What you see in this image above is the result of water getting beneath the roof membrane and residing there. While it may not even show up inside the building, the thermal signature of the water becomes evident with the temperature differential shown in a thermograph. The orange/reddish areas at the inspector's feet are warmer than the other areas. The water captured in the roofing stays warm longer than the surrounding area and is immediately visible through thermography.
The wet insulation has a higher thermal mass than the rest of the "dry" roof structure. As a result of this difference, the "wet" areas will hold heat energy longer than other areas providing the infrared inspector with a clear picture of the potentially damaged area. The temperature difference between damaged and "dry" roof is very small (typically 2-4 degrees). Because of this, a camera with high thermal sensitivity is desired for roof inspections.