Architects specifying secondary glazing for the first time, or for a complex heritage or acoustic brief, face a range of variables that significantly affect performance. Picking the wrong product type, glass specification, or installation method can mean a unit that underperforms acoustically, fails thermally, or creates problems with heritage consent. This guide covers every specification decision.
Why Architects Specify Secondary Glazing
Secondary glazing sits inside the existing window, fixed to the reveal or surrounding wall, without altering the primary frame or external appearance. That makes it the preferred intervention across several distinct project types.
In heritage and listed building work, replacement glazing is often not permitted. Secondary glazing is reversible and non-invasive, which means it preserves the original fabric of the building and typically does not require Listed Building Consent. Historic England recommends it for listed buildings where double glazing replacement is inappropriate.
On acoustically sensitive sites, secondary glazing outperforms both double and triple glazing. A properly specified unit can achieve up to 54dB noise reduction, compared to approximately 35dB from standard double glazing. This makes it the standard specification for projects adjacent to transport infrastructure, flight paths, or high-traffic urban environments.
Thermally, secondary glazing can reduce heat loss by up to 65%, making it relevant to Building Regulations Part L compliance in refurbishment projects where upgrading the primary window is not feasible. In healthcare and education settings, where acoustic and thermal comfort are both specified to a defined standard, secondary glazing frequently provides the most practical route to compliance.
Choosing the Right Product Type
Granada Glazing manufactures six product families. Matching the correct product to the window type and performance requirement is the first specification decision.
Horizontal sliding units are the most widely specified. Two to five independently sliding sashes suit casement or hinged primary windows across offices, education, and mixed-use refurbishment. They are not suited to traditional sash windows; for those, the correct product is the Balanced Vertical Slider.
The Balanced Vertical Slider (BVS) is designed specifically for sash windows. A spring-balance mechanism holds the sash at any position without additional hardware. Tilt-in (TBVS) and slide-past (SPBVS) variants are available depending on access requirements. View the vertical sliding secondary glazing range for full specifications.
Hinged units offer the highest acoustic and thermal performance in the Granada Glazing range. They are the only product to support double-glazed sealed units (24mm or 28mm), making them the preferred choice for healthcare environments. At the Hallamshire Hospital hyper-acute stroke unit in Sheffield, hinged secondary glazing achieved a centre-pane U-value of 1.9 W/m²K.
Lift-out panels are removable rather than operable. They suit heritage and decorative windows, arched openings, and applications where the lowest visual impact is the priority.
Fixed units are permanently fitted and the most economical option. They are not suitable for fire escape locations.
Bespoke units are available for arched, curved, bay, circular, and angled openings that fall outside standard size ranges.
Glass Specification for Acoustic Performance
Acoustic performance depends on three variables: glass specification, cavity depth, and installation method.
Granada Glazing’s tested Rw figure of 49dB is achieved with a 6mm toughened primary pane and 6.8mm acoustic laminate on the secondary unit, with a reveal-fix installation and 150 to 200mm glass-to-glass cavity depth. The guiding principle is to use different glass thicknesses on the primary and secondary sides: this prevents sympathetic resonance between the two panes and improves overall attenuation.
For most acoustic applications, recommended glass thicknesses are 6.4mm, 6.8mm, or 8.8mm acoustic laminate. The maximum tested figure of 54dB is achievable at a 200mm cavity with an optimised glass specification.
A reveal fix installation is preferred for acoustic work because it allows a deeper cavity than a face fix. Where reveal depth is insufficient, a secondary subframe can extend it. All performance figures above apply to the revealed fixed configurations.
Compliance with Approved Document E should be assessed against the target Rw rating for the building use, confirmed at the design stage.
Glass Specification for Thermal Performance
For thermal applications, the specification logic differs.
The best achievable U-value from the horizontal sliding secondary glazing range is 1.5 W/m²K, using 4mm Low-E glass and an 80mm air gap with face-fix installation. Low-E glass should be positioned room-facing, with the hard coating on the inner surface, to maximise radiant heat retention in the cavity.
Cavity depth for thermal performance is optimal at approximately 80mm. Beyond that, convection currents within the cavity begin to reduce the insulating benefit. The hardwood timber subframe supplied with every Granada Glazing unit acts as a thermal barrier and prevents cold bridging at the frame perimeter, which is a common point of thermal loss in secondary glazing without a purpose-made subframe.
Where Building Regulations Part L compliance is being assessed, secondary glazing should be modelled as part of the overall U-value calculation for the window assembly.
Installation Method: Reveal Fix vs Face Fix
These two installation methods determine cavity depth and, by extension, acoustic and thermal potential.
A reveal fix places the secondary unit within the window reveal, set back from the plane of the primary window. This allows cavity depths of 150mm or more, the range required to achieve the highest acoustic attenuation figures. A reveal fix also produces a neater visual result in most window configurations.
Face fix mounts the secondary unit to the wall or architrave surrounding the window opening. It is appropriate where the reveal depth is too shallow for a reveal fix, but it limits the achievable cavity to approximately 80mm. That is sufficient for thermal performance but restricts acoustic specification.
The choice of installation method should be confirmed at the pre-specification stage, during a site survey where revealed dimensions can be measured accurately.
Heritage and Listed Building Specification
Secondary glazing is a recommended intervention in listed building and conservation area work precisely because it meets the reversibility requirement that replacement glazing cannot.
Granada Glazing has a substantial track record in this sector. Bishopthorpe Palace in York, the Grade I listed residence of the Archbishop of York, was fitted with Balanced Vertical Slider units that preserved the original sash mechanism while delivering measurable thermal and acoustic improvement. Porchester Court in London, a Grade II-listed Victorian terrace, achieved 54dB noise reduction and 65% thermal improvement. Both projects are among our project case studies.
Secondary glazing does not alter the external appearance of the building, which is the primary test for Listed Building Consent. In most cases it can proceed without consent, though this should be confirmed with the relevant local planning authority early in the design process, particularly for Grade I listed buildings or those in sensitive conservation areas.
Historic England’s guidance on double glazing in historic buildings specifically identifies secondary glazing as the preferred approach in most cases.
Specifying via NBS Source
Granada Glazing is listed on NBS Source, which allows architects to specify products directly within the NBS platform using the NBS Source specification, without the need to write a bespoke specification from scratch.
The listing covers the full product range, with performance data, installation guidance, and product codes integrated into the platform. Granada Glazing also holds RIBA-Approved CPD status. A structured CPD session is available for practices requiring a formal introduction to secondary glazing specifications.
For project-specific technical support, the Granada Glazing team provides specification assistance directly. Contact our team to discuss project requirements.
Frequently Asked Questions
Does secondary glazing comply with building regulations?
Secondary glazing can satisfy the requirements of Approved Document E (acoustic performance) and Approved Document L (thermal performance) when correctly specified. Compliance depends on product type, glass specification, and cavity depth.
Can secondary glazing be specified for Grade I listed buildings?
Yes. Secondary glazing is non-invasive and reversible, which satisfies the requirements that prevent replacement glazing in listed building contexts. Grade I applications should be discussed with the Local Planning Authority at the design stage to confirm Listed Building Consent is not required.
What cavity depth is needed for acoustic secondary glazing?
The optimal acoustic cavity depth is 150 to 200mm glass to glass, achieved using reveal fix installation. At 200mm with an appropriate glass specification, Granada Glazing units can achieve up to 54dB Rw.
How do I specify secondary glazing via NBS?
Granada Glazing is listed on NBS Source. Access the full product specification directly via the NBS Source specification.
What U-value can secondary glazing achieve?
The horizontal sliding range achieves a centre-pane U-value of 1.5 W/m²K with 4mm Low-E glass and an 80mm air gap. The hinged unit range achieved 1.9 W/m²K in the Hallamshire Hospital installation.
