Holy crap it’s cold! What do you do for the trunk?

[Motrukdriver]

These will keep your feet and toes warm all night long.

https://www.amazon.com/Livefine-Adh...coding=UTF8&psc=1&refRID=0AK3HBP2GZSEM1KBKX4E

 

[akrvbob]

The single most important thing about warm feet is that they stay dry. Do NOT wrap anything around them that can make them sweat like plastic or reflectix.

Before you go to sleep, take off your damp socks and dry them with a warm dry cloth and rub them till they are warm If you go to bed with them wet and cold—it will be extremely hard to warm them up.

Once they are warm and dry, put on thin, fleece socks with very heavy wool boot socks over them. The fleece will wick the moisture away the wool will keep them warm even if slightly damp. NEVER WEAR COTTON SOCKS TO BE IN THE COLD!! The socks must be loose enough to allow good circulation but still not have pockets of air.

Find a decent down coat and put your feet into it and zip it up. Tie something around the neck so it’s closed. It needs to be a little loose, but no air pockets.

 

[MrAlvinDude]

To limit the heat/cold exchange with the outside, in the trunk area, I would follow some of the suggestions already made, that involves cardboard as a building material.  

I might try to make the cardboard into a shape where my sleeping bag/feet would fit into, without making it a tight fit at all.

The goal would be to limit the exchange/escape of heated/cold air with the exterior of the car/outside,  but still allow plenty of space around your body, so any moisture/sweat from you body, has somewhere to go, and will not be trapped close to you, or in the walls of your sleeping bag.


Very close to your skin, I would follow Bob's advice, to use layers that would aid in keeping damp/sweat/moist away from you skin, but allow heat to remain with/close to your body/skin.

Water can absorb a lot of heat. So any dampness or moisture very close to you will be able to simply suck heat away from your skin.

On the other hand, that heated/moist air, should remain in the car/your sleep box, long enough to exchange the heat, and just let the remaining moisture get out. Or everything becomes damp, and near impossible to heat. So a certain level of ventilation is just as important, as not trapping the moisture entirely. 



Wool is a magic material when it comes to managing/handling things like heat/cold and sweat/moisture. So I would use wool socks and wool sheets/plaids/sweaters, in combination with other modern materials.
Wool can do that magic trick, of trapping heat, and allowing moisture to condensate, without the wool becoming soaked in or absorbing the water. So you could say that wool allows the moisture to escape, but it keeps in the heat. 
Again Bob has listed the good clues, of how to use first fleece (or similar) and then wool.


I have all my life had issues with keeping my feet cold/warm, so I only use socks with a percentage of wool in them.


To keep my feet cold/warm, when in my bed, I alternate my blanket/duvet. Meaning, I get under the covers, and allow my body to heat my cover (for a few minutes), then I spin my cover, so what used to be at my feet, are now over my chest.
This gets my feet warm!
And I repeat this process a few times, until my feet and legs feel warm.

This is one reason I never use a sleeping bag. And one reason that I would need to build a cocoon, if I were ever to sleep many nights, in my car or tent.  I would need that cocoon, so I could keep all my sleeping tools warm.

It is very likely that I would make my first car-cocoon from cardboard and duct-/package-tape. To get a good idea about optimal shape. To find that balance between a limited space to keep warm, and enough space to freely move and allow moisture to get away from all my sleeping tools. 


Whenever I can not use my usual methods of how to manage my feet/leg temperature, then I am a great fan of electric heating blankets.
So I own several I can use when on shore power, and several of those loose car seat heaters.

Adding one of those car-seat heaters into the mix of layers I use to wrap around my feet and legs, does wonders for me. I only need to use it for maybe 10 to 20 minutes at a time, and it will help to produce the same result as when I use the cover-svirvel-trick to use torso-heat, to bring a warm duvet down to my feet/legs.

 

[jonyjoe303]

rtech insulation cut it with a boxcutter and just use gorilla duct tape. It works extremely well. Its foam doesnt weigh much, use alot of it. 


the coldest its got so far is 46 degrees, with just the foam, my thermometer. Chilly but not even cold enough to use a sleeping bag. I'm still waiting for colder weather to test it more fully. My goal is not to use the mr heater this year.

 

[WanderingCanuck]

I've done a bit of cold weather backcountry camping, and some of the advice I see here is good while some is not quite there. For starters, down insulation is hands-down the best. It's also the most expensive and somewhat difficult to care for, so if you're living in a vehicle out of necessity it's probably not in your budget. Synthetic insulation is very good though, just heavier and bulkier for the same R value so it should be fine for your use.

Regarding moisture, especially your feet. Your feet will sweat all night long, even if they aren't hot. In fact, your entire body will - and more than you'd think. That moisture, as steam, travels from your warm body through your insulation until it gets cold enough to condense. This is called the "dew point", and with a sleeping bag the temperature where water condenses is often near or at the outside surface of the bag. This means that the water starts to soak your sleeping bag from the outside, in. This lowers the insulating value and traps even more moisture because wet insulation doesn't pass vapour as easily. This is why being wet and cold is so dangerous. Most insulation is useless, or at least severely compromised, when wet. There are other reasons, like evaporative cooling, but that's less the issue here.

What to do about it? Use a vapour barrier inside your sleeping bag. A garbage bag was mentioned earlier, and this is not a bad idea. Yes, you will get damp _inside_ the sleeping bag, so you'll want to minimize what's between you and the vapour barrier - either bare skin or a very thin wicking layer. It isn't the most pleasant thing initially, but I guarantee you'll be warmer. Make sure you have a dry set of clothing to change into in the morning or if for any reason you need to evacuate during the night.

Make sure you have lots of ventilation in the vehicle too because the moisture in the air you exhale will build up in the vehicle and condense on anything cold - like the outside of your sleeping bag. Likewise, don't breathe on or into your sleeping bag. Trying to "save" the heat from your breath is a lost cause and will make you colder.

Some other tips to generate and keep in more body heat:
- Eat high fat foods before going to sleep, like chocolate, oil, and butter. If you wake up cold, pop another cube or two and go back to sleep. This keeps your internal furnace running.
- Stay well-hydrated. If this means waking up a couple times during the night and using a pee bottle, so be it. You'll be warmer.
- Wear a hat, even if your sleeping bag has a hood. It keeps your sleeping bag cleaner too.
- Make sure your socks aren't tight. In fact, if you use the vapour barrier method you might find you don't need any at all.
- Prevent your sleeping bag from pressing against anything - like feet squished up against something. You need loft (dead air space) to insulate.
- Check that warm air isn't leaking up past your neck/shoulders. You've burned a lot of calories to make that heat, keep it inside the bag!

 

[MrNoodly]

South isn't much better right now. I'm in Jacksonville FL and it's been mid-30s for the past couple nights.  

One hell of a cold front. Brrr.

The OTHER south. Arizona, California, Mexico.