Bubble wrap makes great insulation because air is a good insulator, and bubble wrap is nothing more than bubbles of air wrapped in plastic. Bubble wrap is cheaper and easier to use than other, traditional alternatives and it can be reused many times.
Insulation is any substance that protects one space or material from the temperature variations of an adjacent space or material. Any substance can act as an insulator, though some are much better insulators than others. The ability of a substance to act as an effective insulator depends on how it resists temperature changes and how quickly it can dissipate heat.
Air and bubble wrap
Air, particularly air in a confined space, is a good insulator because it tends to resist temperature changes. The air inside changes temperature slowly because the air molecules move around, making transfers of energy (heat) between one molecule and another more difficult to accomplish. Bubble wrap is nothing more than plastic wrapped around many individual volumes of air; thus, the air inside helps bubble wrap to resist temperature changes. Bubble wrap shares this feature with cavity wall insulation "by combining with the still captive air, the insulation acts as a barrier to heat loss" according to The Environment Site.
Bubble wrap can be attached to windows simply by spraying water on the window first and then spreading the smooth side of the bubble wrap over the wet surface. A second layer of bubble wrap can be added just by taping it to the first layer.
One of the greatest things about using bubble wrap as an insulation material is that it can be used again and again, so long as it remains in relatively good shape. If you can keep children (or yourself) from popping all the bubbles, then the bubble wrap will last for many seasons. However, if your intention is to use the bubble in summer and winter, you should use a UV resistant bubble wrap to ensure longevity.
Bubble wrap is not clear, so it will block the vision of a window if it's used as insulation. Also, the plastic of the bubble wrap can deteriorate with exposure to UV. It will only last a year or two if it is kept on a window year-round. Additionally, bubble wrap is not a great insulator in extremes temperatures.