By The Windows and Doors Team
Your Local Glazing & Home Improvement Experts
Reading Time: 8 minutes
Quick Summary
For UK professionals, specifying double glazing thickness is a nuanced decision that impacts a building’s thermal, acoustic, and structural performance.
This guide provides a detailed breakdown of the components, their performance implications, and a practical method for measurement.
Moving beyond consumer-level understanding to provide actionable insights for your next project.
For an architect or specifier, double glazing thickness isn’t a simple measurement.
It’s a critical variable that dictates a unit’s thermal efficiency, acoustic insulation, and compatibility with the wider window frame system.
Making the right choice is a direct path to achieving significant energy savings, reducing heat loss, and ensuring long-term occupant comfort.
This comprehensive guide moves past the basics, providing a detailed breakdown of what glazing thickness truly means for a building’s performance.
From new builds to refurbishment projects replacing old single glazed or double glazed windows.
The UK’s commitment to achieving net-zero carbon emissions by 2050 has placed unprecedented pressure on the building and construction sectors.
A key driver of this change is the uplift to Part L of the Building Regulations, which sets new, more stringent performance targets for the thermal envelope.
To put this into perspective, the target U-value for windows in new dwellings has tightened significantly. The mandated U-value is now 1.6 W/(m²K), but the recommended “notional” value for design purposes is even lower at 1.2 W/(m²K) for windows as best practice.
For major refurbishments, the requirement is 1.4 W/(m²K) for replacement windows (Source: The Building Regulations 2010: Conservation of Fuel and Power (Part L), GOV.UK, Published 15 December 2021, https://www.gov.uk/government/publications/conservation-of-fuel-and-power-approved-document-l).
This move towards lower U-values directly translates to enhanced energy efficiency for the end-user. Government impact assessments show that adopting the notional specification for new homes is expected to save households around £170 per year on average on energy bills, varying by property type.
While direct ROI on glazing thickness varies, studies by the Energy Saving Trust show that replacing a single-glazed window with A-rated double glazing can result in average annual savings of £100 to £155 in a typical semi-detached house in England. (Source: Energy Saving Trust, ‘Windows and Glazing’ fact sheet, dated 2023, https://energysavingtrust.org.uk/advice/windows-and-glazing/).
For the professional, this data underscores a clear commercial mandate: high-performance glazing is no longer a luxury but a fundamental asset to meet regulatory, ESG, and occupant-demand metrics.
Before we discuss the overall measurement, it’s essential to understand that a modern double glazed unit (DGU) is an engineered component made of three key parts.
The total double glazed unit thickness is the sum of these parts, not a single value. These sealed double glazing units are often referred to as insulated glass due to their performance.
The formula is straightforward: Total DGU Thickness = Outer Pane Thickness + Spacer Bar Thickness + Inner Pane Thickness.
The two panes of glass are the most visible components. While standard double glazing thickness often uses 4mm glass panes, a wide range of glass thicknesses is available, each with a specific purpose.
For a professional, understanding the role of different glass thicknesses in a double glazed window glass thickness specification is vital.
The spacer bar is arguably the most critical component for a DGU’s energy performance.
It is the sealed, hollow channel that separates the two panes of glass, creating a sealed air gap.
The most common spacer bar thicknesses are 12mm, 16mm, and 20mm. This dimension is crucial because the size of the gap directly impacts the unit’s thermal efficiency.
The gap is often filled with an inert gas like argon to further reduce heat transfer.
Professionals know that a simple “thicker is better” approach is flawed.
The relationship between double glazing thickness and performance is nuanced.
A DGU’s primary function is to reduce heat loss, which is measured by its U-value.
The lower the U-value, the better the thermal performance. The thickness of the insulated glass unit’s air gap is the single most important factor for reducing heat transfer.
For this reason, a standard double glazing thickness in a modern UPVC window frame is typically 24mm or 28mm, using 4mm panes and a 16mm or 20mm spacer bar to achieve an optimal U-value.
This combination offers the best performance-to-cost ratio for energy efficiency. For maximum thermal performance, a triple glazed unit is often used.
When specifying for noise reduction, the rules change. The principle here is mass-air-mass. Sound is a vibration, and to stop it, you need to break its path. This is where thicker double glazing is truly beneficial.
For projects where noise is a major concern—such as in urban developments or buildings near transport hubs. A thick double glazed unit with an asymmetrical glass thickness and a wide spacer bar is the optimal choice.
As a professional, you’ll often need to measure an existing unit to specify a replacement. Here is a reliable, step-by-step method on how to measure double glazed thickness:
For a precise measurement, a professional glass thickness gauge is the most reliable tool, giving you the exact double glazing thickness of each glass pane and the overall unit instantly.
There is a reason why certain double glazing thicknesses are considered “standard.”
Standard sizes like 24mm (4mm glass / 16mm spacer bar / 4mm glass) or 28mm (4mm / 20mm / 4mm) are designed to fit perfectly into the most common UPVC window frames and systems, including casement windows.
Using a standard size ensures a perfect fit, a proper seal, and a streamlined manufacturing process.
Specifying a custom, thicker double glazing unit is necessary when a project requires enhanced acoustic performance, security, or when an existing frame can accommodate a wider unit (e.g., a frame that previously held a triple glazed unit).
The choice of double glazing thickness is a fundamental specification decision that impacts every aspect of a building’s performance.
By understanding the components and their functions, you can move from simple selection to a strategic and effective specification.