An aeraulic crossmember for a motor vehicle includes a beam (12) in the form of a section forming a channel (13) along the length of the beam. The aforementioned channel is equipped with an opening (14) on one of the side faces thereof and contains a duct (15) which is made from rigid insulating material. The duct is also equipped with an opening which is defined by a mouth that is located opposite the opening in the channel and which is slightly smaller than the latter. The opening in the insulating duct is located inside the channel. The crossmember includes an interface device (16) in the form of a sleeve which extends through the opening (14) in the channel in order to connect the mouth of the opening in the duct in a leaktight manner to an air vent that is located on the exterior of the crossmember.