In short: the more the better:
The Building Code of Australia (BCA) identifies eight different climate zones for Australia, but within a zone, there are some locations with slightly different temperature ranges. There can be significant differences between maximum and minimum temperatures in summer and winter and in length and intensity of heating and cooling periods. The house design, the insulation and construction must respond to these variations in order to be able to perform energy efficient.
For simplicity, Victoria is divided in five climate zones, with winter heating as the predominant concern especially in the Temperate Coastal and Cool Inland Zones. Summer cooling is variable but generally less significant. House design in these zones requires attention to better performing insulation, draught proofing, window protection in winter and shading in summer. Likewise, in warmer cities and areas like Mildura supplementary heating is obligatory for thermal comfort. In these regions, it’s advisable to include extra thermal mass, cross ventilation and summer shading, whereas alpine areas may require constant heating for most of the year and cooling requirements are negligible. Consequently, a 6-star home in Mildura wouldn’t comply with the minimum requirements for a 6-star home in Ballarat.
The higher the R-value the better the performance. Consider what insulation is needed in order to build an energy efficient home in a certain climate zone early in the design process. In particular, it’s important to think about the roof insulation. For example, it would be cheaper to use larger rafters in order to fit in sufficient glasswool to fulfil the desired R-value, instead of using thinner expensive extruded polystyrene. Larger rafters would mean that the overall height of the building rises slightly. This is no problem, if the amendments are done early in the design. However, if a town planning permit has already been granted, it’s not that easy any more. It’s necessary to go back to the council with the changes, which can cost a lot of time and money, therefore in most cases, people choose to use the thinner, more expensive insulation instead.
Adding R1.0 insulation can significantly improve the energy efficiency. For example in Melbourne, adding insulation with a R-value of R3.0 to the ceilings and R1.5 insulation to walls can save 12% on energy bills each year and can ensure a higher level of comfort.
One important thing to consider is that the energy requirements as listed in the BCA are minimum requirements only, not best practice. So if someone is telling you to not put any more insulation in as the regulations call for: don’t listen to them. They don’t have a clue.
If you put in anything less, your building would not comply, so if you are after an energy efficient home, why would you be happy to only have the legally required minimum? Rather put in as much insulation as fits into the wall/roof/ or wall and as much as your budget allows.
Keep in mind, while it is quite easy and common to upgrade bathrooms and kitchens every 10-20 years, you will typically not touch the insulation again. So make sure you make your home future proof!