The film I’ve seen is very low tech. Just double sided transparent tape around the frame to sick the film onto - and then a hairdryer to shrink the film to make it taught so that it doens’t look too ugly with creases.
Seems that you do not describe the same thing in both posts. The first one sounds like you put a film over the whole window which entraps some air. Kind of seems bullshit to me, especially if you have already double glassed windows, which are like a minimum standard in Europe.
If it is more like the second one (which seems more realistic), it is just to make sure that the windows are air tight between the frame and the glass, which should be the case in European standard windows. So basically useless except if you have old single glassed windows (everything else should be air tight). If you do not have airtight windows, I would rather look into replacing the joints or maybe checking the hinges. If they are older, than a replacement would help.
This is rather a problem of aeration. When you live in a rather well insulated house, moisture content increases in the atmosphere as you live in there (breathing, cooking, showering etc.). If the warm moist air hits the colder window, you will have condensation, also with good insulated windows. The airthighter the appartment (good for insulation), the worse the problem with condensation. You need to increase the air exchange to evacuate the humidity in the air (either a forced ventilation system, best with a heat exchanger or opening the all flat windows once or twice a day for about 10-15 Minutes to create cross ventilation). The air exchange is not that much of an energy loss, since there is not much “thermal mass” in it. Of course it is not as efficient as a central ventilation system with heat exchange.
If you look at your window, the glass of the window is usually set around 1cm in from the frame of the window. When you put the tape on the frame and seal the film onto it, you effectively get an additional 1cm of air insulation. 2cm if you do inside and outside of the window. Since the air gap in double glazing is a lot less than 1cm, this could be a considerable amount of additional insulation.
Just for info, the air gap in double glassed windows is generally between 10 and 20 mm.
The goal is to make it as large as possible, without creating a convection cell between the glass panels. Apparently, anything larger than 20mm would lead to this effect.
Realistically, you can put the thin film only on the inside, since on the outside the risk of damage is too big (UV radiation degrading the the glue/film, tension through wind, birds etc.)
The effect it should achieve is to reduce the convection transfer on the inside of the window. If you are not constantly blowing with a ventilator on the window, this should not be that much. The film itself has no insulation value, since it is too thin. The air gap between the window and the film adds no diffusion resistance, since in both cases it is normal air in contact. The only advantage is creating another layer of air which cannot move and since this is not as relevant on the inside than on the outside of the window, it does not matter that much.
Furthermore, since you can’t practically put in dry air inside, you will have a problem with condensation/moisture.
For me it is a false good idea which will not bring any relevant advantage. Better than nothing, sure, will you see it on your heating bill, most probably not. You might create other problems though.
-if you have rolling shutters, look how they are integrated into the wall. They are convenient compared to the classic shutters, but they might be a energy passway
-closing shutters as soon as the sun is gone is super efficient (limiting convection on outside is super efficient)
-make sure the windows are really airtight.
-if you can, look into installing a central ventilation system with heat exchange.
The only advantage of the film is to put another layer of non moving air to reduce convection around your window. The film itself does not add any value.
The process of heat transfer on the inside is anyway dominated by diffusion through air because the air is not moving much. So adding a layer to limit the convection will not help much, because there is actually not much convection happening anyway. On the outside however you have wind, so adding a layer to limit convection through wind would actually help. However, I do not think it is a sustainable since the film will break pretty quickly since it is exposed to the elements.
And you still have the humidity problem.
Look, try it out and see for yourself if you have a different heating bill. I would argue you will only see a negligible effect if at all.
It seems to me that it is not only insulating, but also filtering UV ray.
In the link you will find the solution installed in our hotel, Invisible, quiet easy to install and not so expensive.
For sure a new window would be better, but not really changing the condensation problem, imho.
Probably you just need to open windows and change air and also to avoid the “too warm when sunny, too cold”.
If the problem still there, then a good air conditioning system could help you to reduce humidity and your quality of life in the house/apartment will increase.