In new buildings, the question does not arise as the DTU requires the implementation of tightness with Sd ≥ 18m. During renovation, the installation of a membrane is only mandatory in the case of a supporting floor made of small elements with tight joints (for example, wooden slats). Otherwise, this question refers to paragraph §4.3.1 of CPT 3647 regarding “Implementation of thermal insulation processes added to attic floors and lost attics …”. The requirements are a bit complex, but the more general case consisting of a ventilated tile roof, a flat western climate outside the very cold zone, and no distribution surface above the insulation, must be retained. In this case, the sum of the resistance to vapor diffusion of the support floor (ZDP) and the insulation (ZDI) must be> 3 m².h.mmHg / g. Just know that from R ≥ 6 m²K / W in Nita-Cotton, the ZDI alone exceeds the requirement. Therefore, the vapor barrier is not necessary.