Why radiators are often found under windows in our homes: the physical explanation

Often, in our homes, the radiator is positioned right under the windows. Is there a scientific explanation or is it just a matter of aesthetics? Well, obviously there is a specific physical reason why the radiator is almost always in those positions and it has more to do with the air than with the idea of ​​”the heat rises“. In contact with the low temperature surface of a glass, the air in the room cools and sinks downwards: this is the so-called downdraughta descending current that in winter creates that widespread sensation of cold even away from the glass. Certainly, the better insulated windowswith a warmer internal surface, they reduce this current considerably. The radiator under the window instead generates a warm flow that rises and counteracts it: that’s why the radiator is placed right there.

Why the cold current is formed

In winter, near a cold window, the room air loses heat on contact with the glass and slides down the surface: this is the cold air down draft. In 2002, two Swedish scientists measured the phenomenon by comparison windows with different insulation: as expected, those that they maintain the internal surface warmer – isolating more – produce a significantly weaker downdraft. In essence: the warmer the glass remains on the side of the room, the less likely an intense cold draft is. Furthermore, the problem it does not depend on the average temperature of the room: the thermometer can read 20 degrees and still feel that cold current at the height of the legs. But it didn’t end here. There is also another factor that is often overlooked: it is not only the air temperature that counts, but also that of the surfaces around us. A 2001 study shows that the characteristics of the glass influence thermal comfort and that double glazing reduces the discomfort of cold drafts. A large cold surfacelike that of a window, near us can make us cold even if the environment, as a whole, is at a comfortable temperature.

Because the radiator counteracts it

A radiator starts hot air which, being less dense of the surrounding “cold” environment, rises upwardsforming a so-called thermal plume (in Italian “thermal plume”). If we place the radiator under the window, this flow is found exactly where it can oppose the cold current of glass. To understand more, science has mobilized in different ways: some scientists have studied the phenomenon with experiments and simulations, while others have looked for the most effective radiator temperature, reducing the risk of cold current with the least possible waste of energy. It turns out that the best thing is to find a optimal temperature range, avoiding “heating as much as possible” and push the environment outside the limits of comfort.

Differences with modern windows: is it still necessary?

Not always. Modern windows have better performance than ever and this already reduces the problem at its core: warmer surfaces mean less speed and turbulence in the downdraughtto the point that with a well insulating frame the risk of an annoying cold draft in the occupied area can remain acceptable without other measures. In fact, in the most modern homes, other ways for heating also open up, such as radiant floor or wallto be evaluated together with the characteristics of the building.