Localisation of Air Leakage in Building envelopes using Active Thermography

Kurzfassung

Air leakage in buildings - the accidental exchange of air through cracks in the building envelope - represents a significant amount of its energy consumption. Hence, a fast and accurate way to locate these leakages is critical to achieving sustainable building. This project deals with the localisation of air leakages on building facades using active thermography and signal analysis methods. Active thermography is achieved by periodic thermal excitation of the area around the leakage by creating a pressure difference between the inside and outside of the building and therefore creating airflow through the leakage. Pixels are analysed with the Fourier transform or wavelet transform and visualised as amplitude images at the excitation frequency. Additionally, phase filters can be used to reduce artefacts in the amplitude images resulting from unsteady ambient conditions like unsteady wind and solar irradiation or moving objects. In this thesis a novel filter for amplitude images based on a local maximum analysis is presented. The novel algorithm is tested in the laboratory and compared to the established techniques classic thermography, difference images, active thermography with amplitude images, and active thermography with amplitude images filtered by phase. Measurements were performed withpressure differences of 50 Pa, temperature differences of 5 K and samples with direct boreholes from 2 mm to 8 mm and channels from 1 mm by 3 mm to 8 mm cross section and 32 mm to 96 mm. A quantification metric is introduced to quantify the quality of leakage localisation and is compared for the different techniques. The results show that filtering for local maxima in the Fourier space is sufficient to remove artefacts created by residual frequency components at the excitation frequency. Phase-weighted amplitude images show similar filtering, but need to be manually optimised for the best results. The metric compares reasonable with the visual inspection but shows some discrepancies, which need to be evaluated in further research.