ᐅ Insulation of sloped ceilings. Vapor barrier or interior membrane?

Hello,

we want to insulate the sloping ceilings in the office and small room of our settlement house from 1959. The previous owner had roughly stuffed Styrofoam (polystyrene foam) from above on both sides, but it was completely makeshift. It doesn't even fit remotely. The wall itself is coated with straw/plaster. We have already removed this. The roof is now exposed. The wooden beams and Styrofoam are dry, and although the roof is still from ’59, it looks good from both inside and outside. The beams allow for 12 cm (5 inches) of mineral wool. I now want to fix a breather membrane from the top down, insert the insulation, and here comes the question:

Vapor barrier on the inside – yes or no, and why?

My father-in-law, the internet, and my brother say yes. A good friend who is a project manager at a large construction company says no. He argues that you shouldn’t completely seal a house like this, that it doesn’t help much for a house from this era, that everything is dry anyway, and the indoor humidity can escape better. The windows are new. The attic will be insulated.

I’m torn and would like to start the project next weekend. What do you recommend?

Thanks and best regards

Hello! Is there a photo or sketch available?

Well, in a way, both are right.
If you hire someone to do this, in two out of three cases it will be botched, which would make things even worse. So it's better to have no vapor barrier and just consistently bad work.
Of course, a vapor barrier would be better—if you believe it can be installed properly, then include it.
12 cm (5 inches) is actually too little... if the effort is justifiable, I would install double that thickness.

Is there currently no underlay membrane beneath the roof covering? The problem with old bitumen membranes is their low vapor permeability. They do not allow moisture to escape. If you then install a vapor barrier (also vapor-tight) on the interior side and leave small leaks, condensation will form there and cannot dry out.

Vapor retarders (vapor semi-permeable) allow some moisture to pass through in both directions. Vapor barriers hold back moisture more effectively, but no DIYer can achieve truly airtight installation. That’s why vapor retarders are the better choice.

Not using any membrane at all would cause condensation to form on the exterior side. If there is no waterproof underlay membrane sealing that side, the condensation can still dry out.

My preference would be a vapor-permeable underlay membrane and a variable vapor retarder like Isover KM Duplex.

We definitely want to insulate the ceiling of the top floor. The surface in the attic is very smooth and easy to walk on. The previous owner poured something onto the top floor ceiling; it might be lightweight expanded clay aggregate (LECA) or similar—is LECA generally walkable? It could also be concrete, but the material contains larger granules and is gray in color. I actually want to insulate the attic floor on top of this—would you recommend doing that, even if it is LECA?