Building envelope thermal bridging heat loss assessment using infrared thermography
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Reduction of greenhouse gas emissions and energy use is necessary to stop the progress of climate warming, which may have a serious impact on the human environment and the ecosystem. To achieve this reduction, it is essential that energy usage in buildings is limited, as they have been identified as the world's largest energy consumer. In the European Union, buildings-related energy accounts for 40 % of total consumption and the thermal quality of the existing European building stock is below acceptable standards. Thermal upgrading of the building stock gives significant potential for overall energy reduction and is a key concern in current European Union policy. Putting this policy into practice involves, among others, implementation of thermal assessments of existing buildings. Performing such an assessment before and after building retrofitting allows determination of the actual energy reduction achieved, and monitoring of the overall thermal quality of the building stock. In general, the retrofitting process starts with the building envelope as this is one of the major factors determining the energy demand of a building. To assess the thermal quality of the building envelope, the heat losses, via its plain components such as walls and via thermal bridging appearing on the connection of these components, must be quantified. In this project, a non-destructive method for thermal evaluation of thermal bridging located in existing building envelopes, using quantitative infrared thermography technique (ITT), has been developed. This is a significant contribution to state of the art. With this method, the heat loss is expressed by thermal bridging heat flow rate qTB and linear thermal transmittance Ψ-value. The approach employs the surface energy balance principle on a building component surface containing a thermal bridge. Consistent with this principle, the amount of energy transferred to the surface is equal to the amount of energy leaving the surface. The energy leaving the surface is evaluated from surface temperatures recorded during a thermographic survey using an infrared camera. This approach eliminates the necessity for knowledge of the internal structure and materials of the building, which is often unavailable for older buildings. As part of the development and validation of the methodology, an experimental program was designed in consultation with an industrial partner to ensure that realistic test cases of thermal bridging in buildings were considered. The methodology for the use of the indoor and the outdoor ITT was validated in a hot box device where environmental conditions were strictly monitored. Furthermore, in order to verify the suitability of the methodology to thermal assessment of existing buildings, in-situ testing was carried out. The methodology included the establishment of procedures for undertaking the thermographic survey to enhance the accuracy of the data gathered. The analysis of the data from the thermographic survey was implemented in a Microsoft Excel-based tool for convenient thermal bridging heat loss calculations based on indoor or outdoor measurements. The developed methodology has the potential to be implemented into the overall thermal assessment of existing buildings. Based on a thermal assessment involving in-situ measurements such as proposed in this project, a building retrofit strategy can be planned. Repeating the assessment after building retrofitting will quantify the actual improvement in thermal performance.