If you’re looking to update or retrofit your heritage property, you may be focused on the big-ticket items like heating, insulation or roof repairs. While tackling these can certainly help, there’s another aspect that’s often overlooked – and that’s the glazing.
As window materials and technology continue to progress, vacuum glass now offers a compelling option for heritage properties. I’ve followed the evolution of early vacuum glazing systems (and I still have an original sample from Pilkington in the office somewhere!), which have come on in leaps and bounds. But what makes vacuum glass worth considering for older properties? Here are a few reasons based on some of the listed building projects we’ve worked on:
Energy efficiency
In terms of its construction, vacuum-glazed units resemble double glazing but replace the air or gas cavity with a near-vacuum. With almost no medium for heat conduction, they provide exceptional thermal insulation, sharply reducing heat loss in winter. This, in turn, can significantly reduce energy bills. Similarly, in the summer months, the same low-transfer design limits summer heat gain and helps prevent overheating.
The thermal image above is from one of our Grade II-listed projects in Bath – a Jacobean Revival style country house. The image shows the dramatic heat loss difference between historic single pane glazing on the upper floors and the two windows with vacuum glass installed on the ground floor. The brighter the yellow and orange, the hotter the temperature – meaning more heat loss.
Aesthetics
In vacuum glass, the gap between the panes can be small, so the glass can be thinner than traditional double glazing – which makes it a great candidate for historic properties. It can also be manufactured to fit unusual historic window designs and fitted into original sash woodwork. At the Grade I listed Francis Hotel in Bath, one of several of our projects to have the latest vacuum glass approved and installed, the newly glazed windows still have their original sash cords and wheels in place. This means they retain their Georgian charm while performing much better from a sustainability perspective.
Ventilation
Single-pane glazing is prone to interior condensation and many slim double units block moisture migration. In contrast, vacuum glazing keeps the inner pane warmer and reduces cold spots, helping to lower the risk of interior condensation — a key benefit in older buildings vulnerable to damp and timber decay.
Planning permission
There are multiple factors involved in securing permission to update glazing in a heritage property to vacuum glass. While it’s normally more straightforward if a window is considered neither sensitive nor significant, other factors such as glazing bars and the listing status can affect the chances of securing planning permission and listed building consent. Based on many years of experience, we can advise on the likelihood of achieving permission to have glazing replaced in a listed property: get in touch for advice.
Acoustics
As well as keeping the warmth in, vacuum glazing does a good job of keeping the sound out. And with traffic on the rise in most cities, including Bath, the acoustic benefits of vacuum glazing shouldn’t be overlooked. At a Grade II listed property in River’s Street – a residential Georgian terrace near Bath’s city centre – our retrofit project included installing new vacuum glazing. It has considerably improved the sound-proofing of on-street noise for our client.
Recycling
While double glazing isn’t normally permitted in heritage properties, vacuum glass is designed to last for decades and, being free of complex coatings, can be recycled at the end of its life.
Other considerations
While there are many positives to using vacuum glass and the technology is continually improving, it’s not yet a completely flawless solution. Upfront costs, for example, can put off some heritage property owners – although we would suggest considering this in the context of the expected energy savings over the lifetime of the windows. And at some angles, the bridging spacers between the panes can be visible. In the image below, these are reflecting in the sunshine onto the window cill.
Also, vacuum glazing can cause condensation on the outside of the glass, which you can see below in a pattern around the spacers on a cold day. While this is an unusual look, it’s actually a sign of energy efficiency: the outer surface of the glass is cold because the glass isn’t letting heat out. Especially for older properties, it’s much better to have the condensation on the outside than on the inside of the glass.
So in many situations, vacuum glass could be a much better alternative for heritage properties than options such as single panes or secondary glazing.
We’re pleased to have secured permission for and fitted a number of projects with vacuum glazing, with our clients enjoying the benefits they bring.
- Kevin Balch, Director of Heritage and Reuse, Nash Partnership
