Using the calculation or modelling method


If your house does not meet all the R-value requirements, you can use either the calculation or modelling methods specified in NZS 4218:2004.

These methods allow you to use more insulation in some areas to compensate for lower insulation levels in other areas.

The calculation method can only be used if the total amount of glazing is below 50 percent of the total wall area.

The Acceptable Solution for H1 has three different R-value tables that correspond to three distinct types of external wall construction:

  • non-solid (typically timber-framed)
  • solid timber
  • other solid wall construction (typically masonry and rammed earth).

If you are using the calculation method, you need to follow the rules below to combine the different construction:

  • Do not mix and match the R-values (such as for walls, roofs, floors, windows and skylights) from the different tables. For example, the non-solid construction roof R-value must not be used in the place of the solid construction roof R-value.
  • The R-values for each component should be used in the same proportions and should correspond to the different construction types used in the design. For example, if the design has half timber-framed walls and half masonry walls, then the R-values for Table 1 can be used for half the area and the R-values from Table 2(a) can be used for the other half of the area.
  • Alternatively, the highest R-value can be selected from the different construction types and used for each component (such as the highest wall R-value and the highest roof R-value). This approach is inherently conservative and will result in better thermal performance than the Acceptable Solution requires.

The modelling method has fewer restrictions than the calculation method and takes into account other factors such as heat gain from the sun.

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This information is published by the Ministry of Business, Innovation and Employment’s Chief Executive. It is a general guide only and, if used, does not relieve any person of the obligation to consider any matter to which the information relates according to the circumstances of the particular case. Expert advice may be required in specific circumstances. Where this information relates to assisting people: