ᐅ Flat Roof Insulation for a Garden Shed

Hello, I hope you can help me. I bought a weekend property that was only used during the summer. Now I want to make it winter-proof so that it’s possible to spend a cozy weekend there in winter and get away from the city. The floor and walls are clear to me, but I’m unsure about the best and most cost-effective way to handle the roof. The cabin has a corrugated roof with a slight pitch and a suspended ceiling structure. Previously, there was just 6 cm (2.4 inches) of insulation wool laid on the ceiling structure. However, I found dead insects and a dead bee’s nest on it, so there must be small holes on the side (the roof itself is watertight). Now my question is: what is the best way to insulate? Can I do it the same way as before but with 15 cm (6 inches) of insulation and a vapor barrier in between? Won’t it get damp in autumn due to the humidity coming through the roof? I hope you understand what I mean. The pictures are a bit dark.