The gable end of a stone house shows two notably hot areas: a window, and a flue passing through the wall, level with the upper part of the window.

Generally thick natural stone walls are quite insulative, and their outer surface remains close to ambient air temperature, appearing cool. There are two exceptions seen here: although the window is a modern plastic type with sealed double-glazing, its insulation cannot compete with that of the stone. The surface of the glass also has different IR reflectivity/emissivity, and may not compare directly with the stone surface.

However the brightest and warmest area is the flue for the heating boiler (furnace). Although of modern design, so that tries to recover as much heat as possible from its exhaust gases, most boilers still release quite a lot of heat, and some water. The flue is sufficiently warm to heat the wooden eaves of the roof above, which appear slightly warmer than those on the other side of the gable.

The trees behind the house, and the sky, are significantly colder, although areas of the wall on the right (at the front of the house) which have been in shade for some time, are almost as cool as the trees.

IR thermography is an excellent tool for discovering which parts of buildings are warmest, and thus losing heat most, but you must be cautious when comparing surfaces with very different reflectivity/emissivity, and in allowing for warming sunshine.

Image captured using a FLIR One™ IR camera and iPhone 6. Standard ‘Iron’ palette, without further image processing.