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Why Vacuum Glass is the best option for Heritage Sash Windows

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vacuum glass installed in heritage sash window

Why Vacuum Glass is the best option for Heritage Sash Windows

Reading Time: 4 Minutes
Published on: February 4th, 2021

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How Vacuum Glass improves sash windows

Sash windows make an excellent choice. Their characteristics and charm add a traditional air to any property. Especially suitable for Listed Buildings and properties within conservation areas, sashed windows look old fashioned on purpose. However, a well-made sash window can deliver all the benefits of the flush casement window with the Georgian or Victorian style.

One of the major improvements to sash windows is the use of vacuum glass. This is a true meeting of the latest technology in glazing with the traditional aesthetics of Georgian Windows.

What is Vacuum Glass?

Vacuum glass is double glazing, but not as we know it (Jim). Standard double glazing is, as we all know, a unit consisting of two panes of glass separated by a cavity. This cavity is filled with either a noble gas, usually argon, or just plain air in cheaper units. Argon in a poor heat conductor, so the warmth inside a room cannot pass through the window very easily. Whilst a vast improvement in efficiency over single glazed windows, standard double glazing is fairly chunky, with a gap of anything up to 26mm separating the two panes of glass. 

The trouble is, they just aren’t that pretty. That wide gap means that older properties – the aforementioned Listed Buildings – are excluded from having double glazing. This is because the modern units don’t look like the single glazed items that would have been originally installed with an obvious “double reflection” from the street and thicker window sections.

The problem gets even worse when triple glazed windows are installed; though more efficient, these units are thicker – 38-48mm – and heavier, making them highly unsuitable for most sash window properties. 

Vacuum Glass is narrower, lighter and more efficient

Vacuum glass has all the air extracted from the cavity between the two panes. The vacuum created within the unit prevents all almost all transfers except for radiation (and Jean Luc Picard), so vacuum glass is incredibly efficient. Sound waves are also stopped because there are no particles to transfer the sound across the vacuum.

While standard double glazing needs more Argon to be more efficient, vacuum glass doesn’t. It’s a vacuum – you can’t have more of nothing (except with bank statements). It’s hard to explain, but a small vacuum – a small amount of nothing – performs in exactly the same way as a big vacuum. So the tiny amount of nothing in a vacuum glass unit has the same heat and sound efficiency as the vastness of space. The result is a gap of only 0.3mm. Couple that with a good quality low-E glass (which reduces radiation across the cavity) and you have a window that is both highly efficient and at the same time very narrow and lightweight.

Note: Vacuum Glass was initially developed for earthquake zones in Japan to make buildings lighter and therefore, safer.

Here’s a quick comparison between the three types of glazing:


Double GlazingTriple GlazingVacuum Glazing
Overall Thickness28mm38 – 48mm8.3mm
U-Value1.0 – 1.90.7 – 1.00.44 – 0.48
Acousting Reduction (Rw)21 – 24dB21 – 34dB36dB
Estimated Lifespan10 – 25 years10 – 25 years25+ years

It’s hard not to say that vacuum glass is a clear winner here, apologies for the obvious dad joke there.


Diagram comparing types of double glazing showing that vacuum glass is better for heritage sash windows

Why Vacuum glass is better for sash windows

So now I have explained the difference between standard and vacuum glazing, here’s why vacuum glass is the perfect choice for sashed windows:

1 – Weight – vacuum glass is often lighter than standard glass

Because sash windows go up and down you have to physically lift the weight of the window. Of course, there is a weighted or spring assisted pulley system to make this much easier – more on how sash windows work in this article. However, a lighter window will quite simply be even easier to operate.

2 – Efficiency – your house will be warmer and quieter

As already explained, vacuum glass is much more efficient than standard glazing, so your house will be warmer and your heating bills will be lower. The improved sound proofing offered by the vacuum units is also another benefit for those living in cities or near busy roads.

3 – Your windows will last longer and cost less

With an expected glass lifespan of over 25 years – your windows will, in the long run, be much cheaper than those with standard units as you won’t have to replace them as often. Of course, there are other factors to consider such as the material the frames are made of, and the build quality. But as vacuum units have mechanical seals they are less likely to malfunction that standard glazing, which have semi-porous seals.

4 – Your windows will last longer and benefit the environment

Another major benefit from having windows that last much longer that normal units is the positive impact on the environment. When incorporated into a Gowercroft sliding sash window, a relatively small product with vacuum glass unit will removes the equivalent of over 10,000KgCO2 from the atmosphere over the whole manufacture, installation, maintenance and end of life cycle.

5 – The narrower profiles just look better

With less of the frame on view, sash windows with vacuum units are prettier and more traditional in their appearance than standard double glazing. 

6 – Better for listed buildings

The narrow profile means that the units are more likely to be approved by planning officers for use in Listed Buildings and in Conservation Areas where the original windows would have been single glazed sash windows. These are commonly called Heritage Windows. Gowercroft have an award-winning range of heritage windows here.

There you have it – 6 great reasons why vacuum glass is ideal for installation in sashed windows.