Application of Vacuum Double Glazing in the Upgrade of Windows in Listed Buildings
Installing Vacuum Double Glazing in heritage windows is a delicate balance.
The Government has set out strong targets for reducing Carbon Emissions and the impact on our environment. The owners of the historic buildings themselves generally have a strong desire to make their homes more energy efficient – to protect the environment and save money on heating bills.
At the same time, there is also a strong desire to retain and maintain our historic built environment, for the purposes of this article the windows, which are notoriously energy in-efficient.
The problems with single and double glazing in a historic building
Installing Double Glazing historic windows in a listed property is often met with flat denial by local planning officers. The owner faces a difficult and expensive journey to try and achieve what other homeowners see as perfectly normal.
To add to the dilemma, the internal condensation is among the greatest causes of timber window rot and decay. The windows by their very nature are speeding up the rate of decay.
Double Glazing does not look authentic in a listed building
It largely comes down to appearance. It is the duty of the local planning officers to make sure listed buildings and those in Conservation areas keep their original appearance as much as possible. Historic England have made it clear that repair is always preferable to replacement in this report.
Original glass has certain imperfections which give the characteristic look. Modern glass is flat and reflects differently. This is exaggerated with multi-pane Georgian style windows having lots of small panes, all at very slightly different angles.
So how do the owners of historic buildings improve the performance of their windows while keeping their original appearance?
Slim Double Glazing was almost the answer…
Years ago the glazing industry saw the commercial opportunity and developed slim double glazing units that would ‘fit’ into the historic glazing rebates. These have been made ever slimmer with the heat insulation performance boosted by the use of glass coatings and gas filling such as Argon and Krypton. Sadly whilst these have been well adopted the failure rate is alarmingly high.
The width of these units mean that glazing bars are normally stubbier so less well defined. And the familiar ‘double reflection’ from the glass is pronounced. So not good news for the period windows.
Vacuum Double Glazing
So what of Vacuum Insulated Glazing (VIG)? This method of constructing a double glazed unit is technically very different. Two panes of glass, separated by a tiny space where the air has been evacuated to create a vacuum. The same principle as the vacuum flask we are all familiar with. The overall thickness of VIG is typically 8mm but with energy saving values approaching those of triple glazing.
The benefit of two panes so close together is they effectively act as a single pane, so no double reflection. The reduction of width means the glazing bar profiles can be near back to original dimensions.
Using the Toughened LandVac VIG has an additional benefit. The toughening process of glass takes away the ‘flatness’. It is a happy by-product that toughened glass looks more like the ‘imperfect’ original glass.
So is Vacuum Double Glazing the answer?
For larger pane windows, typically sliding sash windows with 1-over-1 or 2-over-2 style then retro-fitting with VIG is definitely a great step forward.
For the small pane Georgian windows then the only way would be to use a larger single pane and applied timber glazing bars to create the multi-pane effect. The advantage is the glazing bars can be as fine as you like (typically 16mm) and it will be thermally more efficient.
We are sure that vacuum double glazing is the best option when upgrading heritage windows. You can read more about Thin Double Glazing in Listed Buildings in this post.