Analysis of thermograms for the estimation of dimensions of cracks in building envelope

In situ determination of global air leakage of the building envelope is presently done with the fan depressurization test. During such test, infrared thermography could also be used to dimension unintentional small openings (cracks). In this study, thermography was used to measure in laboratory the surface temperature of single-layer walls subjected to air flow through surrogates of cracks. Two image-processing methods were developed and applied to a dataset of 36 thermograms recorded in laboratory. First, using the edge detection technique, the opening length and large width (more than 4 mm) can be graphically estimated with an error of less than 8%. Second, for smaller openings, correlations for two image-processing characteristics, peak height and missing attenuation, were established. These relationships result in estimation with a relative error of less than 4% of the widths of small cracks on thermograms. The development of correlations for the spectrum of conditions found on site could be a step towards in situ quantification of air leakage areas.