Thermal curtains are a good way to lower your electricity bills in winter and even in summer, if you use a lot of air conditioning. It's a fact that most of the temperature exchange between the interior of a room and the environment is done through windows and doors. Modern windows, which are very well insulated and which feature two layers of glass filled with an inert gas are very efficient insulators, so you may not feel a significant difference by adding a thermal curtain. However, in case of older windows, with rotten or cracked carpentry and with only one glass pane, by adding thermal curtains you can significantly increase the r value of the respewctive windows. R value is an index which is used for measuring the efficiency of thermal insulations. Its value is in direct proportion with the insulation efrficiency, so you should aim to obtain an r value as high as possible. I'm not a specialist builder, but this is a method for measuring the r value of a surface, in case you are interested.

Back to our thermal curtains, they are nothing else than regular curtains doubled by a special lining which has good insulator properties. For better efficiency, thermal curtains are fixed tightly on sides and at the bottom, in order to prevent the cold drafts to form. There are many methods of fixing the thermal curtain to the walls, one of the most ingenious being with metal stripes and magnets.

If you can sew, it's no big deal to make your own thermal curtains at home. You just need to buy the lining material and to sew it to your curtains, then fix those metallic strips on the walls and make sure you don't forget to sew the magnets on the curtain edges. I found a funny video which illustrates the process of making a thermal curtain with the help of a sewing machine and with a lot of patience. You'll see also how to measure and how to cut the fabric for making the thermal curtain look great, like the ready-made ones. You'll see that before starting to sew the parts together, it's better to pin them, for more accuracy in work.

As far as I've seen, all thermal curtains are opaque. However, if you like to have transparent curtains, you can double them with thermal drapes, which you can either remove or tie back in those periods of year when you don't need them. If you don't want curtains at all, you could buy one of those plastic foil insulating kits for windows and apply it as shown in the above video.