Double Glazing

Many people prefer double glazing to secondary glazing because the end result is simpler and easier to use. Like secondary glazing, new double glazed windows will reduce heat loss through the window by over 50% and will all but eliminate the draughts.

With traditional homes however, the downside can be the loss of the existing windows, particularly if the replacement frames are uPVC.  If you are replacing timber windows, timber is the obvious choice for the replacement. There are a number of alternatives to uPVC, starting with the least expensive:

  • standard timber double-glazed replacement windows
  • slim-profile timber double-glazed replacement windows
  • slim-profile double-glazed panes installed in original window frames

In all traditional buildings in Cheltenham, timber windows will be more in keeping with the building – and probably the street – a good quality product, with proper maintenance is likely to last much longer than plastic windows. Timber windows in Cheltenham have survived for over two centuries.

Timber is also an environmentally-friendly material which locks up carbon for generations, whereas plastic windows are made from non-renewable petroleum products and generate environmental pollution during their manufacture and disposal. The very best ecological choice is a window made from timber from a forest that is certified as being well-managed (look for Forest Stewardship Council (FSC) certification).

There are many different designs of timber double-glazed window to choose from. You should match the glazing pattern of the original windows as far as possible, especially for prominent street-facing windows. If your windows are not original, this may be a good opportunity to return the windows to their original pattern. This requires particular care in Cheltenham, with window design varying across the town from the small paned (eight-over-eight) sash windows found in older Georgian buildings to the larger paned sash windows (often with margin lights) found in the many later Regency buildings in the town.

Most double glazing has a deep gap (up to 24mm) between the two panes of glass. However, this requires deeper glazing bars than those of traditional windows. Slim-profile double glazing has a much smaller gap between the panes, typically 3-6mm deep. Although this is slightly less effective in reducing heat loss, it allows the exact dimensions of the original glazing bars to be retained. However, new slim-profile double-glazed timber windows are likely to weigh more than the single-glazed windows they are replacing. If this is the case, and you have sash windows, you may need to replace the weights as well the windows.

In some cases, slim-profile double glazing units can also be used to replace the individual panes of glass in sash windows. This is not straightforward, as the sashes have to be removed and taken to a workshop for the panes to be replaced, but it ensures that the original windows are preserved for another generation. It is even possible to specify glass for the outer panes that has the appearance of traditional crown glass. However some original windows may have such narrow glazing bars that even this option may not be practical.

For any double glazing, specify the following:

  • a low-e (low emissivity) coating facing outwards on the inner pane to reduce radiative heat loss
  • a gas fill (argon or ideally krypton) between the panes to reduce convective heat loss between the panes
  • a ‘warm edge’ (the material between the panes) to reduce conductive heat loss.