Vacuum Glazing for Listed Buildings: RBKC Retrofit Guide

Introduction: A New Window Policy for RBKC

The Royal Borough of Kensington and Chelsea (RBKC) has introduced a Local Listed Building Consent Order (LLBCO) to simplify the process for upgrading windows in Grade II listed buildings. This initiative reflects the borough’s commitment to balancing heritage preservation with improved energy performance, enabling property owners to adopt modern solutions without compromising architectural integrity.

Among the most promising innovations for window replacement in listed buildings is vacuum double glazing—a cutting-edge technology offering exceptional thermal and acoustic performance within a slim profile suitable for historic frames.

What is Vacuum Glazing and How Does it Work?

Think of vacuum glazing like a thermos flask for your windows. It consists of two thin panes of glass with a tiny vacuum layer in between, which prevents heat from escaping. As there is no air or gas between the panes, vacuum glazing is much slimmer than traditional double glazing, making it perfect for historic window frames.

How does it compare with standard double glazing?

✅ Lower Energy Bills – Keeps warmth in during winter and heat out during summer.

✅ Better Soundproofing – Reduces outside noise, making your home quieter.

✅ Preserves Historic Character – Can fit into existing window frames with minimal changes.

✅ Long-Lasting – More durable than traditional double glazing, with a lifespan of 50+ years.

Benefits: Thermal, Acoustic, and Heritage-Friendly

Durability and Longevity

Leading manufacturers like Fineo (https://www.fineoglass.eu/) and LandVac (https://www.landvac.net/) offer warranties of 20 years (with a predicted service life of 50-60 years) highlighting the material’s resistance to seal failure—a common issue in traditional double glazing. Advanced edge sealing technologies contribute to its extended lifespan. This compares with standard double glazing units which usually come with a 10-12 year warranty and a 20 year service life (Slim-line traditional double glazing generally offers a 5 years warranty and a predicted lifespan of only 10 years). Single glazing can last hundreds of years of course.

Weight Advantages

Vacuum glazing weighs significantly less than standard double-glazed units, which can exceed 20 kg/m². The reduced weight minimizes stress on historic timber frames, reducing the likelihood of frame warping or failure.

Fitting Considerations for Historic Frames

Most older windows were glazed from the outside with a fillet of window putty to seal the small panes of glass within the frame, the distance from the front of external face of the glass to the face of the glazing bars can be small, and when replacing single glazed panes with vacuum glazing the difference in thickness can be challenging, existing glass is normal 3-4mm thick and vacuum glazing starts around 7.7mm. Some windows may therefore not have enough depth to accommodate them without adjustment. However many windows are able simply to be re-glazed, and one check you can do is to measure the width of your glazing bars, if they are 22mm or wider (when viewed from the front) then it is likely that you can use the existing rebate in the frame to fit the glass.

Applied Glazing Bars

This does not sound good and is not automatically allowed under RBKC’s rules but we have found that in some instances it can be a good solution, here, rather than replacing individual panes in a single sash, for instance, one applies a single piece of glass and on top sticks glazing bars – sounds awful. But look at the picture below...most people can't tell the difference and the reason is that the glazing unit is so thin.

What the RBKC LLBCO Allows (and Doesn’t)

The LLBCO allows listed building owners to upgrade windows without requiring individual listed building consent, provided the replacements:

• Use materials and finishes consistent with the building’s character.
• Maintain the existing frame and external appearance.
• Ensure that replacement glazing does not alter window dimensions or profiles.

Vacuum glazing aligns with these requirements, offering a practical path to compliance while improving energy performance and occupant comfort.

Performance Comparison: Vacuum vs Standard Glazing

Property                        Single            Double                  Vacuum

U-value (W/m²·K)        ~5.5                1.1–1.5                  0.5–0.7

Thickness                    4–6mm          20–28mm            ~7.7mm

Sound Reduction (dB) ~25               ~32                       Up to 39

Weight                          Low               High                      Low

Lifespan (years)           ~100+         ~12-15                   ~50+ 

Visual Considerations: Dots, Dimensions, and Applied Bars

Dimensions: Sizes and Shapes

If we take Fineo Glazing as an example:

• Fineo 8 7.7mm thick 1.2m x 1.2m max dimensions (annealed glass)
• Fineo 10 9.7mm thick 1.375m x 2.500m (toughened glass) or 1.605m x 2.270m (toughened glass)

Shapes are available (e.g. a semicircular arch) but the glass must be flat, it cannot be curved in plan.

Dots spacing

Micropillars are provided between the two glass panes to keep the panes apart as there is a vacuum inside. The typical dot spacing for Fineo glass is 20mm, they look like black dots and can be seen when close up. Most people do not see them once a step or two away from the glass, but it is worth noting that on a sunny day they can cast a series of dot shadows across internal surfaces which some may find distracting.

The images below show how the dots fade away as one gets further away…

Vacuum point

It is also worth noting that where toughened glass is specified because of safety concerns then there will be a vacuum point which is a small hole in the corner of each pane. See picture at the end of this article

Acoustic performance Backed by data

Comparing the sound insulation performance of single glazing (29 dB) to vacuum double glazing (39 dB). This is why that 10 dB difference is really significant:

Decibels Aren’t Linear—Each 10 dB Cuts Noise by Half. The key point is that the decibel (dB) scale is logarithmic, not linear. A 10 dB increase in insulation cuts the amount of noise transmitted through the window by about 50%. So if a loud traffic noise or busy street sound outside is coming through a single-glazed window (29 dB reduction), replacing it with vacuum double glazing (39 dB reduction) would make the noise feel half as loud inside.

Real-World Example: From Noticeable to Barely There

Imagine you're in a room near a busy road or a train line:

• Single glazing (29 dB insulation) → You can clearly hear passing cars, conversations, and distant sirens.
• Vacuum double glazing (39 dB insulation) → These sounds are dramatically reduced, making it much easier to sleep, work, or relax.

This means that the extra 10 dB of insulation makes a major difference in comfort—turning disruptive noise into something much more manageable.

Why This Matters in Homes & Offices

In a home, this could mean the difference between being able to sleep through the night or being constantly disturbed. In an office, it could mean the difference between a quiet, focused workspace or one where background noise makes it hard to concentrate.

The Bottom Line

Going from 29 dB to 39 dB is not a small improvement—it’s transformative. It’s the difference between: still hearing external noise vs. barely noticing it. And the difference between muffled background sounds vs. true peace and quiet.  So while it might just look like "10 more dB," in reality, it halves the noise you hear and dramatically improves indoor comfort

Energy Impact: Real-World Example from Chelsea

We show a worked example (very rough but it gives you an idea). This house is in Chelsea. A very lovely example of domestic historic architecture (please see the photograph below). We have done some calculations:

• Window area (single glazing) : ~30.5% of the total envelope with a U-value of 5.5 W/m²K.
• Wall area (9 inch brickwork) : ~40.7% of the total envelope with a U-value of 1.84 W/m²K.
• Roof area (assumed insulated) : ~25.4% of the total envelope with a U-value of 0.3 W/m²K.

The overall U-value of the building envelope is approximately 2.50 W/m²K, heavily influenced by the poor insulation of single-pane windows.

Comparison of Overall U-Values

Current house with single-pane windows: Overall U-value: 2.59 W/m²K

House with vacuum double glazing applied to all windows: Overall U-value: 1.07 W/m²K

All other things being equal this means with a vacuum glazing upgrade this house would be around 59% cheaper to heat.

Key Takeaway:

Upgrading all windows to vacuum double glazing reduces the overall U-value by nearly 59%, dramatically improving the thermal performance of the house. This would result in:

• Significant reductions in heat loss through the building envelope.
• Lower energy bills for heating and cooling.
• Improved indoor comfort year-round.

Vacuum glazing makes the windows perform better than the walls, bringing the overall insulation of the house to a much more efficient level.

Conclusion and How Cagni Williams Can Help

Vacuum glazing represents a technically robust solution for improving the energy efficiency of listed buildings in RBKC while preserving their architectural character. With its slim profile, high thermal and acoustic performance, and long lifespan, vacuum glazing is uniquely suited to the challenges posed by historic window upgrades.

The introduction of RBKC’s LLBCO provides an ideal framework for homeowners to implement this technology without the administrative hurdles of individual listed building consent. As professionals experienced in both heritage conservation and energy efficiency, we are well-positioned to advise and support property owners in this process.

For more technical insights or to discuss your project, contact us at https://www.cagniwilliams.com/...

References and Research

• "Advances in Vacuum Glazing Technology," Applied Energy (2018) This peer-reviewed article explores the thermal and acoustic performance of vacuum glazing compared to traditional glazing systems, emphasizing its suitability for heritage applications.
• "Energy Efficiency in Historic Buildings," Historic England (2021) Guidance from Historic England supports the use of modern glazing technologies like vacuum glazing in listed buildings to improve energy performance while preserving heritage value.
• Fineo: Reports U-values of 0.7 W/m²·K and sound reduction up to 38 dB.
  
• LandVac: Reports U-values as low as 0.4 W/m²·K in optimized configurations, with extensive durability testing to ensure a 25-year warranty.
  
• "ImprovingThermal Efficiency in Traditional Buildings," SPAB (2020) The Society for the Protection of Ancient Buildings highlights vacuum glazing as a leading solution for energy efficiency improvements in conservation areas.