Thermal characterization of defects in building envelopes using long square pulse and slow thermal wave techniques

Building infrared thermography is a well-established but still qualitative technique that is used mostly for detecting insulation deficiencies, airleaks, moist areas, and thermal bridges in envelopes. Characterization of buried defects requires replacing steady-state thermography with a transient technique. It is proposed to borrow for this purpose the inspection methodology developed in the thermal nondestructive evaluation of materials. This methodology is based on recording the image sequence in both the heating and cooling stages, with the thermal stimulus being delivered onto the surface with a Dirac, square, or harmonic heat pulse. The dedicated image treatment allows the substitution of a sequence of any length with a pair of images called the “maxigram” and “timegram.” These images contain information needed for the characterization of defects. Some explicit inversion formulas discussed in the paper enable the determination of defect depth and thermal resistance. The proposed approach is illustrated with experimental results. Evaluation of internal corrosion in concrete and detection of underplaster delaminations are reported.