Many pre-insulated pipes use rigid polyurethane foam and a HDPE casing.
Over time, the insulation efficiency of the pipe structure decreases, caused by the permeation of air (oxygen, nitrogen) into the foam cells and the diffusion of cell gases (blowing agent, carbondioxide) out of the foam cells.
This reduces the adhesion between the PUR-foam and inner pipe, weakening the structure and compromising the mechanical integrity of the entire pipe system.