Term: Sarking Membrane (WRB)

A layer of flexible insulation typically installed under roof tiles when a home is being constructed.  It consists of aluminum foil laminated onto paper or plastic backing with a flame retardant adhesive and fiberglass reinforcing mesh, and is manufactured in sheet form. Sarking performs a dual function, helping to keep your home insulated from the heat of summer while retaining warmth in the winter.

Source: http://www.homeimprovementpages.com.au/article/reflective_foil_sarking.