Whilst searching for information on double glazing performance I came across this company and sent away for a sample and more gen…

http://www.slimliteglass.co.uk/slimlite-double-glazing.html

The sample came this morning and I’m impressed. The system is ideal for use on conservation and Listed Building jobs (if you can get past the hidebound, knee-jerk, jobsworth CO’s incapacity for imaginative thinking) and can be used as a direct replacement for single glazing in the existing rebates. It’s a brilliant system.

Relatively expensive though. The sq.m cost is roughly double that of a conventional unit and, because of the way the unit is packed, you can add a significant chunk to the cost. But it does offer a solution to the age-old problem associated with bringing the older building into line with the current requirements of energy saving and introducing a welcome level of human comfort.

Don't know what you're looking at, Rog. My figures for the basic costs are:

Both types 4mm, low 'e', gas-filled.

Standard unit    £40.63 per sq.m + £6.50/sq.m for gas + VAT

Slimlite unit    £83.85 per sq.m (inc gas) + VAT

The delivery cost of £130 is a one-off per delivery, so cost per item will reduce with quantity (although still a bit stiff).

It is a possible way around the knee-jerk reaction of some COs against double-glazing per se, especially on Grade 2 listings, and at least makes the point arguable.

As for the conversion of single to double-glazed - when you factor in the cost of machining the existing rebates (or re-making the whole sash) it's not going to work out any different in the long-run and at least makes the process feasible and the retention of original features possible.

It's at least worth considering as another weapon in the arsenal.