Rafter insulation with Armaflex?? Question!!!!

Right... we have thick insulation between the rafters and then thin insulation between the battens. There is no foil in between. Just that - I don't have a more detailed plan on the topic.

First, describe the complete current state

The current state would be above the ceiling, the shell construction, since I had a roof window installed, I know that there is roofing felt on the outside. Below the ceiling, the rafter is insulated with Styrofoam (which will be removed).

In a Bauhaus video, which seemed quite professional, they showed insulating rafters, putting a foil on, sealing it tightly, putting battens for drywall panels on, and then adding insulation between the battens again. That also seems strange to me.

I'll try to attach a picture of the current state.

The question now would be to leave out Armaflex completely.... only use 160mm wool. Would you then need ventilation?

The structure is problematic but not uncommon. That doesn’t mean you can or should just do it like that. The insulating felt between the battens for the drywall is not a problem if the insulation behind the vapor barrier/permeable membrane is sufficiently thick. You can calculate when that is the case, for example, yourself at u-wert.net. I think it’s a very good tool! Basically, it’s sufficient when even at the coldest outside temperatures the dew point is not reached in the insulating felt in front of the vapor barrier. I entered it into the calculator once, also out of personal interest. GK 12mm Insulating felt 40 mm Vapor barrier Insulating felt 160mm Spruce 40mm Bitumen sheet 5mm Brick including battens. Then you have a condensation problem at the sheathing/roofing felt. You won’t get rid of that even with stronger insulation. I think that’s logical because nothing passes through the roofing felt. If there is still an air layer, or better said, ventilation with outside air between the sheathing and insulation, the problems disappear. That’s also clear, I think, because the moisture that could enter the insulation from the inside can dry out. The only question is how to implement something like that and whether this is a common method. I don’t think it is. I believe most old buildings have insulation directly on the sheathing. Maybe they just rely on the vapor barrier/permeable membrane being tight and the problem being solved anyway. From what it looks like in your case, you have ventilation holes in the gable wall. Therefore, I assume you once had a cold roof? That means the upper floor ceiling was insulated and the space between the rafters was not. It also looks like the Styrofoam only slightly extends over the floor of the upper story. Why do you want to insulate up to the ridge beam now, or did I misunderstand something? It would be easier to just insulate the upper floor ceiling again; then the moisture from the insulation can escape and the roofing felt on the sheathing doesn’t pose a problem. I advise you to consult a specialist. Get advice on which structure is right, and that’s it. The consultation won’t cost that much...

The U-value calculator even knows Armaflex. If you enter all your layers there, such as gypsum plasterboard, glass wool with battens, vapor retarder, glass wool, Armaflex, spruce, roofing membrane, slate, you can see 4 things (if you want to use the Armaflex).

You have to be extremely careful that the vapor retarder is really airtight, so that no moisture can get into the wool (if using Armaflex and without ventilation as in your original proposal).

It must not be too airtight either, because the top is vapor-tight (if the Armaflex is behind the insulation).

Without ventilation due to your existing spruce/bitumen layer, it will not work.

And regardless of that, I would not clamp the vapor retarder between both insulation wools. Under unfavorable indoor humidity conditions, you have condensate in the 40mm insulation layer.

Actually, the approach should be:

Gypsum plasterboard, vapor retarder, glass wool with battens, glass wool ventilation, roofing membrane, slate

First of all, thank you, I'll write this evening, now it's going to be a lot via phone...

Hello, it took a bit longer.

As it looks with you, you have ventilation holes in the gable wall. So I assume this used to be a cold roof with you?

It was a cold roof, there was wool between the ceiling joists, and on top of that was complete Styrofoam.

It also looks like the Styrofoam only extends a little over the floor ceiling

Yes, the rafters are currently insulated with Styrofoam, and on the left in the picture you can still see a piece of the old slope, which is supposed to stay like that, with Heraklit in front of it.

Why do you want to insulate all the way up to the ridge beam now or did I misunderstand something?

The plan is as follows: the beams in the ceiling should remain visible, it is the bedroom, which might not get too hot in the summer thanks to the high extension, possibly I have already thought about installing a fan under the ceiling leading outside...
Where can I find a professional who can also be trusted?
How could one realize ventilation behind the insulation, because if I put wool into the rafters, it automatically presses down below, even if it were 2cm narrower, so to speak.
I can probably forget about Armaflex, although I think I could make it airtight. I’m an insulator, just in industry and not in house construction.
Best regards, Kevin