<p>In situ evaluation of thermal transmittance using infrared thermography is challenging, particularly for heavyweight concrete envelopes where surface temperature does not directly represent heat flux. A reinforced concrete building with design U-values of 0.181 W/(m²·K) for the wall and 1.268 W/(m²·K) for the window was examined to compare three non-destructive approaches: the heat flow meter method, the ISO 9869–2 infrared thermography procedure, and an exterior infrared method based on the Jürges equation. The heat flow meter technique provided the most reliable reference, with deviations of 4.9% for the wall and 4.7% for the window, but required long stabilization and interior access. The ISO-based infrared approach produced reasonable estimates for windows on both sides, whereas it failed to converge for heavyweight walls. This behavior is attributed to the high thermal mass of concrete, where transient heat storage decouples surface temperature from actual heat flux, even when heat transfer coefficients are measured directly. The exterior infrared method showed close agreement for walls under low-wind night-time conditions, while large deviations were observed for windows because radiative transmittance and frame–glass interaction cannot be represented by a single exterior surface temperature. The applicability of infrared thermography depends on component type and boundary conditions. The ISO-based procedure is more appropriate for windows, whereas the exterior method can support wall assessment under controlled environments. These findings contribute to a thermophysically consistent framework for non-destructive U-value evaluation of heavyweight and lightweight envelopes and help clarify the conditions for method application.</p>