You can use heating and cooling numbers from AS-2627.1 here. These calculations differ from those in AS-2627.1 Section 3. The approximation I have used is very close (between 2 and 4 significant figures) to the real solution of the quartic equation.
If you set back your heater thermostat to 12 °C (or lower) at night and your cooler thermostat to 24 °C (or higher) during the day (avg 18 °C) you will have significant energy savings. Note that daytime temperatures 15 to 27 °C (avg 21 °C) are generally regarded as comfortable, however people with certain medical conditions may find temperatures outside 18 to 24 °C unbearable. The BoM data above is over 24 h so only the 12 and 24 °C values are useful for these calculations.
No allowance is made for the fact that insulation will slow a buildings response to temperature change. Thus this will underestimate the most economical R value if such an R value could actually be achieved with the product being considered.
On the other hand some types of insulation may have gaps and experience Thermal Bridging [2][3] thus the actual benefit will be significantly reduced. The 'Incremental insulation cost' parameter (b) used should therefore relate to the actual insulation value achieved including any gaps.
If heat capacities are lumped in the conditioned space (which is valid for light weight construction) then heating and cooling numbers will be improved (i.e. made smaller) with the addition of insulation. Thus this model could simply be improved by using 2 sets of heating and cooling numbers: the uninsulated ones; and the improved ones.
EnergyPlus software that does a complete thermal simulation is available from the LBL Simulation Research Group. This work was sponsored by the US DoE. This software can be licensed in source code form making it suitable for research and in support of product or other claims. This is an alternative to the Australian closed House Energy Rating Scheme.