Section J – Glazing System Subframes and the NCC 2019 – Vol 1 – Speckel Support

Subframes are the supporting elements at the head and sill of a glazing system. As a component of the glazing system, the framing has high thermal losses and thus sub sills are required to be assessed within the NCC 2019 – Vol 1 as per Schedule 3 – Definitions for Glazing and Total System U-Value requirements.

Glazing, for the purposes of Section J in Volume One, means a transparent or translucent element and its supporting frame located in the envelope and includes a window other than a roof light.
Total System U-Value (W/m².K), for the purposes of Volume One, means the thermal transmittance of the composite element allowing for the effect of any airspaces, thermal bridging and associated surface resistances.

When providing Total System U-values from a framing subcontractor or if you are collecting them from online, such as the Window Energy Rating Scheme (WERS), or from within a software package, it's imperative that you request confirmation that the calculation of the sub sill has been included within the overall performance of the system. Simply go to your supplier, and they should be able to confirm that the sub sills are accounted for, or it is not possible to state that they meet the performance requirements of JP1 - Energy Use.

For those looking to investigate the impact of sub-framing on a glazing system, Speckel allows you to toggle subframes on and off.