Secondary glazing is, in most respects, a specification decision. The system you choose determines acoustic performance, thermal improvement, aesthetic compatibility, and long-term maintenance liability. But before any of those decisions, there is a more fundamental one: the frame material.
Aluminium and uPVC are the two materials you will encounter most often when specifying secondary glazing in the UK. They occupy very different positions in the market, and the difference between them matters far more than most general window comparisons suggest. Secondary glazing is a precision-fit system installed against an existing window, often in a listed or heritage building, and the material properties of the frame have direct consequences for performance, sightlines, longevity, and compliance.
This article sets out the technical and practical differences between aluminium secondary glazing and uPVC secondary glazing, with the specific considerations that matter to architects, specifiers, facilities managers, and trade installers.
What the Frame Material Actually Affects
Before comparing the two materials, it is worth being clear about what frame choice influences in a secondary glazing context:
Sightlines. Aluminium extrusions can be profiled to extremely slim dimensions. The slimmer the profile, the less visual intrusion on the primary window and the more glass area is retained. This matters aesthetically in any context, and particularly in heritage buildings where matching the proportions of the existing window is a planning requirement.
Structural rigidity. Secondary glazing frames, particularly on large commercial units, carry significant weight. The frame material must hold true over time without bowing, warping, or distorting the seal between the secondary and primary window.
Finish compatibility. Paint and powder coat adhesion, colour range, and long-term finish durability vary considerably between aluminium and uPVC. Heritage and commercial projects often require colour matching to existing joinery.
Thermal performance at the frame. The frame itself is a potential cold bridge. Material choice affects how heat moves through the system, separate from the glass specification.
Lifespan and maintenance. The expected service life of a secondary glazing installation varies significantly between materials.
Aluminium Secondary Glazing: The Technical Case
Aluminium is the material of choice for professional and commercial secondary glazing for several interconnected reasons.
Dimensional stability. Aluminium does not expand and contract to the same degree as uPVC under temperature variation. This matters in secondary glazing because the system must maintain consistent contact with the primary window and its surrounding reveal across all seasons. A frame that moves with temperature creates gaps in the seal, compromising both acoustic and thermal performance.
Slim sightlines. Because aluminium has high tensile strength relative to its weight, it can be extruded into far narrower profiles than uPVC while retaining structural integrity. This allows for slim sightlines that minimise visual intrusion. For projects in conservation areas or involving listed buildings, this is frequently the deciding factor.
Finish options. Aluminium accepts powder coating to a wide range of RAL colours with strong adhesion and long-term durability. Granada’s in-house powder coat line covers 200-plus RAL colours, which means the secondary glazing frame can be matched precisely to the existing window joinery or any other specified colour. uPVC is available in a limited range of foil-wrapped colours, but the range is narrower and the long-term colour stability is generally inferior.
Longevity. A well-manufactured aluminium secondary glazing frame has a service life measured in decades. The material does not degrade under UV exposure in the way that uPVC can, and it does not yellow, chalk, or become brittle over time.
Acoustic performance. Achieving the highest Rw ratings requires a robust, airtight seal between the secondary system and the reveal. Aluminium frames, given their dimensional stability and precise CNC machining tolerances, consistently achieve better seal quality than uPVC. Granada’s horizontal sliding system, for example, achieves 49dB Rw with 6mm toughened primary glass and 6.8mm acoustic laminate secondary glass. The overall system headline is up to 54dB noise reduction, which secondary glazing achieves consistently where uPVC systems, with their variable seal performance, typically cannot.
Heritage and planning compliance. Historic England guidance recommends secondary glazing as the preferred intervention for listed buildings because it is non-invasive and reversible. Planning officers and conservation officers generally accept secondary glazing without requiring Listed Building Consent. But the frame material is not irrelevant to this assessment. Slim aluminium profiles that minimise visual alteration to the window are more likely to satisfy conservation requirements than bulkier uPVC frames.
Large-format capability. Commercial projects frequently require large secondary glazing units. Granada’s horizontal sliding system runs to 4,934mm wide and 2,768mm tall. These dimensions are not achievable with uPVC, which lacks the structural integrity for large spans without significant visual framing.
uPVC Secondary Glazing: Where It Is Used
uPVC secondary glazing occupies the lower end of the market and is most commonly encountered in residential applications where cost is the primary driver.
The material is less expensive to produce than aluminium and requires less specialist manufacturing. uPVC systems are widely available from trade and DIY suppliers, and they are frequently sold as self-fit solutions aimed at homeowners.
For straightforward domestic applications, where the windows are standard sizes, the aesthetic requirements are modest, and long-term performance targets are not demanding, uPVC secondary glazing can serve a basic purpose. It reduces draughts, provides some degree of thermal improvement, and costs less upfront.
The limitations become apparent in more demanding contexts. uPVC profiles are broader in section than aluminium equivalents, so sightlines are wider and visual intrusion is greater. The colour range is restricted. The expansion behaviour of the material under temperature variation makes consistent sealing more difficult to achieve, which in turn limits acoustic and thermal performance. And for commercial or heritage projects, uPVC is rarely specified by architects or conservation officers because the slim, powder-coated aluminium profile is the standard against which secondary glazing is assessed.
It is worth noting that most specifiers who have worked across both materials do not treat them as interchangeable options at the same quality tier. They are different products serving different market positions.
The Heritage Question
For any project involving a listed building or a property within a conservation area, the material choice is largely settled. Aluminium is the professional standard, and for good reason.
Secondary glazing specified for Porchester Court, a Grade II listed Victorian terrace in London’s Bayswater Conservation Area, used Granada’s balanced vertical slider and horizontal sliding systems in RAL 9003 satin. The project achieved 54dB noise reduction and 65% thermal improvement with zero snagging. The slim aluminium profiles satisfied Listed Building Consent conditions and preserved the visual character of the original windows.
At Bishopthorpe Palace, the Grade I listed home of the Archbishop of York, the specification requirement was thermal improvement that could not alter the building’s appearance or fabric in any material way. Again, aluminium secondary glazing was the only credible option.
The majority of commercial secondary glazing projects in the UK heritage sector specify aluminium as standard. For an overview of how secondary glazing performs across different project types, the Granada case studies library covers 29-plus projects across heritage, education, healthcare, and hospitality.
Specifying the Right Aluminium System
Assuming the material decision is made, the next question is product type. The secondary glazing system that suits a large commercial office will differ from what is appropriate for a traditional sash window in a listed terrace.
Granada’s range covers six product families, each designed for a specific application:
- The horizontal sliding system is the most widely specified across commercial projects. It runs from two to five independently sliding sashes and suits casement and hinged primary windows. It is available in a heavy-duty variant for units over 2m in height, requiring a 62mm hardwood subframe.
- The balanced vertical slider is designed specifically for traditional sash windows and should always be specified in preference to a horizontal system where the primary window is a sash. The spring-balance mechanism holds sashes at any position. The tilt-in variant (TBVS) allows each sash to pivot inward up to 180 degrees for cleaning access.
- The hinged unit is the only system in the range to support double-glazed sealed units (24mm and 28mm), and it is the right choice where maximum thermal improvement is required in a single frame, as demonstrated by the Hallamshire Hospital installation, which achieved a U-value of 1.9 W/m²K centre-pane.
View our full range of secondary glazing systems for trade and commercial projects. To get an initial indication of pricing by system type, size, and glass specification, the online price generator provides trade-level figures ahead of formal quotation.
