Cover windows with bubble wrap to improve the heating of the house: this is the “home trick” that promises to have warmer homes in winter and avoid heat loss in a simple, fast way and with minimal expense, with possible savings on your bill. But does it really work? As always in these cases, the answer depends on many contextual factors but in general we can say that bubble wrap – packaging material containing small air bubbles so satisfying that they burst – it actually offers a certain amount of thermal insulation but it doesn’t work miracles. For better results (and without disturbing the view from the windows) it is better to use special bubble wrap covered with aluminum or heat-reflecting panels to apply to the walls near the heat sources.
How the “bubble wrap trick” works
The reasoning behind the “trick” is that bubble wrap contains many small air bubbles – an excellent thermal insulator – not large enough to allow heat transfer for convection. Convection is one of the three mechanisms of heat propagation, typical of fluids just like the air. When some of the fluid is heated, it expands and thus becomes less dense. This causes the Archimedean thrust to act on this part of the fluid: the hot air rises, once it reaches altitude it cools, thus becoming denser and consequently sinking. The cycle repeats itself, giving rise to the so-called motions convective – we can see them very well in a pot of boiling water – that they are effective in distributing heat in the fluid and transferring it elsewhere. Bubble wrap has air bubbles that are too small for convection to propagate large amounts of heat, and this makes bubble wrap a thermal insulator.
However, if we look at the numbers, we realize that this material it’s not really a champion of thermal insulation. There is a physical parameter that measures the ability of a material to offer thermal resistance: it is called value R and measures how much the temperature varies in a unit area of material given a given heat flow. The value R it increases with the thickness of the material (the thicker the insulating layer, the more it insulates) and decreases with its thermal conductivity (the more it conducts heat, the less it insulates): higher values therefore indicate better insulators. Now, a layer of bubble wrap has a value R of approx 1. To be clear, the same as you would expect from window glass and about half that of an equally same layer of polystyrene. Considering the dispersions due to the inevitably imperfect “home” application, in practice apply bubble wrap brings the thermal performance of single glass closer to that of double glass. The difference is there on paper, but whether it will be felt in your home will obviously depend on many other circumstances.
How to put it into practice: the advantages of aluminum
If you want something more without too much effort or expense, it is better to focus on solutions that act not only on convection but also on another method of heat transport, often more crucial: theirradiationi.e. the transmission of heat through electromagnetic waves (typically in the mid-infrared for the temperatures we are interested in in this context). A heat source, for example a radiator, emits infrared rays by virtue of its temperature, and this radiation transports heat.
Preventing this heat from passing through walls and windows and then exiting the room is particularly effective in thermally insulating a home. And doing so is simpler than you think: just use thealuminuma material capable of almost completely reflecting mid-infrared radiation, thus ensuring that it remains inside the home. This is the principle behind another “home trick” for home heating, that of tin foil to apply behind the radiatorsalso recommended by ENEA (the national agency for new technologies, energy and sustainable development).
Bubble wrap is made up of polyethylenea material that is almost totally transparent in the mid-infrared and therefore has very low reflective power. However, by combining the insulating effect of air with the reflective effect of aluminum you obtain a rather effective method for insulating your home, and for this there are special materials, such as bubble wrap covered with aluminum foil or heat-reflecting panels consisting of two sheets of aluminum separated by air. These materials are more expensive than simple bubble wrap or simple foil, but they provide values R much superiortherefore making the material significantly more insulating. Just don’t apply them to windows but to walls near heat sources!
