This image shows wet insulation on a roof. During the day, the sun will warm the roof, and this energy is stored in the thermal mass of the water after the sun’s energy is removed. At this point, the wet insulation appears as a hot spot. In the morning, the sun warms the roof again,and wet material takes longer to warm up, appearing as a cold area. At some point during the day and during the night, thermal equilibrium will occur, making the indication disappear.