Any combination of 2 layers of 12.5mm/15mm plasterboards each side of a stud frame will be classed as Severe Duty.

To achieve Severe Duty with a single layer each side of the stud frame would require the use of 15mm LaDura, Universal, Megadeco or Aquaboard.

Of these, LaDura is classed as H1 moisture resistance to BS EN 520, Aquaboard is classed as GM-H1 to BS EN 15283. Siniat recommend Aquaboard for wet areas, LaDura in areas of high humidity.