Sleeping While Car Living
The floor of a car can be a very uncomfortable place. In the summer, it heats up and stays hot for a long time. In the winter, it sucks heat. It's a good idea to insulate the entire rear-living area of your vehicle, but if nothing else, its essential to have a good barrier between your body and the floor when sleeping. Air is the best barrier and thus I highly recommend an elevated cot or bed. I built a bed from dumpster-dived plywood and two by fours (an abandoned shelf). I now have a 14 inch gap between the bottom of the bed and the van floor. I slide my plastic storage bins under the bed. I nailed a camping pad (closed cell foam) to the plywood... and stapled another layer of foam bedding (open celled foam) on top of that. I put a sheet over the whole thing and now I have a comfortable (and firm) bed. I'm protected from the temperature extremes of the car's flooring.
In his carliving bible (10 Years of Living in Cars), Craig Roberts suggests wrapping the floor, side walls, back wall, and ceiling in 6-12 inches of upholstery foam. I price checked this stuff and it was WAY too expensive for me. Plus, I live in the south so such insulation was never necessary. However, I may be moving to Vermont this Fall and will need alot more insulation to survive the Winter.
For cold weather, a good sleeping bag with a low temperature rating is highly recommended. In addition, you can buy fleece sleeping bags which are cheap and can slide inside a regular bag... thus lowering its effective temperature rating even more. And finally, wearing warm clothes to bed is another way to increase warmth. During a week of 7-15 degree weather this winter, I used the fleece bag inside my sleeping bag, wore thermal underwear and wool sox, and wore a wool hat. I cinched the hood on the sleeping bag. With this combination of methods I stayed perfectly warm while sleeping. In even colder weather, you can sleep ON TOP OF a 12volt electric blanket.
Note: Cheap cots can be bought at K-Mart or Wal-Mart. If sleeping in a compact car, where space is a premium, I still advise building a raised platform.. even just a few inches... to keep your body off the car floor. I built a bunk in my Nissan Sentra (hobo experiment 1) that ran from the back window-sill, over the passenger seat (which was lowered back), to the front dash. I had only a few inches of nose room but it worked. In retrospect, I should have removed the back seats and utilized the combined trunk-rear area as a living space (possibly should have removed the front passenger seat as well). Oh well, live and learn.