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Published on: October 23rd, 2019
Heritage Windows are replacement windows in Listed Buildings. Owners of listed building replace their windows with, as close as possible, like for like units. Owners of listed buildings are required by law to maintain as closely as possible the original look of their buildings. Heritage windows evolved in an effort to fulfil the criteria of giving modern day performance to old fashioned styled windows.
According to this Wikipedia page,:
A listed building, or listed structure, is one that has been placed on one of the four statutory lists maintained by Historic England in England, Historic Environment Scotland in Scotland, Cadw in Wales, and the Northern Ireland Environment Agency in Northern Ireland.
Essentially a listed building is a protected building. They cannot be altered, demolished or extended without the express permission of the planning authority local the where the building is located. The local planning authority may refer to the relevant central government agency. However this is usually only in the case of applications to buildings of special interest.
There are 3 types of listed buildings in the UK:
Currently there are around 500,000 listed buildings in the UK, though a listing could include more than one building.
Listed buildings fall under one or more of the following criteria:
Most properties are Grade II Listed, and it is unlikely that any contractor would be asked to work on a Grade I building.
Planning permission is required for replacement windows for Grade 2 listed buildings. Owners of such properties must seek the approval of their local planning authority.
Owners of listed properties may want or need to upgrade their windows. The current installations may be rotten, insecure or the owner may simply want a more comfortable, warmer home. In most older properties, the windows are very likely to be single glazed sash windows made from wood.
The main requirement for the replacement widows being installed in a listed building is that they retain the original aspect of the windows. Ideally, for the planning authority, this would mean the installation of like for like windows. Unfortunately for the building owner, this could mean the installation of cold, noisy single glazed units. But most people want more efficient double glazed windows.
The first criteria is that the window frames should look the same as the ones being removed. UPVC frames can be used but they us a wood effect laminate that looks imitation, even from a distance. There is also the issue of styling – it’s difficult to replicate the ornate nature of older windows.
Another problem is the thickness of the glass in the windows. Standard double glazing comprised of two panes of glass has a cavity of around 20mm between the two sheets. This means that the windows are quite bulky and there is a ‘double reflection’ from the 2 sheets of glass. This solution is deemed unsatisfactory by planning authorities. So how do manufacturers build a double glazed window that looks like it’s only single glazed?
Slim double glazing is an attempt to answer this question. The gap is narrowed to around 4mm. This reduced ‘sightline’ sounds like it’s a good idea. Unfortunately the amount of sealant needed to make the windows air tight for the duration of their life is more than the gap will allow. As a result, in order to pass the planning criteria, manufacturers are producing windows that have a much shorter life expectancy. Double glazed units are filled with heavy gasses such as Argon that make it difficult for heat to pass across. Obviously, if the gas escapes due to the poor sealant then the windows are no longer efficient.
Also, most old style windows are comprised of lots of small panes of glass held in place with thinner wooden frames. It’s these cross frames that cause some problems. The common solution is to add bars to the front and backs of the windows to replicate the olde worlde look. However the cavity in the double glazing emphasises the gap even more, making the units even less likely to pass planning authority criteria.
As the name suggests, vacuum glazing is a set of windows where the cavity between the two panes of glass is a vacuum. Heat and sound have great difficulty passing over a vacuum, which makes these windows very efficient. Whatsmore, the gap between the two panes of glass can be very, very narrow – as little as .2mm. This is because a vacuum is, well, a vacuum no matter how big or small – no matter how big the gap there is still the same amount of nothing there!
This very narrow gap gives the impression of a single glazed window, and currently heritage windows with vacuum double glazing stand a very good chance of being passed by your local planning authority conservation officer. There are few niggles – such as the ‘spacer’ dots that stop the two panes of glass from touching, and the small sealed hole where the air was extracted from the units. Despite these minor negatives, vacuum double glazing does of great job of modernising listed buildings with the appearance of single glazed windows.
Glass manufacturer Pilkington are the foremost supplier of Vacuum Glazing with their revolutionary product – Pilkington Spacia™. Gowercroft Joinery in Alfreton design and manufacture award winning heritage windows.