I recently read a very interesting paper by Natasha L Ginks from De Montfort University’s Institute of Energy & Sustainable Development looking at the attitudes of conservation bodies in the UK to slim double glazing (SPDG) in listed buildings. One of the most striking findings was the difference between the planning authorities in England and Scotland. The Scottish authorities seem much more comfortable with the replacement of single glazed units with slim double glazed units on the grounds of the improved thermal efficiency and that the retrofitting of units, in fact, prolongs the life of the original window.
English authorities have noted that in their eyes the aesthetics of the buildings will take priority over the thermal efficiency and suggest the use of shutters, secondary glazing and heavy curtains as a means of addressing this. The replacement of historic glass with modern float glass, the double register on reflections and the need to use thicker glazing bars to accommodate the double glazed units, have all been mentioned as reasons for this resistance to accept either the retrofitting of these units in existing windows or their inclusion in replacements.
It’s reassuring to know that conservation officers take the preservation of our historic housing stock seriously and I would whole heartedly agree that there are many situations where slim glazed units will never be the answer, however this has to be balanced with protecting these buildings through use. One argument put forward is that “the additional cost of the slim units will not be recovered by the improvements in thermal efficiency” which misses the point for many people living in these buildings and that is the ‘comfort factor’. Many people who choose to live in these buildings share the conviction of retaining their original character but need a certain level of thermal comfort, which slim glazed units will offer.
The challenge that needs to be met by the glass and window manufacturers is how to address these concerns, but progress is being made. More manufacturers are now offering longer performance guarantees on the units, development in spacer bar technology is allowing 22mm glazing bar construction and the use of Crown glass which, more closely mimics the waviness of historic glass, is available but seldom specified when giving guidance on planning applications.