Insulation of the upper floor ceiling in a cold roof

Dear members,

I want to insulate the upper floor ceiling (22cm reinforced concrete ceiling) with 2 x 14cm EPS rigid foam boards.
The foot purlins of the pitched roof (cold roof) rest on a masonry separation layer directly on the floor ceiling. On the eaves sides, there is ventilation of about 2 - 3 cm along the entire length.

Even though it is sometimes said in the forum that no vapor barrier is needed for a reinforced concrete ceiling, I will still install a vapor barrier and edge it up at the gable wall and the foot purlins (so that it looks like a tub). I will then install the EPS rigid foam boards staggered in this.

I have the following questions about this, maybe someone can answer them for me (see sketch).

- Should I stick the vapor barrier (foil) directly to the foot purlins or can I also stick it in the bending area at the edging onto the reinforced concrete ceiling? Which adhesive would you recommend (e.g., Alujet)?

- Does it make sense to tape the joints of the EPS rigid foam boards? Or would that be rather problematic?

- In the area between the edged vapor barrier foil and the foot purlin, there is still a gap, in which I also have to add insulation. This insulation would then lie directly on the foot purlin without a vapor barrier or vapor retarder. So it must be diffusion-open, I think. What type of insulation is suitable for this (mineral wool, wood fiber, rock wool, ...)?

Maybe someone has a picture of a building situation like mine, where I could see how the foil and insulation are installed.

Thank you very much.
Best regards, Sebi

Picture of the roof truss and upper floor ceiling

If you leave out the vapor barrier, lay the boards only loosely, you minimize the risk of building damage and save yourself the work. The vapor barrier is unnecessary. To insulate cavities on the cold side of a component, any insulation is suitable, the more diffusion-open the better. Outside (that is on the EPS insulation) no diffusion-tight layer should be installed (OSB and co.).

Hello Jumi1, thanks for the message. I also think that I can lay the EPS rigid foam boards on the raw concrete ceiling without a vapor barrier. At least without a foil, it is ensured that the moisture (wherever it may come from) can escape through the underlay membrane or the ventilation openings on the eaves side.

Can you lay an EPS rigid foam board directly on the foot purlin, or should another material that is more vapor permeable be used here?

Best regards and thanks again for your answer.

Can you place an EPS rigid foam board directly on the foot purlin, or should you use a different material here that is more vapor-permeable?

Regarding vapor permeability, I wouldn't worry too much about that in this case, but EPS boards are too rigid for the purlin and its unevenness or for the installation situation between the rafters. Better use a layer of flexible insulation mats here. Whether glass wool, stone wool, jute, or wood fiber depends on your preference; otherwise, they all work equally well.

Should the insulation on the ceiling be walkable? EPS boards are not. If so, you would have to lay a load distribution layer on top, e.g., made of chipboard. Unbonded EPS boards are very slippery and gaps easily form. If you use EPS, best with a tongue-and-groove edge.
If it does not need to be walkable, I would lay mineral wool mats (or natural materials as mentioned above) in two layers instead of EPS. For occasionally walked-on areas and storage rooms, wood fiber boards with a compressed surface are recommended, as you don't need an additional load distribution layer and they do not slip as easily as EPS.