Originally Posted by
ian
Going back to Andy's initial post, it looks like he has Gyprock attached one side of his rafters and the roof purlins the other.
I'm not sure that any of the insulation products discussed are designed for that situation. As far as I know, ceiling insulation is designed to provide a thermal barrier between the roof cavity and the living areas. And to be fully effective the roof cavity needs to be able to exhaust the hot air generated inside the roof from solar radiation to the outside, with replacement air drawn from the under the eaves where the air is typically cooler.
I also understand that it is important to maintain an air gap between the insulation and the roof -- otherwise you get thermal coupling with the hot roof which conducts the heat into the ceiling material.
Perhaps Andy's most effective insulation will be to ventilate the roof cavity, so that the air heated through contact with the very hot roof is exhausted outside the house and replaced with cooler air drawn from the shady side of the building. A forced air flow coupled with 25 to 50 mm of rigid foam insulation might do the trick.
The other option worth investigating is installing a sprayer to wet the roof on very hot days. When I lived in Broken Hill the workshop was fitted with a lawn type sprayer system which sprayed water onto the roof on very hot days. The cooling effect of the evaporation reduced the heat radiating from the iron into the building.