A novel experimental method for the detection of thermal bridges in building envelopes is presented. The combination of active infrared thermography (IRT) and singular value decomposition (SVD) allows to detect the spatial variations of insulation of the inspected area. The ambient air temperature is modulated at night and the region of interest is monitored with an infrared camera. A SVD analysis is then applied on the recorded thermographic sequence. The capability of the method to detect in situ thermal bridges was demonstrated at room-scale for different wall configurations and different seasons with a low-cost infrared camera.