Impact assessment of the façades' actual state on the energy performance gap of residential buildings

Abstract

Bridging the gap between the predicted and actual energy performance of buildings is necessary to increase the energy performance of existing buildings and achieve the European Union’s energy efficiency targets in the “2030 climate & energy framework”. Construction is considered the sector with the most potential for energy saving. Specifically, buildings space heating has considerable potential for energy saving. Thermal transmittance is the fundamental parameter to characterise the heat losses of building envelopes. However, several research authors evidenced that assumptions regarding heat loss from a home pre-retrofit and post-retrofit were incorrect. In this sense, accurate on-site measurements are necessary to provide information on the actual thermal transmittance of façades. The purpose of this thesis is to contribute to reduce the energy performance gap of residential buildings. Therefore, the aim of the thesis is to enhance the accuracy of in situ measurements of the actual thermal transmittance of façades of existing residential buildings using the heat flux meter method to ensure successful decision-making during the energy renovation processes of existing buildings, and to confirm energy performance strategies for new nearly zero-energy buildings. The analysis of testing parameters for the in situ measurement process of the thermal transmittance are not fully specified by the standard ISO used extensively. Relating to calculation methods to conduct research on the thermal behaviour of façades, the standardised average method is widely used by authors. Very few initiatives used the standardised dynamic method because it is more complex than the average method. This research assesses the implications of using the different standardised calculation methods in order to verify which best fits theoretical values. Moreover, the usability of the standardised dynamic method is promoted and facilitated through a calculation procedure, defining a programmed spreadsheet accessible to practitioners. The dissertation proposes a classification system of façades to facilitate the selection of façades to conduct a systematic analysis of the thermal performance of façades in the housing sector. The classification of façades of existing residential buildings for subsequent analysis is based on the characterization of the opaque part of façades. The dissertation continues with the exploration of the boundaries of the requirements for using the standardised heat flow meter method to refine the testing conditions in low U-value façades. Façades with low thermal transmittance values are increasingly promoted due to comply with Directive 2010/31/EC targets. However, as several researchers shown, conducting accurate in situ measurement of low thermal transmittance façades is a challenging task. Investigating the limits of application of the HFM method for the in situ measurement of U-values in low thermal transmittance façades would allow to ensure compliance with policies to transition the existing building stock to nearly zero-energy buildings, and to confirm energy performance strategies for new nearly zero-energy buildings. To enhance the usability of the heat flux meter method, the dissertation compares different criteria for determining conditions for stopping the test during in situ experimental campaigns when measuring the thermal transmittance of existing buildings’ façades using the heat flow meter method. This will help to reduce practitioners’ uncertainty about the duration of experimental campaigns for obtaining accurate actual thermal transmittance of existing buildings’ façades. The dissertation concludes by outlining the main contributions of this research. The subjects that were raised during the research undertaken although they could not be addressed are commented and proposed as future work.