More like what wasn't Jun using to keep warm? Have you noticed what he was currently wearing?

"Heating charm, eating hot foods, extra layers of clothing and blankets," Not to mention possibly burning his boring textbooks in the common room fire. And clinging onto Connor because body heat. Though the Gryffindor would shove him off sometimes. Such a rude bestie to let his Ravenclaw friend freeze in his tower. Or anywhere in the castle.