When I lived in Minnesota, local TV station did a story about a latter day hippie, living out in the woods in an old Airstream trailer. He'd built a stove for himself - as I recall, a pretty clever design - and he kept warm during the winter by burning junk mail. He said that he went out of his way to get on every free advertising, catalog, and newspaper list he could, and he seldom had to supplement his fuel with wood. Maybe the paper didn't provide as many BTUs as wood, but he got a LOT of it. Free.