double glazed windows glazing bar thickness

Part of an interesting debate on double glazed windows on historic properties. he full deabte is here.

Biff

posted on 05-11-06

...it's obvious which windows are double glazed and which single. But the real issue over maintaining the architectural heritage of British townscapes is the width of glazing bars. Through much of the 18th and 19th centuries these were tpically about 20mm, sometimes even slimmer. For a standard insulated glazed unit, a rebate of about 12mm is requires to hide the seal, both for aesthetic reasons and to protect the seal from uv-light. A 12mm rebate either side of the glazing bar plus 6mm of timber between the units adds up to about 30mm for the width of the bar. This really is a practical minimum and many manufacturers make the bars nearer 40mm. While this might not look out of place in a Queen Anne or early 18th century building it really makes a difference to the appearance of many later buildings. Applying thin fake glazing bars to the outside surfaces of a large continuous igu, with corresponding bars inserted between the panes, gives a semblance of the traditional pattern but is, nevertheless, a fake and does little for the respect of historic fabric. The glue used to bond the bars to the glass probably has a limited life. The appearance of windows replaced by modern units is further compromised by the ubiquitous use of float glass. Its optically flat and parallel surfaces gives a dull and lifeless character, quite different from the movement created by cylinder or crown glass. Crown glass, in particular, is completely irreplaceable. The skills employed in making it have been lost, perhaps permanently. To remove a piece of crown glass and replace it with float is, in my opinion, akin to throwing away a piece of antique furniture or old masters painting and replacing them with self assembly flatpack furniture and prints from IKEA. Hand made, mouth blown, cylinder glass is still made at a couple of factories in Europe but if folk knew the cost they might be less ready to break their old windows. With the growing awareness that global warming is the greatest threat that humanity has ever faced, there is obvious pressure to throw out anything that is thought to be energy profligate. However, that our irreplaceable historic heritage should be the first victim of our effort to live sustainably, is a tradgedy that cannot be repaired. There are other more energy efficient routes to be travelled first. I would recommend Richard Heinberg's book, Powerdown, and particularly the chapter, Building Lifeboats - The Path of Community Solidarity and Preservation, as a particularly pertinent contribution to the debate. Let us build sustainably, and let us convert our existing housing stock to be fit for sustainable living, but let us not destroy our heritage and the beauty that our forefathers have left us. It was not the 18th century lifestyle not resulted in global warming.