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Double Glazing

Double-Glazed with Clear Glass, Efficient Windows Collaborative, University of Minnesota

Double glazing has the advantage over single glazing in reducing down-draughts which are noticeable if sitting near a window in cold weather, and thermal insulation is improved. However, double glazing is really only effective if the gap between the two sheets of glass is 20mm — in most sealed units the gap is only 6mm. Sound insulation can be improved but in this case the gap needs to be 100-150mm. If the glass is too thin, the vacuum between the two sheets of glass causes deformation, which is clearly seen in reflections from each sheet; there can be a convex image superimposed on a concave image. Also, double glazing will always result in a double reflection.

Sealed units are expected to last 10 years. When breakdown occurs at the edge seal, moist air leaks into the cavity, and condensation occurs, causing misting of the glass and obscuring the view. Inexpert installation can cause the best quality sealed units to degrade more quickly. There are only two methods of installation approved by the Glass and Glazing Federation and which are in accordance with British Standards; neither of these involves the use of putty. From this it follows that, on this simple technical point alone, double glazing and existing sash windows do not go together.

A further technical problem lies in the fact that light also causes the edge seal of a sealed unit to degrade. It is recommended that the timber beads holding the glass in place should be at least 18mm wide; this leads to an overall glazing bar width of about 44mm, whereas most glazing bars since the 18th century fall into the 16-22mm range. Clearly this width might suit very early buildings, but is completely unsuitable otherwise.

Where it is absolutely essential, for whatever reasons, to employ double glazing (as opposed to secondary glazing) in an historic window with glazing bars, the least obtrusive solution is to use one of the proprietary window systems in which the sashes are fitted with false glazing bars on each side of the sealed glazing unit. The false glazing bars should follow the profile of the originals. To avoid the concave/convex double reflection mentioned earlier it is possible to introduce spacers within the sealed unit which align with the bars. However, some installers have found that windows exposed to strong sunlight can shatter where spacers have been fitted as differential expansion causes tension within the glass.

Wrightson, David. The Conservation and Renewal of Timber Windows, Public Information Leaflet, Cathedral Communications, Wiltshire, England [PDF file]


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