Home DIY

I went with the full replacement including the boxes. Yes, it was worth it.

I renovate sash windows and replace the 3 or 4mm glass with 6.3mm acoustic laminate glass. I looked at all the different ways of adding double glazing but to make it work it needs a big gap in the middle and it's near enough impossible to achieve that with conventional double glazing. There are also issues with the edge sealing, if you have an edge seal small enough to be hidden by putty it is also vulnerable to leaking which means blown units. Bear in mind the extra weight of the dg units needs larger balancing weights and even the acoustic glass is approaching the largest weights you can fit in the og boxes.

There is a modern type of dg unit which is very thin the way it's made is complicated but too expensive to produce in small sizes.

I've found properly sealed sash units with acoustic glass are the best option if you really want original frames and boxes. If you can stand to have larger boxes/thicker sashes they can improve on performance but getting good fitters is difficult.

If you can stomach it you are better off stripping the whole lot out and replacing with modern (wooden or ZMG plastic / composite) units. The cost will be similar to a retrofit + service and they will last a lot longer. We looked at retrofitting dg panels to older sg frames and like Airhead knows it's not really feasible or if it is it's not durable. It's almost certainly non-economical.

Secondary glazing seems to be recommended by conservation folks, so if you really want to keep whatever is there now, or you can't remove it for whatever reason, you can add a new layer of glass internally (or even that stick on plastic stuff).

Airhead has covered it better, but when we looked it didn’t seem like a practical conversion because of the many complications.
And then there’s the fact that sliding sash aren’t the most efficient design in the first place - so it can be really hard to service old units to the point that they’re effective (and on top of that it’s hard to find someone with the skills who’s willing to do it).

We ended up with PVC sliding sash at a cost of around 7.5k because we weren’t planning on staying on the flat long enough to justify the 15-20k new wood ones would cost. The downstairs neighbours had theirs retrofitted with double glazing, I’m not sure what the cost was, but I wouldn’t have been happy with the result if it was my flat.