Gowercroft Director Andrew Madge shares why slim double glazing units may not be the best choice for heritage projects and offers a reliable alternative.
What Is Slim Double Glazing?
Slim double glazing refers to insulated glass units (IGUs) that are 14mm thick or less. A typical setup includes two 4mm panes with a 4mm cavity in between.
It’s often used in listed and heritage buildings as a compromise. Conservation officers prefer it because the reduced thickness minimises double reflection, preserving the traditional aesthetic. At the same time, homeowners are drawn to its improved energy performance over single glazing.
However, this compromise isn’t always a good one.
What Are the Problems with Slim Double Glazing?
By law, all IGUs in the UK and Europe must meet EN 1279 standards to ensure durability. This includes:
Most slim units use a 3mm ‘warm edge’ spacer bar. To meet conservation requirements, manufacturers often reduce the sealant depth to just 2mm to achieve a minimal 5mm sightline. Unfortunately, this drastically reduces the unit’s ability to retain gas and keep out moisture.
How Does This Affect Performance and Guarantees?
Many glass suppliers still offer guarantees of 5 to 10 years without clarifying that those warranties only apply to standard sightlines (8–10mm). Architects and conservation officers may unknowingly specify units that cannot meet those guarantees when installed in narrow glazing bars.
Some companies even provide certificates stating that their products have passed EN 1279 Part 2 and 3. But the tests were often conducted on thicker, standard units rather than slim double glazing with reduced sightlines. This lack of clarity causes issues for window manufacturers like us.
Here’s why:
That leaves us facing a tough choice. We either break the slim glazing specification or walk away from the project. In either case, the customer ends up with windows that are either compromised or unavailable.
Is There Evidence That Slim Units Underperform?
Yes. Studies such as the Thermal Performance Report: Double Glazing in Listed Buildings (Changeworks for Historic Scotland, March 2010) show that slim units prioritise aesthetics over performance. Reduced gas retention means the insulating performance degrades early, often within just a few years.
The Glass and Glazing Federation (GGF) has also issued a formal warning. They are taking action against one supplier and have stated they will remove any member offering non-compliant slim units.
What’s the Better Alternative to Slim Double Glazing?
At Gowercroft, we often work on listed and heritage properties. Clients regularly request slim double glazing, but we simply cannot justify offering a product we know won’t last.
Instead, we found a better solution: LandVac™ vacuum glazing.
After testing it extensively — including on a major project in Richmond — we committed to using LandVac™ across our heritage range. This technology combines a traditional appearance with outstanding performance.
To learn more, see the products we developed using LandVac, see our Heritage Range.
This range won the Best Windows award at the Build It Awards 2017. They continue to outperform slim units across all key areas: appearance, energy efficiency and durability. It has also won multiple awards since then.
Interested in Vacuum Glazing for Your Heritage Project?
If you’re planning a restoration or listed building project and want to see the difference for yourself, we’d be happy to help. We offer on-site demonstrations and factory tours at our Derbyshire workshop.
You can also explore our full Guide to Replacing Windows in Listed Buildings for more technical advice.
Contact us today to discuss your project and discover how we can help preserve history with better windows.