A thermal imaging camera is a powerful tool which can help find many hard to detect problems in a home. It can sometimes reveal hot spots in active electrical circuits, areas of poor or missing insulation and wet areas in walls.
In each of the image pairs below, the image on left is a normal photo of an area. The right photo is a thermal imaging photo, sometimes called an infrared (IR) image, of the same area. By looking at the temperature scale at the right of the IR image you can see that bright areas tending to yellow or white indicate a warmer temperature and darker areas ranging from purple to black indicate a cooler temperature.
Wet areas, through evaporation tend to be cooler and show darker in a thermal image. This is a basement ceiling and there might be a leaking water line or drain line where the dark spot is.
In this pair there are some areas that are much warmer. Many factors affect the interpretation of this situation. During the summer, this might suggest that there is poor insulation in the attic. During the winter, if there is poor insulation, the same area might appear dark.