Hi original poster
Go for it: I think it is definitely worth it - some energy savings for sure, but more significantly every time you look at a window that used to have condensation but doesn't anymore, you will be reminded of the improvement you made.
(But in any case, get rid of any unflued gas heater you may have!)
Retro-fitting double glazing onto existing aluminium frames offers most households a significant increase in energy performance, without the much higher cost option of replacing standard aluminium frames with thermally-broken aluminium frames. The additional costs include higher labour costs (getting a builder involved) and significantly higher material costs. The suppliers of retro-fit double glazing send their own glaziers in: a one-stop shop.
I have double glaze windows at home, using NZ standard frames, without argon fill: halves the heat loss through the glass area compared with single glazing, no condensation, and in our own limited experience, no mold growth on the frame or perimeter of the double glazing units. Sure the aluminium frames are losing some heat: so does opening windows, kids leaving doors open, uninsulated concrete floors, lack of good thermal drapes, wrong position of heat pump, etc.
A few of the people who post on this forum keep repeating that NZ has junky windows, why don't we do this/why don't we do that - NZ is behind the times, cost is too high, why don't we do it the way they do in Europe, etc.
Importing a house-worth of windows is probably worthwhile to an end customer, without adding up their own time and labour. Our market size (small) dictates our costs structures (higher than elsewhere). The reality is that if it really was HEAPS cheaper or even cheaper-ish for a business to import all these European imports and stay in business and keep their prices down, then someone would do it as a business so the handful of Ecobob posters including the ones on this thread would be able to buy off them rather than have to import their own.