I opted for Triple Glazing, they are Argon filled 4-12-4-12-4 Clear Softcoat Triple Black Thermal.

We still have some more to do as funds allow, but the ones that have been done are great, glad that I got them done.

U values are about 0.4 for 2 x 4mm panes with a 0.8mm vacuum gap.  This low total thickness allows the units to fit in normal frames.

2G Pilkington K was until recently the standard low-e coating, and would give U=1.5 with 2G in a decent frame with Argon fill, 4/16/4 

A more modern Pilkinton Optitherm S1 plus now gives U=1.0 with the same sizes.  SGG makes planithem 1, which is the same as this.

The coating doesn't seem to change the price particularly, even though it's so influential on overall window performance.

I would focus on the U value; centre pane U=1 is achievable for 2G, U=0.5 is possible for 3G (this needs 2x coated soft coat low-e glass panes).

The uPVC framed double glazing we installed back in the 1980’s and 90’s had virtually come to the end of its’ serviceable time; we have had a few ‘blown’ units replaced over the years but we then reached the point where worn fastenings, seals and locking mechanism were becoming a little irksome. Advice on the internet seemed to suggest that average life of such units is in the region of 15 - 20 years, so I suppose we have had our money’s worth out of them. 

Over the last few months, we have had all windows and the French Doors replaced with triple glazed systems that use argon gas fillings. My only quibble is that changes in regulations require trickle vents be fitted and although they may be useful, their design does not exclude noise as well as an otherwise sealed frame would have done.

We now have underfloor foam insulation (Q-Bot) on the ground floor, cavity wall insulation, and something over 300 mm of loft insulation plus foil faced boarding between the rafters (we also had roofing felt installed some years back as this was also absent at the time.) The remaining chimney breast (one was removed years ago) has a shuttered vent which remains shut as we have a bathroom extractor, a cooker hood extractor and normally have at least one window slightly open in the winter. The ASHP should have an even lighter load this coming winter and I don’t think there is any further insulating treatments that we could carry out! MVHR is not something I wish to contemplate on this 1930’s semi., we just need to encourage the neighbours to warm the party wall a little more! 😉.

Rewards, Toodles.

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 

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.

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