Architects and specifiers working with secondary glazing know the headline performance claim: up to 65% heat loss reduction. What is harder to find is a product-by-product breakdown of the U-values achievable at different specifications. Without that data, thermal specification becomes guesswork.
This guide presents verified U-values by product type, glass specification, and installation configuration, drawn from test data and live institutional projects. The figures are traceable and documented for use in SAP assessments and building regulations submissions.
What a U-value Means for Secondary Glazing Specification
Thermal transmittance, expressed as a U-value in W/m²K, measures the rate at which heat passes through a building element. Lower figures indicate better thermal performance. A typical single-glazed window carries a U-value of approximately 5.0 W/m²K. Standard double glazing achieves approximately 2.8 W/m²K.
Secondary glazing does not replace the primary window. It creates a second thermal envelope on the inner face, trapping a layer of still air between the existing window and the secondary unit. The combined system achieves U-values that, in optimum configurations, match or exceed standard double glazing.
Building Regulations Part L sets requirements for thermal performance in new-build and refurbishment projects. For heritage settings where full replacement is not possible, secondary glazing provides a compliant route to demonstrable thermal improvement.
What U-value Can Secondary Glazing Achieve?
The optimum achievable figure is 1.5 W/m²K. This is reached with Granada’s horizontal sliding secondary glazing using 4mm Low-E glass, an 80mm air gap, and face-fix installation. That figure represents approximately 10% better thermal performance than standard double glazing and a 65% reduction in heat loss versus a single-glazed baseline.
Three variables determine the U-value of a secondary glazing installation:
Glass specification: Low-E glass (hard-coated, room-facing side) is the primary thermal variable. The hard coating reduces infra-red heat radiation through the glass surface. 4mm Low-E glass is the standard thermal specification.
Cavity depth: The air gap between the primary window and the secondary unit is the second critical variable. For thermal performance, the optimum cavity is up to 80mm. Face fix installation allows deeper cavities; reveal fix is constrained by the reveal depth.
Installation method: Face fix mounts the secondary frame against the surrounding wall. A reveal fix sits within the window reveal. Face fix is preferred for thermal specification, as it accommodates the full 80mm cavity.
U-values by Product Type
Performance varies by product family. The following figures are verified data from test reports and documented installations.
Horizontal Sliding: 1.5 W/m²K with 4mm Low-E glass, 80mm air gap, face fix installation. The most widely specified product for thermal improvement at scale. Available in two to five sash configurations for casement and larger commercial openings.
Hinged Unit: 1.9 W/m²K centre-pane, recorded at Hallamshire Hospital Sheffield (Low-E glass, slimline hinged units on the NHS hyper-acute stroke unit). The hinged secondary glazing range also supports double-glazed sealed units (24mm and 28mm) for applications requiring maximum performance.
Balanced Vertical Slider: Designed for traditional sash windows. Thermal improvement is achievable with Low-E glass. Optimal cavity depth may be constrained by the sash reveal, which can limit performance relative to face fix installations. The preferred product for listed building sash window specification.
Lift Out: Removable panels with no operating mechanism. Appropriate for heritage and decorative openings where a fixed secondary panel is acceptable. Thermal performance is determined by glass specification; Low-E glass applies the same improvement factor as other product types.
Fixed: Permanently fitted panels. The most economical option. Not suitable for fire escape locations. Low-E glass achieves comparable thermal improvement to other product types at the same cavity depth.
The Role of Glass Specification
Low-E glass is the single most significant variable in secondary glazing thermal performance. The hard coating, applied to the room-facing surface, reduces radiant heat transfer through the glass without reducing light transmission.
Granada uses 4mm Low-E glass as the standard thermal specification across all product types. The hardwood subframe, supplied with every unit, provides an additional thermal barrier at the perimeter, preventing cold bridging and improving the installed U-value relative to the centre-pane figure.
Detailed specification data, including tested U-values by glass type and configuration, is available from Granada’s technical downloads.
Cavity Depth and Installation Method
The thermal performance figures cited above assume optimum cavity depth. The relationship between cavity depth and U-value is non-linear. Performance improves from 40mm to approximately 80mm, then plateaus. Cavities deeper than 80mm do not improve thermal performance and may introduce convective movement within the air gap.
For specifiers who also need acoustic performance, this creates a trade-off. The optimum acoustic cavity is 150 to 200mm, which far exceeds the thermal optimum. A single specification cannot maximise both. Where both are required, the brief should define which performance characteristic takes priority or whether the project warrants separate thermal and acoustic specifications on different elements.
Case Studies: Verified U-values in Practice
The figures above are not theoretical maximums. They have been achieved in commissioned installations under normal working conditions.
Hallamshire Hospital, Sheffield: An NHS hyper-acute stroke unit. Low-E glass hinged units installed across a live healthcare building. Centre-pane U-value: 1.9 W/m²K. Integral venetian blinds fitted within the cavity.
Premier Inn, Llandudno: A grade II listed hotel. Secondary glazing installed to preserve original timber windows. Result: 65% heat loss reduction, 80% noise reduction.
Reuben College, Oxford: A grade II listed Radcliffe Science Library. Result: 65% thermal improvement, 54dB noise reduction. The installation supports the University’s decarbonisation strategy.
A full record of completed projects is available through our project case studies.
FAQs
What is the best U-value achievable with secondary glazing?
1.5 W/m²K with Horizontal Sliding secondary glazing, 4mm Low-E glass, 80mm air gap, face fix installation. This figure is verified by test data and has been achieved in live commercial installations.
How does secondary glazing U-value compare to double glazing?
Secondary glazing in optimum configuration achieves approximately 10% better thermal performance than standard double glazing (typically 1.6 to 2.8 W/m²K depending on specification). For heritage applications where the primary window cannot be changed, secondary glazing provides comparable performance to modern double glazing while preserving the original fabric.
Which secondary glazing product gives the best thermal performance?
The Horizontal Sliding achieves the best U-value at scale (1.5 W/m²K). The Hinged Unit supports double-glazed sealed units for applications requiring maximum performance. The choice depends on the primary window type and installation constraints.
Does cavity depth affect the U-value of secondary glazing?
Yes. The optimal thermal cavity is up to 80mm. Below 40mm, performance drops significantly. Above 80mm, there is no further thermal improvement and convection may begin to reduce performance. Acoustic performance peaks at 150 to 200mm, which creates a conflict if both performance criteria apply.
Can secondary glazing help meet Building Regulations Part L?
Yes. Secondary glazing is a recognised thermal improvement measure. Specifiers can demonstrate compliance using verified manufacturer U-value data. Granada is NBS Source listed. Specification data for SAP assessments is available through technical downloads.
