Thickness of the insulating layer determines its ability to ward off cold, in general. The difference between down and synthetics is that for a given insulating value, the synthetic fill will be appreciably heavier.

This matters in a bag used for backpacking, but not for something you will keep in the car. Since I like to backpack and get outdoors a lot, a good (albeit expensive! down bag is worth the bucks. I might as well store it in the car. If I didn't backpack, I would save big and get a synthetic.

"Apparently it can be attributed to the "polar vortex"
How odd. I assumed it was Global Warming!

Just a thought on keeping your best camping gear in the car-

If you come upon an accident and a victim going into shock, you might use your supplies to help. If you do, getting your expensive down bag back might be problematic.

I'd be willing to donate pretty much any of my bags or blankets at an accident scene.

Excellent point, Unimogbert. Among my considerations in stocking my vehicle (backpack, bike bags, etc.) with safety and survival gear has been that this is not just for my benefit. It is also for the benefit of passengers and strangers who we may encounter.

For instance, I believe it is more likely that my fire extinguisher and tools such as seat belt cutter are more likely to be used at an accident scene in which I'm a bystander and not to save myself.

I'd like to be able to help others in a crisis.

To your point on the sleeping bag, my primary concern with expensive gear in the car is the potential for it to be stolen. That's why most of what I have in the car is older camping gear, older clothes and boots, etc.

I've been such a gearhead over the past decade that I have plenty of "older" gear which is very nice. But as soon as it arrives, I am putting the new 20-degree LL Bean sleeping bag I've purchased in the car because it will come packed in plastic and will be as compressed as it can be. I also got it on sale.

LL Bean is currently having a 15% off sale on everything -- the sale ends today. Shipping is free.

The way I see it is that a vehicle of most any sort is pretty good rain and wind protection. Not much in the way of insulation though.

I have some extra clothes including a warm coat, hat, and gloves, and a wool blanket. I also have some chemical hand warmers. I figure with what I am wearing that is adequate.

I have been considering some kind of heating means but none of them seem real safe to me.

I got a few cans of sterno and a large multi-wick candle for that. It's about as safe as I can find. The sterno even comes with it's own little pot stand.

A down bag will lose over 60% of its' insulating property in 6 months if kept compressed.
qjs

Where are you getting this info from?

I would imagine a good shaking would return the loft.

I've heard the same advice about keeping your sleeping bags compressed. I wonder if that's more for synthetic bags? The shake it out methods seems like it should work for a down bag.

On the expense note, it might be worth a trip to your local thrift store to have a peek for a down bag. I haven't had any luck with one yet but I haven't lost hope, and have found some great wool and down garments in the process.

sleeping bags, candles, matches, holder, food, chemical heaters...hat, gloves.