Insulation boards as inter-rafter insulation in a cold flat roof

Hello everyone,

I would be very grateful for an expert answer. I am in the process of converting a garden shed. The roof consists of 23mm OSB boards. Above that, rodent protection and roofing membrane. Now to my question: I would like to clamp normal insulation panels between the rafters as inter-rafter insulation (see photos). Does that make sense, and especially, does condensation form anywhere there? The roof is ventilated through holes in the roof box (about 5cm in diameter and arranged approximately every 60cm). I want to hang the ceiling with a wooden structure. In other words, bring up a battens level and lay insulation wool (100mm) on top. Under the battens, a vapor barrier and another cross battens to which I want to screw the plasterboards. Does my construction make sense, or am I completely off track? I thank you for your answers

Foreword: I am just an amateur too!

You can forget about the EPS boards. If your ventilation concept actually works (I’m skeptical), the air layer between the mineral wool and the EPS would be ventilated through, and you would already have outside temperature above the mineral wool, making the EPS useless.
Why am I skeptical? As far as can be seen from the photos, it is a gable roof with a very low roof pitch. You want to ventilate through the holes below the eaves. Normally, you would ventilate through the ridge, but in your case it seems to be closed. Therefore, no air flow arises (not even between the eaves, how should it form?) and any moisture cannot be removed.

What remains? A construction similar to the so-called “self-composting flat roof” (just google it). Penetrating moisture will neither be ventilated out nor can it escape upwards (OSB) or downwards (vapor barrier). The consequence: your roof will rot away.

If I were you, I would get a professional involved with whom you can develop a suitable (functioning) concept together. Carrying out the plan as intended would be foolish.

Hello Dr. Hix. Thank you for your answer. Correct, it is a gable roof with a low pitch. The remaining piece of wall visible at the ridge beam will of course be removed. I had forgotten to mention that. This way, the air can circulate in the area of the rafters (above the ridge beam) and below the ridge beam (I think). I thought that the EPS boards might help reduce the heat in the ventilated space during summer. I'm just unsure whether condensation or mold could form there. Does this make things a bit better now? Thank you in advance for your response.
Regards, Ronny

PS: here are a few pictures where it might be easier to see.

As already mentioned, I currently do not see how the ventilation is supposed to work. For an airflow to occur, the air must somehow be set in motion.

-Thermals?
Warm air rises and in this case it stays there because the ridge is sealed.
- Wind?
It would have to blow from below into the holes, especially since in our latitudes it usually comes from the west, but as far as I can tell, the garden shed is positioned with the gable facing west.

Maybe with the help of fans at the holes?! But I still maintain: get professional help.

That sounds plausible. I thought the airflow occurs automatically when two sides of the roof have holes. I will probably consult a professional. Thanks for the quick answers. Regards Ronny

I would also say a professional. Although a few ventilation tiles at the very top might be enough. But it looks like a metal roof, I don't know what you do there, but there is definitely a solution. Otherwise, I think your plan is fine. Only the ventilation still needs to be properly resolved. And connect and seal the vapor barrier cleanly at the corners ;)