We enclosed our previously screened-in carport, placing two small conventional windows on the SW side, facing our tiny sideyard. Both the SE side (facing our back garden) and NW side have sliding glass doors, the NW doors having glassed-in miniblinds. Both sliding doors have thermal drapes that can be used to cover them completely in weather that is too hot or too cold, and I have both a portable dehumidifier in the SW corner and and a large "end table" automatic quartz heater in the NW corner. I placed three large area carpets over the concrete slab, giving me essentially three different areas in which to arrange things. The slab absorbs both heat and cold, so if I am attentive to the room's needs it is entirely possible to keep it comfortable year round by adjusting the drapes, ventilating, and running the dehumidifier and heater. My sliding glass doors are screened, so on a good day I can open the doors at both ends and have a wonderful breeze through the entire room, and almost have the look and feel of a sun porch *and* a screened in porch, but without the inconveniences. The NE side opens into our tiny dining area, and we have an air conditioner in there. On very hot, humid days, our "sun room" is cooled sufficiently from that and a ceiling fan we added. This is a fairly low cost solution for anyone with a roofed-over carport or garage that they never park in; it's incredible the amount of living space that can be added to a home--it's now the largest room in the house, and the most attractive in terms of view, lighting, and general comfort.

We used to have a storm door that the screen always stayed in, but the plexiglass (or whatever it was -it wasnt heavy glass) was in a frame that we would mount on the door in winter and take off during the summer. It was easy to put on the door and had little hardware that turned to go over the frame and hold it. I am unaware of any ready made product on the market like that for porches, but I think it would be reasonably easy to make.

You don't want just a space heater. When we bought our house 33 years ago, it had what had been a screened in porch that the owner had put in sliders and turned into a 4 season room. It had a small gas furnace; that got struck by lightning after we'd lived there a few years, and one that size was no longer made, plus the gas line was outside and had corroded and the gas company turned it off. It would have cost nearly $1000 to run a new gas line, even for this short distance. So, we tried the space heater/ceiling fan thing for many years. It was cold in the winter and hot in the summer so we rarely used this nice porch room. After my husband died, I decided I wanted to bring plants in from the terrace for the winter and I needed heat. I found electric baseboard heat, hardwired with a thermostat online and my SIL installed it for me. I quickly discovered that the AC for the first floor of the house (we have a zoned system) did an excellent job of cooling the porch as well, especially with the ceiling fan. I was lucky that there was already a screened porch there and the roof line was an extension of the roof of the house. That saved the previous owner a LOT of money. Last summer, I replaced the cracked plaster ceiling with painted beadboard and it looks terrific. I love this room and enjoy it with all its plants in the dead of winter. In MI, you may want to include a skylight on the roof to bring light into the sunroom and the house. I don't think this will be cheap, even with an existing slab for the foundation. If you do this, be sure to have a plumber install a faucet (like your outdoor ones) so you can easily water plants. We did this years later and what a great idea it has been. In England, one sees conservatories on the back of lots of houses, even modest ones. Unfortunately, in this country they are very pricey.

ptreckel - that's a great question. I'm not sure I know though :) I kind of want it to be both, if that's possible. I do want it to be a porch. But want to make sure it's cozy and comfortable as well, like a "room" in our home. I'm worried that slate flooring would feel cold. I originally thought of doing a grey washed oak floor with white walls, and a dark ceiling. I also liked the idea of a pretty blue ceiling like the ocean. I'm leaning towards neutrals (grays, whites, darker ceiling) and then doing fun things with couches, pillows, etc.