First, some back-ground...

Until the 1960s windows were single glazed, central heating was only for the well-off, and winter mornings would usually find ice on the inside that had to be scraped off to see what the weather was doing.

That started to change with the arrival of double glazing in the 1960s. Expensive and elite at first, it gradually caught on until it really got going with, first aluminium frames, then pvcu in the 1970s.  Wood window manufacturers struggled with the technical issues for decades, and many traditional joiners have never caught up with them to this day.

The concept was good, but it took many years - decades, in fact - to iron out all the technical issues to get the long-term highly efficient double or triple glazing we see today. The technical issues of cavity depth, spacer bar, glazing bar, sealant type, surface coating, and weight are still with us, and some of those in the industry are happy to avoid the tricky detail that they don't properly understand.
Too many are still happy to take your money and run.

What the experts say...

When things go wrong it often ends up in court, and how does the judge know of the rights and wrongs? Of course, he summons the best technical advice.

Throughout the 1980s and 1990s that advice was provided by the Fenestration Associates, a group of industry specialists whose independent advice would advise the judge in his conclusions.

Sadly, the passage of time has seen the passing of all but one of that team, and he must be getting on a bit by now.
However, some of their writings are still to be seen...

The late Philip Rougier, on learning of the new slim sealed units puttied into shallow rebates in sash windows wrote...
"This is just astonishing. Awful! But if they do sell, we're quite likely to have a busy 2011-2012 picking up failed installations and explaining what's gone wrong, all over again."

He continues...

"A few years back I noticed that contractors at Poundbury, Prince Charles model village near Dorchester, had forced IG units into Boulton & Paul frames with shallow rebates. And to add insult, had puttied them in. They're failing routinely of course. I don't know what HRH makes of it all, but very likely blames the 'double glazing'."

So keep digging.
To last, double glazing has to be properly drained and vented with the edge seals completely buried in the frame. There are wood windows out there that do everything necessary and still look good. Ask the right questions and keep looking until you find them.

Good luck

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