Calculating thermal mass from NZS 4218:2009

Issue 52 – August 2013

As NZS 4218:2009 Thermal insulation – Housing and small buildings is not cited in the H1 compliance documents, designers using it as a means of compliance must submit their thermal calculations in their consent documentation as an alternative method – currently the H1 cited standard is NZS 4218:2004.

Design rules in the 2009 version of NZS 4218 are more definitive than the 2004 version.

When using the 'buildings with high thermal mass' provisions of Table 4 of NZS 4218:2009, calculations for the thermal mass can only be used where:

  • the thermal mass is exposed to the interior
  • the thermal mass material has a surface density of at least 215 kg/m2.

For wall construction, this means that the wall insulation must be located on the external surface of the wall. This means that heavy mass walls that are strapped and lined with insulation on the inside cannot be used to provide a thermal mass. The thermal performance required of such walls must be determined using Table 2 of NZS 4218:2009 'any wall type'. When using a bulk insulation such as glass fibre, wool or polyester with strapping, the strapping will need to be at least 75 mm deep to meet the 'any wall type' schedule method requirements.

Summarised from BRANZ Guideline, July 2013.

Published in building.

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