To provide a heat insulating material for a roof, capable of preventing dew condensation while improving heat insulation performance of the roof.

A heat insulating material for a roof includes a fitting portion 25 which has a lateral dimension determined in such a manner that both side surfaces abut on opposed side surfaces of both rafters with predetermined pressure in incorporation, and a pair of folded portions 27 for forming a ventilation channel between the fitting portion 25 and a sheathing roof board. Each of the folded portions 27 comprises an extended sheet portion 28a which is formed by extending a top surface sheet 21 of the fitting portion 25, and an extended heat insulating layer portion 28b which is formed by using a heat insulating layer 23 bonded to an inner surface of the extended sheet portion 28a. The extended sheet portion 28a is folded back centering on a boundary portion B between the extended sheet portion 28a and the top surface sheet 21, and fixed to abut on the top surface sheet 21.