The complicated nature of secondary glazing when it comes to meeting prescriptive design requirements.

Design responsibility; descriptive, prescriptive and secondary glazing


The Built Environment



It is becoming ever more important in project workflows for responsibilities at each level to be well defined, during knowledge exchanges if using the RIBA 2020 Plan of Work and to follow the ‘Golden Thread of Information’ practices. This is to make certain that all involved know the part they must play in managing the successful outcome of the project for the client. A fundamental role which needs to be clarified in detail is design responsibility and at what level. You would expect that the architect or specifier assumed this, however, they do not for all aspects of a project. With increasingly complex buildings being designed and constructed, more specialist sub-contractors are being involved at earlier stages of the process to make sure standards are met, performance requirements are achieved, and that the integrity of the whole design is safe and sustainable. A way of managing this, along with accountability, is via a design responsibilities matrix, which can be shared for all involved to follow. It is also forming part of the 'Employers Information Requirements' on projects where BIM is being adopted. The RIBA Design Responsibility Matrix to help outline accountability on construction projects and exhanges of information between designers, contractors and sub contractors Example of a DRM – Design Responsibility Matrix for the RIBA Plan Of Work 2020 (downloadable document – 2020RIBAPlanofWorkoverview.pdf)

Descriptive and prescriptive design responsibility – the confusion clarified

This is an area that can cause confusion and one that needs to be clarified at an early stage, as it has an impact on deliverables as well as the overall specification of elements of the construction. Prescriptive design is where the designer provides a detailed specification with reference to the exact industry products required, certifying that their solution will satisfy the project deliverables. Descriptive design is where the designer provides a design intent and the final design outcome including the performance requirements with little level of detail. This will be passed to the main contractor who in turn will engage specialist sub-contractors, consultants, or their own design team to complete the design and provide the necessary general arrangement, detailed drawings, and specifications for approval.

Selectaglaze secondary glazing and descriptive design

As a specialist sub-contractor, we are often asked to assist in the specification of secondary glazing to meet a required performance. Although all our systems are designed in-house and performance is verified by 3rd party organisations, there are still many variables which will influence performance – some which are out of our hands. Secondary glazing is an independent window system fitted on the inside of primary glazing. In the main it is used to raise performance levels in Listed and heritage buildings, where the original windows cannot be changed. However, it is equally important in new-build projects where elevated levels of noise attenuation are required yet cannot be met by new double glazing alone. Selectaglaze secondary glazing with primary window makeup in the content of a Listed building project - Banqueting House and new build Stansted Airport College Grade I Listed Banqueting House : Secondary & primary glazing configuration : New Build Stansted Airport College Whether a Listed building or new build, there are many things which will influence the final performance of the installation; size, configuration and makeup of the primary windows; size configuration and make-up of the secondary window; the fixing substrate and the relationship between the primary and secondary window to name a few.

How are secondary glazing performance levels worked out?

Thermal efficiency U-values are desk-based calculations, which are worked out as a combination of both primary and secondary windows. They are produced based on the assumption of a typical window and secondary glazing combination. With such a vast range of variants, it is prohibitive to formulate figures for each and every case and therefore the performance for a specific project will only ever be an assessment. Acoustic dB ratings are a physical test, again carried out in combination with the primary and secondary glazing set in a rig. As with the U-value calculations assumptions must be made about a typical window and again the range of variants is so great that it is prohibitive to test all combinations. Off the page extrapolations can be made to provide a broader performance assessment but there are limits to how far this can go. Acoustic tests can be carried out on site, but without isolating the noise break through from other parts of the building it is almost impossible to assess the performance of the window assembly alone. Blast rating is assessed by subjecting a sample to a blast load on a range. The assessment is made with a sacrificial primary pane although this does little for protection. Test samples are a standard size but in practice, the size of unit can impact on performance and this needs to be considered carefully when specifying for a project. Selectaglaze secondary glazing on a blast range where primary glazing has been sacrificed, yet secondary remains in tact Size of a blast and testing of a Selectaglaze unit; primary window sacrificed yet secondary glazing intact Physical Security is again done by test, these tests consider the size range of the products and apply a standard attack method so in this respect they can be far more absolute when determine if performance requirements are met.
Attack testing on Selectaglaze secondary glazing LPS1175 SR2 unit  Fire performance like physical security is a physical test which considers the size range of the product so again it is easy to determine if performance requirements have been met.

Meeting the performance Specification

The performance of secondary glazing is intrinsically linked with that of the primary window yet in older, and in particular heritage and Listed buildings, the primary window performance is relatively unknown. Experience and skill allow assessments to be made, but they are only best judgements. Setting an absolute performance requirement is easy, verifying that the proposed solution meets that requirement is less so.