ᐅ Insulation between roof rafters using Armaflex?? Question!!!!

Hello, I have a question about insulation between rafters.
We removed the ceiling on the upper floor and want to insulate and cover the sloped roof in some rooms.
From the outside, there are slate tiles, underneath are boards with bitumen membranes attached. My idea is to install 19mm Armaflex from the inside, then add fiberglass insulation on top of that.
I am an insulator but mainly in industrial settings, not roofs. Armaflex is vapor diffusion tight, which also concerns me...
The assembly from outside to inside would be: slate – bitumen membranes – boards – Armaflex – 140mm (5.5 inches) fiberglass insulation – vapor barrier – counter battens – 30mm (1.2 inches) fiberglass insulation – interior cladding.
Maybe someone has an opinion on this and could kindly share their thoughts.

Best regards

Kiwi&Tina schrieb:
The construction from outside to inside would be: slate - waterproof membrane - wooden boards - Armaflex insulation - wool (140mm (5.5 inches)) - vapor barrier - counter battens - wool (30mm (1.2 inches)) - interior cladding.

This setup already suggests a risk of mold forming in the 140mm (5.5 inches) insulation. The airtightness should decrease from the inside to the outside. Also, why use wool twice? And what kind of interior cladding are you using? If it’s gypsum board, then the 30mm (1.2 inches) wool might also develop mold.

I’m already really liking it here... well, I thought I’d just give it a try and ask… maybe there are others like you who write such great answers… Your questions (if that was even a question) I won’t answer because I’m not really interested in that type of discussion.

Kiwi&Tina schrieb:
Maybe someone has an opinion on this and would kindly share their thoughts.

Kindly, I have shared my opinion on this.

Kiwi&Tina schrieb:
I won’t answer your questions (if that was even a question) because I’m not really interested in this type of discussion.

So what are you looking for then?

All right, as long as we keep a normal tone.
I want to write something about this, not just repeating "mold, mold, mold!!!" I thought that seemed contradictory, so I’m trying to educate myself. What confuses me, or what I don’t fully understand, is about the waterproofing membranes on the boards... aren’t they airtight as well? Or are they vapor-permeable? (I have no knowledge about that.)
I wanted to avoid adding extra thickness to the rafters, so I was thinking of adding an additional 30mm (1.2 inches) to the substructure, which I’ve seen done before. You often read that it should be as airtight as possible, so I thought Armaflex might be an option since I have the material available.

Please first describe the complete current condition. The structure in the first post is a mix of old and possibly new parts; it is not clear which parts are old.

Then there is the question whether it is a real waterproofing membrane or just an underlay membrane.

In the first case (i.e., waterproofing membrane on boards), the boards are probably ventilated from behind. So it’s good that the waterproofing membrane is sealed, but moisture is removed through the ventilation.

Kiwi&Tina schrieb:
Man liest immer möglichst Dicht

Yes, that is often said. But basically, from inside to outside, the layers should become more vapor permeable. Moisture should not enter the structure; and if it does, it should have a way to escape.

And your setup does not provide that in two places. With only gypsum plasterboards, moisture quickly penetrates into the 40mm (1.6 inches) layer and passes very slowly through the vapor retarder; then it accumulates completely at the Armaflex.

Kiwi&Tina schrieb:
I wanted to avoid adding thickness to the rafters, so I planned to add another 30mm (1.2 inches) in the substructure, which I have seen done before.

In that case, though, the vapor retarder was not placed between the two layers, right?