Make a wooden frame against it and insulate it with room insulator. If you cant afford that newspapers or something less water soluable would work fine. I think.
Grey being insulation
Brown being wood
Black dots being nails.
Remember to mylar over the wooden surface at the end. Or you could add another layer of uslation.
Bewarned tho, if you dont user proper insulation and use something like newspaper, if there is a fire, do not expect it to go crazy. Newspaper burns like a bitch. So do it right and get the right insulation.