Everything above absolute zero radiates infrared energy in proportion to its surface temperature. When the objects are hot enough they radiate visibly and our eyes can see them glow. As they cool their radiation becomes invisible to the eye. We then can use infrared thermal sensors and scanners to measure their self-emitted infrared radiation and relate it to surface temperature. When the inside and outside of a structure are at different temperatures thermal energy flows through the walls and ceilings. The better the insulation, the less energy flow and the more is conserved. Changes in wall and ceiling surface temperatures are an indication of the thermal energy loss. This paper introduces the basic physical laws that make infrared thermal sensing instruments work, and explains how they are used to detect and measure heat loss in structures.
Herb Kaplan, Herb Kaplan,
"Infrared Thermal Sensing And How We Use It", Proc. SPIE 0313, Thermal Infrared Sensing Applied to Energy Conservation in Building Envelopes, (12 March 1982); doi: 10.1117/12.932872; https://doi.org/10.1117/12.932872