According to the This is Bath, a local Grade I Listed property is set to get new double glazing.
The St John’s Hospital charity reckons its city centre almshouses will be among the first grade one historic listed buildings in England to be fitted with double glazing.
As with all historic properties, it is hard to make them energy efficient as they have solid walls and glazing is such an important feature. Approved by Bath and North East Somerset Council and backed by the Bath Preservation Trust.
It seems the new timber sash windows will replace Victorian timber sash windows, back to more appropriate Georgian styled sash windows, using 11mm sealed units which is reported to,
cut out the kind of reflection that was a feature of modern glazing.
Pity nobody has explained these slimmer sealed units are also made of 4mm Low Emissivity reflective glass, (either float glass or reproduction crown sheet), gas filled and warm edge technology spacer bars, as other, slightly larger sealed units. Still, if “double glazing” is accepted in this Grade I Listed property, then it provides hope for many owners of listed properties who wish to improve the thermal performance of their properties, whilst retaining their character and those living in conservation areas.
According to slimlite a company that specialises in ultra slim sealed units the optimum cavity for krypton or Xenon gas filled double glazed sealed units is 6-8mm, ie a 3mm cavity has a U value of 2.1, 4mm=1.9, 5mm=1.7, 6mm=1.5.
Of course in giving permission for these new windows, the council would wish to avoid setting a precedent, so they point out that these modern double glazed replacements sash windows are replacing the Victorian replacements, and that it is an entirely different situation to replacing the Georgian originals, because there’s no historic glass involved.
Had the windows been of historic glass then different criteria would have applied. As with any grade one listed building, consideration has to be on a case by case basis.”
So the message is clear, buy a property that the Victorians knocked about a bit, because you can probably install new double glazing, just avoid buying one that still has original sash windows because you’re unlikely to have permission to improve its thermal efficiency!