Insulation, too. A lot of sheds and cabins (and single-family homes) could do with more. Saves on cooling bills in the summer, too.
Anyway, in a cold climates like mine an acre would not be enough to keep you warm because trees grow slow when it's winter six months out of twelve. It takes a lot of wood to keep you warm, and one tree gives surprisingly little wood. After a handful of years you'll have clearcut your measly acre and still have another couple of decades to go before it becomes harvestable again. I'd say you want at least 10 acres if you manage it well and the soil and drainage are at least half decent.
They grow faster since they already have a root structure, the size is consistent and the wood is generally straigter (the japanese have a similar technique for extremely straight cedar logs)