Advice on warming a room

Our dining room is very cold- has a sloping ceiling varying from 7'6ins to 14ft approx. -it used to be the garage and now one side wall is the dividing wall to the present garage. We would assume there is no lagging. in the roof of this room AND when lifting up the carpet one finds chipboard flooring beneath which is just earth. My husband spends a lot of time in there on his laptop and finds he needs an electric fire on in addition to the radiator. Can you give us any advice please on a reasonably priced way to improve the warmth in this room?