New panes in old plastic windows

Hello, in our house built in 1992 there are plastic windows, the U-value is given as 2.8 in the documents. Since I am still hesitant about a facade insulation (including new windows) due to the costs, which will not pay off even if I were to exceed 100, the idea of replacing the panes as a cost-effective DIY measure comes to mind. The window frames as well as the sashes including seals are still perfectly fine, no damage, and I have already removed the window trim on one window as a test – that works. Of course, the U-value of the old frames is worse than that of new windows, but replacing only the sashes is not possible and replacing the "simple" double-glass panes from the 90s with modern insulated double glazing roughly halves their U-value. The problem: the pane thickness is an unusual 21mm. Common modern thicknesses are 4+14+4 or (better) 4+16+4. At the same time, I have noticed that the window trim does not lie somewhat higher than the frame, as seen in all related videos about pane replacement so that they can be knocked out with a suitable tool (or wedge/block), but rather "coincidentally" (?) about 1mm lower than the frame. As mentioned, I can still remove the trim with a spatula etc., but it is clearly more fiddly than shown in the videos. Naturally, I cannot fit 24mm panes with 16mm spacing there (and I also do not know who the actual window manufacturer is to ask if they also offer thinner trims). My question is therefore whether it could be possible to install at least 22mm panes (for 21mm I would only manage with 5-12-4 or 4-12-5, but on the one hand that is more expensive and heavier than 2 x 4mm glass and on the other hand 14mm spacing insulates better) and whether these trims that lie 1mm lower than the frame are not because this is necessarily the case and corresponds to the sash construction, but only because (due to cost reasons, obviously) 21mm panes and not 22mm panes were installed. Ultimately, whether something like this was common earlier, in the 90s.

I would commission the glass replacement at a specialist workshop if I were you and am tagging in the discussion.

I would recommend having the window pane replaced by a professional company in your place and I’m tagging into the discussion.

Why should I do that? That would cost so much more that it would make more sense to have new windows installed – which I simply cannot do myself. But since the windows, apart from the U-value due to the installation time, are still completely fine, the whole thing has to pay off within my lifetime, and that is definitely not the case with a window replacement carried out by a professional company. The lesser improvement from replacing the pane alone can be worthwhile, but only if no labor costs are incurred. And with plastic windows that have only clipped/clamped glazing beads, even a normal DIYer can easily do that themselves.

I can't contribute much to the windowpane replacement, but I can tell you that the DIY installation of windows is not a big deal as long as they aren't monster-sized. Usually, 2 people are enough; for larger windows, it should be two men (as a woman, I sometimes reached my physical limits here, and we needed male carrying assistance). Our old windows were from 1992, and we completely replaced them with [EL]. We didn't want to invest money in 30-year-old windows. Ours were neglected wooden windows, yours are made of plastic, so I would rather take new ones straight away.

Why should I do that? It would cost so much more that it would make more sense to have new windows installed – which I simply cannot do myself. But since the windows, apart from the U-value caused by the installation time, are still completely okay, it has to pay off within my lifetime, and that is definitely not the case with a window replacement carried out by a professional company. Even the lesser improvement through glass replacement can be worthwhile, but only if no labor costs are incurred. And with plastic windows that have only clipped/clamped glazing beads, a normal DIYer can also do that well by themselves.

Keyword: shimming. You can do that yourself; replacing the glass is not the problem – provided the seals on the profiles stay in place when removing it and do not stick to the glass and tear out. Then you need new seals (which might be sensible anyway after 30 years – but takes longer). Especially with large elements, the new glass must be carefully shimmed again. Otherwise, after opening the door twice, it will sag downwards and no longer close properly or becomes wobbly, etc. You can learn that too, even as a DIYer – but you should have shim plates of the correct thickness and a tube of silicone for sticking it on. Whether 22mm windows fit with the same glazing bead, I cannot say. My experience is limited to the matching replacement.