Period windows and double-glazing

[Flyfisher]

If I have an old (Georgian-like) window frame that is really beyond repair, is it possible to have a replacement made that can accomodate double-glazed units? I don't mean overlaying wood onto a large double-glazing unit to make it look OK (as if!), I mean a like-for-like replacement except for being slightly deeper to accommodate multiple DG units.

I've read that it can't be done because of the thinness of the wood between the glass panels (not sure what these bits are called ), but I don't really understand why this should be.

I do realise that any new glass wouldn't have the same character as old glass though. Are there any companies that make crown glass anymore, and shouldn't it be possible to make a DG unit from such glass?

Or is the thought of adding DG to a period property too heinous a sin to even consider - even if it looked the same as the original window? Surely it must be more acceptable than that dreadful secondary stuff?

 

[Nemesis]

No you can't buy crown glass, you can buy cylinder glass, yes you can get DG with it BUT you can't fit it in thin bars as the DG edges have to be buried or they degrade.

If you have existing historic joinery - hang on to it.

It's precious. Repair it.

 

[Flyfisher]

Thanks for that; I agree about the preference to repair where possible.

A couple of other questions:

Does cylinder glass have a similar 'look' to crown glass?

I understand that fitting DG into existing old windows might be a problem, but if a new window is being made (or at least just the part holding the glass) could the bars be made to be deeper (to accommodate the DG edges) whilst retaining their original widths? In other words, the X & Y dimensions would be identical but the Z dimension would be slightly larger (if I remember my technical drawing lessons correctly :wink: )

 

[biffvernon]

No, you need a bigger X dimension as well. The problem is that the sealed unit has a spacer bar and glue round the edge. This needs to be covered up by a rebate in the timber glazing bar so that you can't see it and so that the sunlight doesn't degrade the glue. The rebate needs to be at least 12mm. A rebate on each side of the glazing bar plus, say 6mm for the part of the bar that goes through from front to back means that the bar has a width of 30mm. That really is a minimum. I prefer to make them about 32 or 33mm.

32mm is noticeably slimmer than many of the profiles found on the mass produced softwood windows, which are typically nearer 40mm, but is still a good deal thicker than the 20mm typical of late 18th and 19th century windows. On the other hand, many old windows have so much paint haphazardly sloshed on them that the effective glazing bar width is a lot greater than the wood. Early 18th century glazing bars, a feature of the Queen Anne period, were much thicker, often well over 30mm.

Painting the windows in anything other than white, especially the very dark green that was often used in the 19th century, has a remarkable effect. The woodwork seems to shrink away.

Cylinder glass and crown glass can be distinguished by the patterns of their imperfection, crown glass having distortions running in concentric arcs, the sheet having been cut from a disc perhaps a yard or more in diameter. Cylinder glass has a more haphazard pattern of distortions. Very high quality hand made glass is almost distortion free so the two types are difficult to tell apart.

No crown glass is made anymore but mouth blown cylinder glass is still made in Germany, France and Poland. It's four or five times the price of float glass but does look beautiful.

My own view is that once an old window has really reached the end of it's life and can no longer be repaired then one should seriously consider replacement with a modern well designed window rather than a reproduction. You won't want to unbalance the appearance of a facade but in many situations, double glazing can sit happily in an old building, along side the garage, flushing toilet, electric lighting and other non-original features.

 

[Flyfisher]

No, you need a bigger X dimension as well. The problem is that the sealed unit has a spacer bar and glue round the edge. This needs to be covered up by a rebate in the timber glazing bar so that you can't see it and so that the sunlight doesn't degrade the glue. The rebate needs to be at least 12mm.

Yes, of course; I'd forgotten about the width of the DG spacer

Painting the windows in anything other than white, especially the very dark green that was often used in the 19th century, has a remarkable effect. The woodwork seems to shrink away.

Our windows are all painted black, so maybe that would help, visually at least.

My own view is that once an old window has really reached the end of it's life and can no longer be repaired then one should seriously consider replacement with a modern well designed window rather than a reproduction.

Sounds eminently sensible. Do the COs generally take the same view?

 

[Nemesis]

No, they have often the view where listed buildings are concerned that you replace like for like.

Windows are such an important part of a building's facade, and messing about with proportions can greatly affect that.

http://www.gov.je/PlanningEnvironment/Planning/Historic+Buildings/History+of+Jersey+Windows+and+Doors/

 

[biffvernon]

From the Jersey link that Nemesis posted, a nice example of glazing bars that are easily wide enough to cope with double glazing (though I wouldn't paint them white):

One of the earliest sash windows in St Helier at Hue Street - the building is dated 1756. Note the very wide and flat glazing bars, typical of this early style

 

[Nemesis]

I think that the point is made that it is an early example.

Joinery details are important in a period building, and changing them thoughtlessly can rob it of much historic interest.

 

[Penners]

double glazing can sit happily in an old building, along side the garage, flushing toilet, electric lighting and other non-original features.

Thanks, but I think I'll do without the window sitting alongside my flushing toilet. :shock:

 

[plumbers mate]

I was Ventrollad last week and where they had to replace a couple of panes (been broken for years) they used eastern European glass.
It looks Ok ----- well actually it looks odd because it's clean and the rest of the windows aren't but that's nothing to do with the type of glass, more my slothefulness! ....