1/ eal the beams with a round-edged flat of at least 15 cm.

2/ Attach the rafters lengthwise along each beam.

3/ Install the prefabricated ceiling panels on the beams, between the rafters.

4/ Cover with the desired thermal insulation.

5/ Attach other rafters perpendicular to the existing beams and rafters.

6/ Install and attach pressed wood plates or other boards on the rafters in order to use the attic.
(You will need a design study to determine the sections of the various wood components.)

Installing under an attic between existing beams

Installing under an attic between existing beams