ᐅ New Glass Panes in Existing Plastic Window Frames

Hello,
in our house built in 1992, we have plastic windows with a U-value listed in the documents as 2.8. Since I am currently hesitant to carry out exterior wall insulation (including new windows) due to the costs, which would not pay off even if I live past 100, a cost-effective DIY approach that comes to mind is replacing the glass panes. The window frames and sashes, including seals, are still perfectly fine, with no damage, and I have already removed the window trim on one window for testing – 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 switching the “simple” double glazing from the 90s for modern energy-efficient double glazing roughly halves the U-value. The problem is the glass thickness, which is an unusual 21mm. Common current thicknesses are 4+14+4 or (better) 4+16+4.

At the same time, I noticed that the window trims are not, as shown in all the videos about glass replacement, slightly higher than the frame so 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 trims using a spatula and so on, but this is definitely more fiddly than in the videos. Naturally, I cannot fit 24mm glass with a 16mm spacer in there (and I don’t even know the actual window manufacturer to ask whether they offer thinner trims).

My question is whether it might be possible to install at least 22mm glass panes (for 21mm I would only get 5-12-4 or 4-12-5 panes, which are not only more expensive and heavier than two 4mm glass panes but the 14mm spacer also insulates better) and whether these trims that sit 1mm lower than the frame are not deliberately designed that way for the sash construction but simply because (clearly for cost reasons) 21mm panes, not 22mm ones, were originally installed. In other words, was this common practice in the 90s?

If I were you, I would have the glass replacement done by a professional company and I’m tagging @profil65 in this discussion.

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11ant schrieb:

If I were you, I would have the glass replacement done by a professional company and I’ll tag @profil65 in this discussion.

Why should I do that? It would cost so much more that it would make more sense to have new windows installed—which I can’t do myself. But since the windows are still completely fine apart from the installation-time-related U-value, it needs to pay off during my lifetime, and that is definitely not the case with a window replacement carried out by a professional company. The smaller improvement from just replacing the glass could be worthwhile, but only if no labor costs are incurred. And with PVC windows that have only clipped or clamped glazing beads, a regular DIYer can easily handle this themselves.

I can’t contribute much about replacing glass panes, but I can tell you that installing windows yourself is not a big deal as long as they aren’t massive units. Usually, two people are enough; for larger windows, it’s better to have two men (as a woman, I sometimes reached my physical limits and we needed male assistance for carrying). Our old windows were from 1992, and we completely replaced them in energy-efficient double glazing. We didn’t want to invest money in windows that were 30 years old. Ours were neglected wooden windows; since yours are made of plastic (uPVC), I would recommend installing new ones right away.

MarkFalk schrieb:

Why should I do that? It would cost so much more that it would make more sense to install new windows—which I can’t do myself. But since the windows are still perfectly fine apart from the U-value related to their installation time, it needs to pay off within my lifetime, and that definitely isn’t the case with a window replacement done by a professional company. Even just replacing the glass panes can be worthwhile, but only if there are no labor costs involved. And with plastic windows that have only clipped or clamped glazing beads, a regular DIYer can easily do that themselves.

Keyword: setting blocks.
You can do it yourself; replacing the glass isn’t the problem—as long as the seals on the frames stay in place when removing the glass and don’t stick to the glass and tear off. Otherwise, you’ll need new seals (which might be sensible anyway after 30 years—but that takes longer).
Especially with large units, the new glass must be carefully reset with proper setting blocks. Otherwise, the door will sag downward after opening it twice and won’t close properly anymore or will become loose, etc. You can learn this even as a DIYer—you should have shims of the right thickness and a tube of silicone sealant for sealing.
I can’t say if 22mm (about 7/8 inch) windows fit with the same bead profile. My experience is limited to exact replacements.