Australian Standard AS 1562 – 'Design and installation of metal roofing'
Sarking and roof blankets
A layer of sarking (reflective foil laminate, or RFL) is then put in place. While this isn’t strictly compulsory in all cases, sarking’s generally considered to be very important for metal roofs to help prevent condensation. Condensation forming on the underside of the roof can cause roofing sheets to rust prematurely, and can also drip down to rot timber framing, wreck plaster and cause any number of other problems you'd be far happier not having to deal with...
A lot of the time, a foil-backed ‘roofing blanket’ that incorporates a layer of glasswool or rockwool is used instead of simply a layer of RFL. This serves the same purpose, but also provides insulation against sound transmission (e.g. to dampen the sound of rain) and adds a little more thermal insulation.