Timber frame builder replaces OSB with gypsum fiber boards - equivalent?

..."OSB is a universal weapon" because ... more or less breathable,

"Breathable" is absolutely nothing at OSB.

I could have expressed it in numbers too: the diffusion resistance realistically lies between values of µ = 70 (OSB/3) and µ = 200 (OSB/4). Gypsum is between 5 and 10, concrete at 150. According to your definition, diffusion either exists = 1 or not = 0.

However, I do not want to deepen the topic.

Thanks to everyone who participated!

Diffusion != Air permeability but whatever

After months of searching for timber frame house manufacturers offering credible wall constructions, I decided on one from Bavaria.

Wtf whatever a "credible" wall construction is supposed to be, you found a Dane - Danes don’t lie, says Otto - in Bavaria. I suspect it’s about the structure of the exterior wall. And if I understand correctly, the OSB is now to be replaced under the room-side cladding with gypsum fiberboards by gypsum fiberboards as well. I still assume the exterior stiffening cladding is OSB, and it doesn’t seem to me that vapor diffusion is at all the issue here, since the replacement of a membrane is not even mentioned.

Building without a vapor barrier in the exterior wall is not unusual nowadays. I had already asked about the exact wall construction.

The builder has not specified more precisely either! I only received the letter a few hours ago and wanted to equip myself with your combined pool of knowledge before I ask about it.

I am no longer so horrified by now. When I opened the letter, I felt a rush of heat at the thought of moving into a gypsum board house.
By now, I actually see the advantages of the alternative and am much more relaxed: more diffusion-open, just as load-bearing for hangings, non-combustible, no emissions, vertically load-bearing, so also structurally, statically load-bearing, more soundproof compared to OSB, more resistant to summer heat, omitting foil POSSIBLE.

and am much more relaxed

Then you can now get down to work, answering questions or commenting on assumptions.