Thermal Performance and Materials

Australian Standard AS 4859.1
Materials for the thermal insulation of buildings Part 1: General criteria and technical provisions (2006).

Insulation, what is it?
Insulation, simply put, is a quality of a substance to 'resist' transferring heat (or cold, basically energy) through it. In building construction high insulation is used to allow the internal environment temperature to maintain constant independently of external temperature changes. This not only makes the inside a more 'pleasant' place to live, it helps reduce heating and cooling costs by making it easier to keep at the same temperature; which in turn means a building is more ecologically friendly, and consumes less environmental resources.
So what does the R-Value have to do with this?
The R-value of a substance is its direct measure of its resistance to transferring energy or heat; R Values are expressed using the metric units (m2.K/W). Basically the higher the figure the better it is at resisting energy transfer, so the easier it is to maintain a difference in temperatures across it for a longer time.

In the metric system, the R value measures per meter squared the amount of degrees kelvin temperature difference required to transfer one watt of energy. So an R value of 1 means per meter squared a single degree difference will transfer one watt of energy. So an R value of 2 will transfer half a watt of energy for a degree of difference.
Usually the R value is given for a certain type and thickness of material as installed (often known as the 'added R value'); i.e. a low density glasswool batt would need to be 130mm installed to achieve an R of 2.5, but only 100mm thick of medium density. Note: We say 'as installed', taking a low density batt that is designed to work in 130mm as installed and squashing it to fit in 100mm will not be the same as using a medium density batt in the first place.

http://www.ecowho.com/ard/?u=http%3A%2F%2Fwww.ecowho.com%2Ftools%2Fr_value_calculator.php&s=92604
All main building materials (be it wall, floor, ceiling, loft or roof components) have known R-Values. This is a handy calculator for adding together all the components R-values to see if your specifications for walls, roofs and floor meet the minimum requirements as specified in the BCA.

What is the U-value?
The U-value is the inverse of the R-Value; i.e. you divide 1 by either the R or U value to convert to