Cooling house facade manipulates heat radiation to great effect

A research group at Princeton University has looked into the main cause of warming roofs and facades. This is not necessarily a completely new topic, and there are also some very old solutions. Completely whitewashing houses to radiate heat became established centuries ago.

It also seems logical that this is effective. However, heat radiation lies outside the visible range in the infrared. Infrared light can be felt in the form of heat from the sun's rays or from an infrared heater. This light is only partially impressed by facade paints, even though light-colored surfaces often reflect non-visible light better.

In addition, the researchers were able to determine that the heating of the roof and the radiation behave differently than on the sides. The roof typically only looks towards the sky. Facades and walls are also heated by radiation effects from the ground and surrounding houses.

Unlike roofs, however, the stored heat hardly radiates upwards. And this is precisely what is now to be changed by limiting the heat radiation from facades to a small range of infrared light.

Instead of the wavelength range of 2.5 to 40 micrometers, only 8 to 13 micrometers are emitted back into space, which corresponds to just under 13 percent of the entire spectrum. The remaining light and its thermal energy are distributed between the ground and the walls of buildings, heating up houses and cities. Incidentally, visible light has a wavelength of 0.4 to 0.8 micrometers.

Even more astonishing than the realization that only a narrow strip is ideally suited to better radiate heat was the search for the right material. Polypropylene (PP), for example, radiates heat quite precisely in the desired frequency band. The plastic can be found almost everywhere, from yogurt pots to baby drinking bottles or as cable sheathing.

In the experiments, temperature differences of several degrees were subsequently measured, simply due to the use of a widely used plastic and the fact that the heat reflected from it can be radiated very well towards the sky. In addition, the heat radiation from the inside is also blocked, so that at least a few tenths of a degree can be gained in winter.

The purely passive use of an inexpensive material that is available in large quantities is particularly remarkable. According to the study, the effect produced is comparable to the difference between a dark and a white-painted roof - but now also on the facades.