ISO 15758:2004 specifies a method to calculate the density of water vapour flow rate in cold pipe insulation systems, and the total amount of water diffused into the insulation over time. This calculation method presupposes that water vapour can only migrate into the insulation system by diffusion, with no contribution from airflow. It also assumes the use of homogeneous, isotropic insulation materials so that the water vapour partial pressure is constant at all points equidistant from the axis of the pipe.
ISO 15758:2004 is applicable when the temperature of the medium in the pipe is above 0 °C. It applies to pipes inside buildings as well as in the open air.