Hi all,
The final part of my renovations will be to replace some failed double glazing units.

The windows were installed in 1995, and are uPVC with 28mm units, 3 chambers, external shuffle-bead and espag locks. Every window functions and looks fine. The previous owner replaced the two south-facing front windows in 2009.

The biggest issue is with the two north-facing bedroom windows, which are approx 1800x1000, with a large 1200x1000 panel which has failed on both. Cost-wise, I could order the best specification double glazed units, and come out at around half the cost of replacing the whole window.

Is this false economy? Are newer frames going to be significantly more efficient? I've never liked waste, although presumably the uPVC goes to recycling, so it's not so bad. The security of the old frames could be better, which is another plus for replace.

Going further, I could reglaze, and then fit secondary glazing outside the reveal? Wouldn't this perform better than even the best replacement window both in terms of heat and noise insulation, and no waste other than the failed glazing units.

Windows from 1995 will have a U value of around 2.7, and new planitherm glass will get you down to a U of 1.1. I don't think changing the plastic work will have much impact on the overall insulation value - you may even be constrained in what glass coatings are available, and end up with a worse U value.

Compaired to this better glass( well, its the magic metal coating and gas fill), secondary glazing (just glass/plastic) doesn't get you much in improved insulation - though it will help noise reduction if important. 2ndary glazing can be a condensation magnet.

I'd just change the glass, though there are other factors: security, fitting (are these big windows fitted from the outside?), will the new ones be the same quality and last 20 years+ ?

You could go 3g on the 28mm big north facing ones - they'll be heavy though. Probably 4:8:4:8:4,(edited, oops) exotic gas fill, prob U=.7. Check out how heavy, and how you'd fit them first!

2nd the drainage comment, might be why they failed?

Thank you for your advice.

To be honest, I hadn't even considered 3g, but will look into that if it's worthwhile. It's a bungalow, so there aren't any serious difficulties with us fitting heavier units, and because they're not openers, no strain on hinges. It sounds like a significant improvement on the glass that's already in there.

As for the drainage, I think inadequate packing is the problem, which is what I noticed when I changed the door panels. The big panels appear to be sitting very low in the frames, possibly even sat at the bottom, since I can see around 10mm of the bottom external glazing bead when looking out from inside (and less than 5mm of the top bead). When I replace, I'll fit proper sized packers to lift the units up and away from drainage water.

Narrow spaces would need to be filled with krypton rather than argon I think, to improve the U-value.

edit: to refer to a superhero's weakness rather than a comedy robot/android