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The Challenge of Replacing Heritage Windows

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The Challenge of Replacing Heritage Windows

Reading Time: 6 Minutes
Published on: May 30th, 2019

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Owners of Listed Buildings and searched long and hard for a viable update for their old single glazed units when replacing heritage windows.

The problem with traditional single glazing

When old window frames are beyond repair and it’s time to replace them, home owners want their replacement windows to closely match the looks of the previous units. But how do you add the benefits of modern, more efficient windows? Without doubt, modern units offer greatly improved levels of performance in sound proofing and insulation, not to mention durability and sustainability. All these benefits add up to creating a more pleasant living space.

Listed property owners who opt to simply replace their old single glazed units with like for like have to compromise. They reject the thermal and acoustic benefits of modern double glazing in favour of keeping the aesthetics of the property consistent. Some even go as far as to install secondary glazing, contradicting the whole point of maintaining the aesthetic integrity of the building.

Why most Modern double glazed units are unacceptable with replacement heritage windows

A modern double glazed window can achieve higher levels energy efficiency. However these factory fitted ‘standard’ units come with a number of problems when fitting them in listed buildings.

The ‘Double Shadow’ effect

The main problem is that the sheer width of the units – typically 16 – 20mm – creates a tell-tale double shadow. This is almost universally considered unacceptable for period projects and an alternative to these thicker profiles is required.

Low Sightline slim double glazing

‘Low sightline slim double glazing’ is a popular option. It aims to combine the the benefits of better thermal performance with the improved looks of a slimmer window.

The improved energy efficiency is achieved by filling the cavity with a more dense inert gas such as Krypton or Xenon. Standard double glazed units are generally filled with Argon. The reduction in thermal conductivity allows for a narrower gap between the two panes of glass.

The sightline – the area from the edge of the glass to the top of the spacer bar – can be reduced to as little as 5 – 6mm, allowing thinner window sections.

A less than perfect solution

Unfortunately, these window units are the focus of much debate within the industry, not least from the Glass and Glazing Federation (GGF).

To achieve the reduction in the sightline, window manufacturers are can reduce the amount of sealant and desiccant used in the perimeter of the units. This sometimes causes the units to be unstable and increases the likelihood of failure. Some manufacturers have even questioned as to whether these units even comply with the Construction Products Regulations (CPR).

In light of these ongoing problems, owners of listed properties are finding it a more challenging and complicated job than ever when choosing which replacement windows to install.

The New Alternative – ‘Vacuum Glazing’

There is increasing interest from the Heritage Sector in ‘vacuum glazing’. In this alternative option, all the air is extracted from between and pane of low emissivity (low-e) glass and a pane of clear float glass. With a vacuum between the two panes, literally nothing can pass between them, neither heat (or cold) or sound. The vacuum means that the efficiency of the units greatly exceeds that of the other options.

Vacuum Glazing was originally developed in Japan around 20 years ago as a lightweight, energy efficient solution for buildings in areas that suffered from earthquakes.

Pilkington SpaciaTM is the UK Option for Vacuum Glazing

Pilkington Spacia™ is the first commercially available vacuum glazing offered in the UK. It offers U-values of 1.1 on its standard units. These values can be reduced as low as 0.9W/m2K on their higher performance Spacia™ Cool units.

What is a U-value?

A U-value, or Thermal Transmittance, measures the transfer of heat through materials and structures. Measured in W/m2K , thermal transmittance take the effects of conduction, convection and heat loss. The lower the U-value, the better the insulation. You can read more about U-values here.

Pilkington Spacia™ is roughly a quarter of the thickness of a conventional double glazed unit, and about half of the thickness of a so-called slimline unit. The total thickness is just 6.2mm with the vacuum cavity being a mere 0.2mm). Spacia™ is approximately two thirds of the weight of a typical slimline unit. All this means that it is light enough and narrow enough to fit into most timber window frames, and without any discernible double reflection.

A minor flaw in the process

Of course, nothing can be perfect and vacuum glazing is no exception. In order to create the vacuum, a hole has to be made in the inner pane of glass. After the air has been extracted, the hole is plugged with a permanent 12mm plastic cap. This imperfection, located about 50mm from the edge of the pane, is considered by some to be a distraction, while others barely notice it.

Also, the units’ 0.25mm micro spacers that appear every 20mm to maintain a fixed distance between the two panes of glass are visible on close inspection.

Lower the carbon footprint

Whilst the units are imported from Japan, it can be argued that this more than offsets the pollution from the production of inert gasses that the majority of the window manufacturing industry creates, making vacuum glazing arguably the more ecologically sound choice.

Heritage Projects are embracing Vacuum Glazing

Period home owners and specialist joinery companies are starting to use vacuum glazing, despite its minor imperfections.

It certainly does help create traditional looking windows with the benefits of state-of-the-art modern performance for those that want to preserve the original joinery of the property as much as possible.

Gowercroft and our use of Pilkington Spacia™

Gowercroft is one of a handful of window manufacturers who have incorporated Spacia™ into its range. It’s available in our Winston Sliding Sash windows and Richmond Casement windows. We build the units into Accoya™ timber frames designed to maintain the aesthetic integrity of heritage properties.

Why we use Accoya™

Accoya™ is a modern modified timber. It has been treated at a cellular level to increase durability, stability and longevity. It is then coated with a unique formulation of protective paint that guarantees zero maintenance for up to 10 years.
You can read more on our Accoya Windows here.

Overcoming manufacturing challenges to offer improved looks and performance

It is a challenge for any manufacturer to combine traditional looks with modern day performance. Great lengths are taken by Gowercroft to ensure that the overall aesthetic that conservationists are striving to preserve is not ruined by modern manufacturing techniques.

For example, we have replaced the putty used to secure the glass with the outer profile. This is more ‘regular’ looking than traditional putty, and significantly improves security and coating longevity. Other modern seals designed to keep the windows resistant to the weather are neatly concealed within the joinery.

In conclusion – vacuum glazing is a viable option

Converting any heritage property into a modern living space involves some level of compromise. We feel that we offer owners of period properties a viable option to maintain, as far as possible, the aesthetic integrity of their homes. We also strongly feel that benefits of using our products – length of life, lack of maintenance, quality of build – far outweigh the somewhat minor imperfections that vacuum glazed units have.

Types of Glazing Compared

Single Glazing

4-8mm thick glass that can be float, crown or cylinder depending on the production method.


  • Traditional, with no double reflection
  • Inexpensive
  • Long Lifespan


  • Poor energy efficiency
  • Poor acoustic performance

Double Glazing

Two sheets of 4-6mm float glass separated by a spacer bar. Space between the panes (cavity) typically 16-20mm and filled with inexpensive inert gas


  • Inexpensive
  • Moderate Lifespan
  • Good energy efficiency


  • Not traditional with pronounced double reflection
  • Heavier and thicker typically 24mm requires larger frame sections

Slim Double Glazing

Two sheets of float glass separated by a spacer bar. Cavity between glass panes is reduced to 4-6mm and filled with a more expensive inert glass


  • Limited double reflection
  • Moderate Energy Efficiency


  • Poor lifespan
  • Expensive
  • Limited Unit size

Vacuum Cavity Glazing

An inner pane of clear float glass and an outer pane of low-e glass separated by microdots. Cavity between panes is reduced to 0.2mm and air is extracted to give a vacuum


  • No discernible double reflection
  • Good energy efficiency
  • Better sound insulation
  • Long Lifespan
  • Ultra thin glazing: Pilkington Spacia™ is 6.2mm


  • Plug on face of unit and tiny microdots within glass
  • Expensive

You can find out more about the Gowercroft range of Heritage windows here. Visit the Pilkington Spacia™ page here.

Vacuum Glazing Case Study: Harvington Hall

Gowercroft used Pilkington Spacia™ in Harvington Hall Farmhouse, a Grade II listed property dating from the early 17th century. We fitted our Richmond Flush Casement windows that offered modern performance without compromising the unique looks of this unique heritage property. Read the full article here.

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Adapted from an article in Listed Heritage Magazine, membership journal of the Listed Property Owners’ Club

Read the original article here