Section J - System U-value Glazing Requirements – Speckel Support

The System U-value requirements are located within J1.5 Walls and Glazing within the NCC 2019 - Vol 1.

In comparison to the NCC 2019 – Vol 1 2016, high levels of glazing or high window-to-wall ratios are generally being discouraged via higher thermal performance requirements within NCC 2019. This is a great solution for better performance in buildings and also to provide higher levels of occupant comfort. Lower levels of glazing, however, remain very lenient from an energy efficiency perspective.

Specifically, J1.5 Walls and Glazing provides System U-value Performance Requirements for two groups of buildings. Buildings that comprise Class 2 common area, Class 5, 6, 7, 8 and 9b buildings or a Class 9 building other than a ward area are defined as daytime buildings. These daytime buildings have lower performance requirements than night-time buildings, comprised of Class 3 or 9 c buildings or class 9A Ward areas.

In order to understand these performance parameters and how they meet the performance requirement of JP1 - Energy Use, Schedule 3 - Definitions is often a great place to start and required by the code.

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.

Breaking down the above definition for glazing, we can see that the requirement from the NCC 2019 – Vol 1 2019 – Vol 1 is focused on the transparent or translucent element and its supporting frame located in the envelope. By virtue of this definition, we need to account for all transparent or translucent areas and the frames that support such elements.

Looking at the second definition for Total System U-Value, we need to account for the thermal transmittance of the transparent or translucent element, alongside the effect of any air spaces, surface resistance and the impact of thermal bridging. The impact of thermal bridging is associated with the frame of the system, where the majority of the heat loss occurs.

What is clearly missing from this definition is Standard Calculation methodology or a Primary or secondary reference document.…. So how do you calculate Total System U-values in line with the NCC 2019 - Vol 1?

In order to calculate the Total System U-values, the primary requirement from an NCC 2019 – Vol 1 is to meet the definitions within Schedule 3 through a recognised repeatable calculation methodology and evidence to support compliance with a relevant ABCB protocol. There is no ABCB protocol for this subject matter.