Heat loss. Thermogram of a house showing the distribution of heat over its surface. The colour coding ranges from white to yellow for the warmest areas (greatest heat loss) through red to purple and green for the coolest areas (greatest insulation). This thermogram shows that the roof and windows (yellow) are poorly insulated, while the walls (red, purple & green) are losing the least heat. Thermograms are often used to check houses for heat loss, so that they can be made more energy efficient through improved insulation.