What can I do with these roof trusses to cover them up a bit?
In the US, you can find rigid insulation panels of polyisocyanurate foam



and polstyrene foam



These can be cut to fit tightly between studs or rafters, can be flush with the face of the wood and are thick enough to be self supporting.  They are usually sealed to the wood using broad tape, such as aluminum.

They are not cheap and not very strong if hit, but have a very high R value.

They also could be covered with either very thin drywall or the new ultralight drywall.