The number one most very important thing you need to get through the winter warm and happy in your van is down. You need a few good down comforters (like this one). I layered mine and then shoved them all into a fancy duvet cover that my aunt gave me so that it’s like one big, very floofy, blanket. I was going for an eight inch thickness, but ended up closer to sixteen inches, and I must say it’s very warm.
It is not enough to have feathers above you, though. You also need feathers below you, in the form of a feather bed (like this). This does much more than you’d think. It insulates you from any cool drafts or frozen memory foam that might exist below you. It holds your body heat and keeps you warm in a way that frozen memory foam and futons never will. Perhaps most importantly, it poofs up around you to seal tightly with the comforters. No cold air leaks here.
All these feathers will keep you extremely warm, to the point of sweating stuffiness and pushing back the covers to let in cool air, down to about ten above. From ten above to about ten below you’ll be comfortably warm, without the sweat. You’ll probably still wake up once or twice to push back the blanket and cool off. When the temperature drops below ten below you’ll start feeling more serious and glad for all those feathers. Below zero, you might want to braid your hair or tie it back from your face to avoid waking up with a face full of frozen frosty hair from your breath.
In the Tara method of winter vandwelling, this is the extent of special warming/insulating. Of course you need reflectix on your windows to prevent cold drafts, but you probably have that already. To insulate further you would need to make a thick wall of reflectix behind the seats, and that just wouldn’t work for me or Bro (he’s gotta jump on the front seats to get in bed). For this to work you would need to be incredibly efficient and airtight, AND you would need a way of heating the van from the rear. All my heat vents are in the front, and I’m not inclined to add propane heat to the rear when I know I could be perfectly warm under all that down without the lost space and added risk of burning gasses.
(Soon to be followed by: Winter VanDwelling: Electronics)