Concrete slabs are a fantastic way of evenly distributing mass throughout a house in a cost effective way. It also locks the house to the mass of the earth for stable internal temperatures. Thermally however, slabs however have one weak point - their sides.
During winter under the slab, the deep ground temperature is a reasonable 14 degrees. We are typically trying to maintain a temperature of 20 degrees internally, which means we only have a 6 degree temperature differential. This is not an overly large differential as things go, but because concrete is a good conductor and the area of the slab base is large, heat loss to the ground is significant. That is why we always recommend that we insulate under our slabs, and this usually gains at least 1/2 a star in the rating. A cost effective way to do this is to design your integrated slab with a waffle pod (polystyrene void former) construction, in lieu of the traditional sand bed. This effectively puts at least an R0.7 between the slab and the ground, significantly slowing the rate of heat loss to the ground. This means that the sunlight that came through the northern windows in your passively designed house on a winters day, stays radiating into the living space well into the early morning hours.
On the side of the concrete however the night air in winter could be much lower than under the slab, (with air temperatures of 5 degrees, or even zero degrees not uncommon in Melbourne). This could make for a 15-20 degree temperature differential, which would create a much faster heat flow to the slab sides. Worse still, slab sides typically go uninsulated, creating a thermal bridge around the footprint of your house.
How much of problem is this? Unfortunately this is one area that the current house energy rating software does not accurately calculate, so it is currently impossible to quantify. It is definitely visible to an Infrad Red camera, and you can feel the difference with bare feet on a slab near the external walls if the night is cold, but to put a rating value on it is currently impossible. If you are installing in-slab heating, slab edge insulation is a compulsory element.
The CSIRO have earmarked it as something to improve and include in future software versions of their Accurate TM software, so hopefully it wont be long until the benefits to quantified.
Until then, it is still however something worth considering, especially if you have high slab sides, inslab heating, or are in a colder climate - and the budget allows.