The prices aren’t particularly high and it’s a bit of a hassle to fix, but it’s doable since you don’t need any special tools. But the crucial question is: is the whole slide worth it?
In principle, you should not expect miracles from foils, advertising promises should not be tenable. A good insulation of the facade by craftsmen is certainly the best solution.
Frank Ebisch of the Central Association for Sanitary, Heating and Air Conditioning also sees it that way. “Anyone who sticks pure aluminum foil behind the heater gets no insulating effect. A very poorly insulated house can save up to 4% by using foil with an additional insulating layer. With modern construction, reflective sheets are not worth it”.
If you assume annual heating costs of 2000 euros, you could save a maximum of 80 euros. That would probably have pretty much covered the cost of the slides the first year.
Reflective sheets are not a placebo and their effect should not be overestimated. In well-insulated homes without heating niches, however, you can certainly save on sheets.