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Infrared Thermal Imaging Method for Evaluating Building Thermal Bridge
The use of multi-layer walls with high thermal resistance values is widely used to reduce heat losses in buildings during wintertime. Nevertheless, it is extremely important to treat also weaker components of the envelope such as doors, windows and all the various thermal bridges. If the improvement of the thermal performance of windows and doors has reached a significant development and a high level of standardization, the same is not true for the corrections related to thermal bridges, which therefore require a specific analysis in the design phase.
Thermal bridges, of whatever nature, are therefore a crucial point in the energy analysis of the building envelope. The study and reduction of buildings heat losses play an important role on an integrated energy-saving policy. The infrared (IR) thermography imaging technique results useful to conduct in situ analyses, since it allows a qualitative survey to evaluate the surface temperatures of the envelope surfaces. The analysis on existing constructions can be performed on site with thermographic techniques that describe in first approximation the qualitative energy performance of the building and put in evidence the main heat losses.
Figure 1. Example of a structural thermal bridge and window thermal bridge at the coupling of glass and frame.
The paper proposes a methodology to perform a quantitative analysis of some types of thermal bridges, through simple thermographic surveys and subsequent analytical processing. From the simple measurement of the air temperature and the analysis of the thermogram, the thermal bridge effect can be estimated as a percentage increase of the homogenous wall thermal transmittance.
The analytical methodology – which was validated with experimental and numerical analyses – is described and the results of surveys on different types of thermal bridges are reported. This method represents a quick and effective tool to define the actual heat loss of high-insulation buildings and to evaluate the benefits in treating thermal bridges.
The IR thermographic analysis is influenced by many factors affecting the accuracy of the absolute temperature reading. After a quick analysis on the determination of the parameters involved in the IR camera temperature output, the analytical description of the quantitative incidence factor of the thermal bridge is introduced. The validation of the methodology has been executed with the realization of a thermal bridge in a controlled environment, so giving the possibility of comparing the proposed technique to other experimental data and to a finite volumes analysis.
Figure 2. Measurement of the reflected temperature.
The application of IR thermography in buildings may be characterized by different levels of deepening: a first step qualitative analysis is described, with diagnosis oriented not only to buildings energy losses, but also to non-destructive tests of ventilation, heating, air conditioning and electric systems. Quantitative IR thermography has to take into account a number of variables linked to the heat transfer phenomena.
Active thermography is suitable for the detection of internal defects, as explained a methodology for the evaluation of the thermal transmittance of an opaque component, through the IR thermography and the contemporary measurements of the external and internal air temperatures, comparing the results of three cases with data obtained from heat flow meters.
Figure 3. Thermogram of the non-insulated beam.
In this work a quantitative factor was introduced, at the aim of evaluating in a simple and effective manner the effect of thermal bridges on the global dispersions of buildings. It was analytically defined the incidence factor of the thermal bridge that depends from the internal air temperature and the internal wall surface temperature, read by the IR camera. It emerged that the thermal field obtained with the three methods is very similar, producing close values of the incidence factor of the thermal bridge.
The quantitative thermographic imaging constitutes also a useful mean to analyze, improve and validate 3D simulation tools especially designed for the evaluation of building energy performance, since they are capable of evaluating the outdoor and indoor walls thermal fields.
Francesco Asdrubali, Giorgio Baldinelli, Francesco Bianchi. A quantitative methodology to evaluate thermal bridges in buildings. Applied Energy. 97:365-373, 2012.
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