Insulate windows from the outside, insulation wool and panels

Hello,

I have been reading along for a while and have already received various suggestions. Since I now want to ask something myself, I have registered.

My issue is as follows:
I have a window front about 5m wide and 2m high with a balcony door to the terrace. The previous owner obviously saved both on the price of the windows and on the installation, so that when going up the stairs along this window front you really feel the cold air streaming down. The same applies to another single balcony door in another room. Since our gas bill is already too high and these windows are not needed for brightness, I wanted to close everything on the wide window front except for the balcony door and completely close the balcony door in the other room. However, this should only be temporary to keep the winter cold out. In a few years, we will receive a larger sum of money and will then renew both the uninsulated facade and all the windows. Until then, I just wanted to seal these windows as cheaply as possible.
The first idea was simply to put insulation wool on the window from the inside and then cover it with drywall or panels. However, I fear that moisture could accumulate there. Then I thought about just doing it from the outside with insulation wool and panels. No one can see that from the outside anyway, and since we hardly ever use the terrace, it wouldn’t be a problem.

Does anyone have a suggestion or maybe even a better idea how to temporarily close this with simple means?

Hi, the whole undertaking would have dubious success if your sole goal is to reduce heating costs. Purely economically, it doesn't pay off, but if you have other reasons you need to assess that differently ([draughts] etc.)
Brief explanation:
Currently, with the old windows, you will have a maximum U-value of 2W/m²K.
Afterwards, with about 5cm of 0.35 mineral wool and cladding, about 0.34W/m²K.
That makes a delta of 1.66W/m²K - with about 8m² area, an estimated average temperature difference of 17°K (inside/outside) and a 100-day consideration period
I come to a saving of about 540kWh - calculated in city gas that would be roughly around 40€. If you can get the wall done for that money - fine :)
However, since a roll of decent tape for the vapor barrier costs 2/3 of that, I don't even need to calculate further...
I wouldn’t do it and would save the time and money for the renovation.
A floor-length thicker curtain will serve the same purpose and is easier to remove later.
ks

Currently, with the old windows, you'll have a maximum U-value of 2W/m²K.
Afterwards, with about 5cm of 0.35 mineral wool and cladding, about 0.34W/m²K.
That makes a delta of 1.66W/m²K – with about 8m² area, an estimated
average temperature difference of 17°K (inside/outside) and a 100-day period,
I come up with a saving of about 540kWh – in city gas calculated, that would be
about 40€.

Well, the calculation is quite sobering. It's just that this window front is on the upper floor, and when you go up the stairs you really notice how a kind of waterfall, only made of cold air, rushes down. Therefore, my fear is that it’s not just the poor window, but also that it’s poorly installed. Or would you only have that feeling if the air in the house cools down through the large window front and therefore sinks down, so it’s not outside air at all?

If you can get the wall done for that money – fine :)
But since a roll of decent tape for the vapor barrier costs 2/3 of that,
I don’t even have to calculate further...
I wouldn’t do it and would save the time and money for the renovation.
A floor-length thicker curtain will serve the same purpose and is
easier to remove later.

Vapor barrier and such are definitely beyond my skills. I have lived my entire life in rental apartments and therefore have zero experience and am only now trying to teach myself step by step – so I didn’t want to start right away with internal insulation. That’s why my idea was to add insulation from the outside, because I hoped I wouldn’t need anything more than insulation wool and panels since I already have both anyway. (The previous owner left them in the house when selling.) Only I didn’t want to try it myself and then have something happen that I hadn’t foreseen because I lack experience – so I hoped someone could also say something about the outside option.
But you’re definitely right about insulation from the inside, because I definitely don’t have such tape in stock.

A floor-length thicker curtain will serve the same purpose and is
easier to remove later.

I googled that now and buying a thick curtain and a correspondingly sturdy curtain rod is not exactly a bargain. :( Does anyone have any suggestions where I could get that cheaper?