Envelope thermal transmittance strongly influences building energy consumption, so there is a significant interest in using methods that assess it accurately. The quantitative infrared thermography method is among the most studied methods to assess thermal transmittance as this method could be used to analyse building envelopes qualitatively and quantitatively. However, its main limitation is the great variety of approaches. Their greater differences are the convective heat transfer coefficient and the place from which the measurement is carried out. This chapter comparatively analyses the experimental results obtained in previous studies. The analysis showed that the approaches from the interior using expressions of adimensional numbers allow accurate characterizations of thermal transmittance to be obtained.