Secondary double glazing is most likely the way to go as the openers may have warped and will be drafty. It may also be possible tho difficult to re-glaze with stepped units or megathin ones.

Sorry for ore slow response on this one

Our 1928 house has replacement steel Crittal windows from around 2010. They are double glazed and look appropriate. However, there appears to be no thermal break in the steel frames, and we get heavy condensation on the frames, but not the glass, during the winter months. The house was also retrofitted with a MVHR system.

I note that the magnetic strip used for secondary glazing does stick to the windows, but I do not necessarily want to fit another layer of glass/acrylic, although a sheet of polythene and double-sided Sellotape on the outside of the wooden frame has cured the problem in a bathroom over the last winter.

So, I am looking for ideas to try and insulate the steel frames.

Thanks.

You can't beat the look of Crittalls, bronze fittings and all, where appropriate/original in period. No real alternative sadly.

Tho back in the 90s we did a succesful job with the basic old fashioned variety of (weakly) thermally-broken aluminium, double glazed. These did have a wider sight line than Crittalls, especially at opening lights, but we were careful to not put any opening lights alongside ea other, only corner to corner (three or four bays wide with fanlights, all in the same frame).