So now all the slanted roofs are done

damn, what a difference in temperature it made, it was 25°C inside when I got home from work yesterday.
But I have a question, I've now put in wind paper and battens, but do you have suggestions for what to use in the slanted roof? I was first considering MDF that is grooved nicely, looks very nice
And then on all the walls in there, can you put something against them? Or do you have to frame and have an air gap there too? Because now it's just rough boards, doesn't look very nice, so the floor will be painted as well.