We've single glazed sash windows (old flat).
Is there a 'thermal' glass that could replace the old single panes?
Have you looked at secondary glazing? Or are you wanting to replace the whole single-glazed units with double-glazing?
You can get slim double glazing that looks like a single glass panel. We had some in some of our new doors (listed building so we're limited as to what we can do).
I'd also recommend well fitting blinds alongside lined curtains - stayed in a single glazed holiday let and closing the blinds as well as curtains really helped to keep the heat in in the evenings.
Technically you are right but I've not found a glazer near me that uses it in a single pane window, always as a component in double or triple glazing.
I think the thinnest double glazing you can get is vacuum glazing. Works as well as triple glazing but it's skinnier than all non vacuum double glazing.
I am enjoying this at the moment too - trying to get the sashes replaced with double glazed units (keeping the old frames - long story) - a few companies have said they can't fit new sashes as the frames are too shallow (35mm) but have found one that does (waiting for them to come and quote).
I also have secondary glazing on some of them and I hate it.
I'm not suprised, when it came out forty years ago the company I worked for stopped supplying it to domestic customers after about 2 years, the internal coating wasn't up to the gentle treatment it received!
Pilkington Spacia. Usually used in listed buildings. There’s a risk of knackering the window as you put it in tho
I'll have to check with my builder for our supplier but something like this.