where the fibres serve to reinforce the material by using two materials having different heat insulating properties it is possible in an economical feasible manner to arrange the pipes 1 2 very close whereby the pipe assembly as a whole only occupies little space in general the distance between the pipes 1 2 can be reduced from the conventional distance d shown in fig1 to the small distance d shown in fig2 the economical feasibility resides in that only the relatively small space between the pipes 1 2 needs to be filled with an expensive high insulating material whereas the larger part of the insulation u2014 the outer insulating element 11 surrounding the pipes 1 2 and the inner insulating element 10 u2014 can be made of a less expensive conventional heat insulating material in a preferred embodiment the outer insulating element 11 is made of mineral wool such as glass wool stone wool or slag wool it may be semi rigid like conventional pipe sections of mineral wool but preferably it is made as a flexible mat that is cut to size and subsequently wrapped around the pipes 1 2 and the inner insulating element 10 the abutting ends of the mat can be interconnected by any means such as by a binding wire or adhesive tape not shown in order to receive the pipe bearings 3 4 see fig2 a slit 12 is cut in the mat before it is wrapped around the pipes 1 2 the inner insulating element 10 arranged between the pipes.