical heat loss coefficient (HLC) will illustrate the limitations of the standard calculation methods by assessing the impact of a varying infiltration rate, thermal bridges, and heat exchange through the party walls
By comparing the theoretical and measured envelope performance of the post-retrofit building envelope of a case study, this paper aimed to illustrate the limitations of the calculation and characterization methods
Summary
In order to adhere to the European climate targets [1] set to reduce greenhouse gas emissions and increase energy efficiency and the share of renewables, the total energy use needs to be decreased.Since households in Europe are responsible for circa 25% of the total energy use in Europe [2], mandatory criteria prescribing a minimum insulation quality, energy efficiency, and a minimum share of renewable energy [3,4] were imposed on the energy performance of buildings. The actual energy performance of newly built or retrofitted dwellings, can strongly deviate from the theoretical design value This effect, commonly known as the energy performance gap, is identified in literature for several case studies. The energy performance gap was assessed in Germany for 60 renovated apartments, which showed the impact of user behavior, the malfunctioning of the heating system, and a mismatch between the observed and expected domestic hot water use [7] In addition to these case studies, a Dutch large scale study of all energy-labeled dwellings [8] showed that the energy performance gap depends on the energy quality of the dwellings: for energy-efficient dwellings, the actual energy use is underestimated by theoretical calculations, while for energy-intensive dwellings, the actual energy use is overestimated
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