I had a screen-porch built off my kitchen. It follows the model of the "screen-door" porches that TOH ran this spring. The walls of the porch are wooden screen doors fixed into place, and we have glass storm windows to replace the screens. The floor is wood, and has 1/8 inch gaps between boards. Underneath the room is a four-foot high crawl space, built of plywood, housewrapped, and covered with fibercement siding. I could insulate the crawl space.
We like the porch so much that we want to use it all year round, but we live in Chicago, and the uninsulated nature of the room is going to make heating it a job. We would only need to heat the porch for about three or four hours a day.
My idea is to install a big electric or gas heater in the crawl space and to let the hot air rise between the floor boards to heat the room. Is this a reasonable plan?