From http://www.greenbuildingforum.co.uk/forum/index2.php?DATEIN=tpc_hnovqhocr_1163970597&LP=true

>brand new sashes can be fitted within existing sash windowcases, allowing optimum-thickness (24mm, with 16mm airgap) DG units in best-practice deep rebates using dry glazing tapes, rather than bedding in mastic (which is death to dg units).

New sashes can be 56mm or more deep, in place of the usual 44mm. The bit that runs in the frame remains 44dp; extra depth spreads inboard and outboard within the depth of the staff bead and the fixed outer facing (whatever that's called); it's surprisingly unnoticeable. The sightline width remains 44mm, as normal.

The sashes are glazed-in, not glazed-out. The normal bevelled putty profile is reproduced as part of the outboard rebate platform. Inboard there's a timber bead incorporating the usual mould shape. Hell, difficult to describe. Anyone who wants, I'll post a drawing - email me at info at space-and-light dot co dot uk. I've failed to send same to 2 people who've requested same previously - I will do it, promise.

Sash weights will have to be doubled in weight, to counterbalance 2 panes instead of one (the timber weight is negligible compared with the glass)<

Here at last is the detail - hope it works here: http://i30.photobucket.com/albums/c301/fostertom/Waterheadwindows.jpg

It shows newbuild sash windowcase but such sashes *can* also be fitted into existing windowcases, with a stepped-edge to the sash - I'd describe further if nec.

This is it, for me: http://i30.photobucket.com/albums/c301/fostertom/Waterheadwindows002.jpg . If doublesided tape can hold car rubbing-strips.... never had any trouble with bars falling off, but requires scrupulous de-greasing, no fingerprints. Also shows the hinged sash version, with nice fat draughtstrip grooved into the sash not the frame, and wide non-jamming gap around the frame, just like Scandinavian-type windows.

So just one dg unit per sash. Someone on this forum said they hated that but I'm totally OK with it and only one client's ever objected, and she objected to everything so doesn't count! I wouldn't do without sash bars - and they must be correct, narrow, where appropriate. Being able to see 'through' them when you squint sideways I kind of like - subtly-visible evidence of 21st century priorities valuing and incorporating what's important from the past, rather than having to choose one or the other.

Yes Tom, stick on glazing bars can look the part from a distance but it's a fake, like repro-furniture. I kind of sympathise with yor client who doesn't count:)

Slimlite have a rather slim air space which means their U-value seem to, er, challenge the laws of physics.

I agree

Of course the difference between whole window and centre pane values depends on the size of the window. The bigger the window, the less important is the edge effect.

It's the U= 1.4 centre pane value for a 16.4 mm unit that intrigues me. I'd be interested to learn who measured this and how.

all you double glazers have a look at my suggestions under 'high quality secondary glazing'.