Natural gas.
The house is right at 100 years old. Beautiful oak woodwork downstairs, pine upstairs, but with the same 10" baseboards and pattern in the doors and windows. Probably 50 years ago somebody had the bright idea of painting it all white. 25 years ago the downstairs was stripped. We moved in 2 1/2 years ago, and our intention is to remove all the paint from the upstairs woodwork, returning it to what it was originally. We started out almost 3 months ago on one of the bedrooms, and after several weeks of trying to strip the paint with the wood in place, I removed all the baseboards and molding around the door and windows, taking it out to the garage to strip. The whole process has taken a lot longer than we ever expected (it won't take as long to do the other rooms, I've learned how everything fits together, and how to get it off without busting it up, and I've picked up some very handy tools), and I've been using a kerosene heater in the garage to take the chill off, in order to work comfortably, and keep the temperature where the stain and varnish will dry. The past few days the outside temps have dropped enough that I have added a propane heater at times. The project will be done by this weekend, and we won't need those heat sources anymore.
Related to heating costs, when I had the windows out, including a picture window that is about 5'x6' and three other standard windows, it was very obvious that the storm windows were pretty much useless as is. Not that there was anything wrong with the windows, but the storm window frames were not sealed against the window frame. You could see daylight almost all the way around on some of them. They are now all well caulked. When I get done with this project I will be checking the remainder of the windows to see if they are in the same condition. This room seems warmer already. It's my daughter's bedroom, and she is almost always cold, so it will be interesting if she is warmer when she gets back in the room.