J1.5 – Façade U-value
One of the biggest changes in NCC 2019 was the introduction of new benchmarks and notions regarding minimum performance requirements for wall and glazing components of a wall-glazing construction. Using an area-weighted average between the wall area and glazing area, combined a walls façade must have a maximum U-value of 2.0 for most buildings and climate zones.
The wall components of a wall-glazing construction must achieve a minimum total R-Value of Rt1.0 where the wall is less than 80% of the WGC and Rt1.4 where the wall is more than 80% of the WGC.
AS4859.1:2018 and AS4859.2:2018
Another change in NCC 2019 that had a significant impact was a reference to AS4859.1:2018 and AS4859.2:2018. The AS4859.1:2018 release introduced changes to the way a product is tested and how a materials thermal performance (R-value) is determined and declared. Where AS/NZS 4859.2:2018 represents a prescribed procedure for calculating the overall thermal resistance of the construction assembly, including a new reference to the New Zealand standard, NZS4214:2006 when calculating the effect of thermal bridging.
The definition of total R-value also changed in NCC 2019. Total R-value, for the purposes of Volume One, is now defined as the sum of the R-values of the individual component layers in a composite element including any building material, insulating material, airspace, thermal bridging and associated surface resistance. The main difference between NCC 2016 and NCC 2019 definition is the inclusion of thermal bridging.
Thermal bridging now needs to be accounted for within thermal calculations and design. NCC 2019 has adopted an Isothermal Plane Method to determine total R-value. An Isothermal Plane Method breaks the components in an assembly into two types: (i) a component which has parallel paths of heat flow (ie; glasswood in between steel studs), and (ii) continuous layers of homogeneous materials which are included in assembly (ie: plasterboard lining).
This method of calculating a total R-value allows for better accuracy when determining heat transfer thus in turn reducing energy costs, rising emissions and negatives impact upon the thermal comfort of building occupants. Using a continuous layer of insulation will help mitigate thermal bridging, find out more here.