In our recent renovation we put in triple-glazed bifold doors and I have been very impressed. During the recent cold spell there was frost on the outside of the glass which didn’t melt for several days, while the inside pane of glass didn’t even feel unpleasantly cold to the touch. 

They weren’t cheap though (about £3600 for a 2.1 metre 3-door unit) although I don’t believe triple glazing added a huge amount to the cost. 

My previous point of comparison was horrible 30 year old aluminium sliding doors which were constantly dripping with condensation in winter due to cold bridging problems. 

Our upstairs is triple glazed and the windows are noticeably warmer to the touch than the downstairs double glazed. Only a few years between them, so I'd say very comparable. 

We're in the process of replacing all of ours. They are about 18 years old and whilst some are still ok, some aren't so we're going to do them all at once

We're replacing with Triple - my thinking is do the best you can now, as it will probably become the norm in the next 10 years or so (and already is in colder climates)

However we did find some companies sell 3x glazing and its not much better than 2x. We went to Green Building Store and they only sell 3x and it's very good - so just be careful to compare the U values to check what you're getting.

We changed the glass in our circa 1990 uPVC windows 11 years ago.  The double glazing units that were in were the standard of the time: 4mm glass/16mm air /4mm glass, aluminium spacer, air fill - giving a pretty ropey (by todays standards) Ug of 2.6W/m^2/degC with a lot of cold bridging, but all of the frame still seems fine. 

The replacement sealed glass units are 4/16/4, insulating spacer, argon fill, St Gobain Planitherm 1 coating on the inner pane - this is a "softcoat" giving an overall Ug-value of 1.0.  I think it's still pretty much the best that you can get, barring replacing the frames to get a bigger gap, and fitting 3G.  The plastic frames and windowsills themselves are now mostly foam filled by me, to improve their insulation further - being very careful not to fill anything associated with keeping the DG unit seal exposed to outside air, for a good long life.  Pilkington make a similar "softcoat", but I couldn't find anyone that would sell it - I was only offered Pilkinton-k, which is not quite as good.

Pilkington and St Gobain have free calculators, that will work out the glass performance of the glass - U value, clarity, sound attenuation, UV.  The St Gobain one now even gives the upfront CO2 emissions for making it.

https://calumenlive.com/

https://spectrum.pilkington.com/Main.aspx

It's funny how things come around but we are just about to have new windows installed and have been told that new regs are that they must now have vents on them.

We were also told by 3 different salesman that there would be no great benefit going to triple glazed unless we lived near a road or noisier environment.

Windows getting replaced are about 20 years old with the seals degraded leading to condensation so if new windows don't help, I'll be changing my extractor fan to a blauberg decentralised MVHR unit to get rid of humidity in the bathroom.

We’ve changed the majority of our windows to triple glazed.

The original windows were only 11 years old but hardwood and had been neglected by the previous owner.  They were however very high spec units being 22mm rather than 17 or 19

The difference is phenomenal for insulation and sound with the added benefit of improved security 

I have a thermal picture I’ll dig out as that shows the heat loss difference as @bob77 above we had frost on the outside that didn’t melt even with 23 degree internal temperatures 

another plus for us on the south facing side was slightly lower solar thermal gain from triple as we have a lot of glass on that side and it’s too hot in the summer 

edited to add the cost difference was 12% over double glazed

pictures attached, see the heat loss through 100mm hardwood and 22mm glass compared to the triple unit on the same room 

Thank you all for the replies and information, food for thought, interesting JPST21, a cost difference of 12% over double glazing, I suspect that the insulating properties will be greater.