Sloping roofs, double-layer OSB and gypsum board for thermal insulation

I want to improve the summer thermal insulation during the roof conversion and therefore clad with two layers of OSB and plasterboard. An Isover KM UV with variable Sd value was used as a vapor barrier.

Now I fear that due to the diffusion-tight OSB boards, the vapor barrier can no longer perform its function as intended, i.e., to be diffusion-open in summer. Above that, 240 035 glass wool and a diffusion-open underlay membrane are installed.

I see the following alternatives
1. OSB and plasterboard as planned
2. double-layer plasterboard
3. single-layer plasterboard as usual.

Well, basically you write it yourself: "as usual"!

I have another option.

Diffusion-open wood fiber insulation boards instead of OSB. Then gypsum board on top. Has anyone ever installed something like that?

OSB is indeed a vapor barrier. However, I see no problem with that. OSB has mass, and mass helps with heat protection. I have not installed wood fiberboard yet. But it could be a possible solution.

OSB is indeed a vapor barrier. However, I don't see any problem with that. OSB has mass, and mass helps with heat protection. I haven't installed wood fiberboard yet. But it could be a possible solution

On the one hand, OSB could reduce diffusion into the insulation in winter, but on the other hand, it could also worsen the drying back to the interior in summer.

Therefore the underlay membrane is diffusion-open