what you are talking about is a glazing bar. on a trad sash it has a core of about 8mm running the thickness of the frame 35,40 or 45. off that on either side is the moulding again another 8mm or so. giving your imperial inch in effect. on the georgians it can go down to 20mm. now, ordinarily a dg unit has an edge band of about 10-12mm in black, which is the silicon that forms the vacuum seal and shows through the glass. put that into a glazing bar and the black edge shows. not good. which is why you see new timber dg sash windows looking a lot chunkier than the others. for glazing bars on dg units i make up overlay beadings and stick these in the right place with silicon mastic. this goes onto an entire dg unit in the frame. from the street it looks the same as any other house. equally i bury the black edge into the wood of the frame by about facing the conventional cuts and making a putty finish out of the timber frame. a joy to paint and maintain. once an amateur has done a basic paint job, the windows i make will look just like they have been there for nigh on a 100 years.