The same insulation that retains heat in winter should deflect away the heat from the sun in the summer months. If your insulation isn't up to scratch you will notice that the room that is colder in winter is the same room that is warmer in the summer. I have such a room with a cathedral ceiling. The roof insulation in that room is just below the roof shingles and is some kind of white flattish covering. The only reason I know this is it showed when the roof was replaced some years ago. Also, 3 of the 4 walls are exterior walls, which doesn't help.