I was hoping someone had run across some miracle insulating tarp that could replace, or be added over, the rain fly. Although the double bag and cheerleader solutions have a certain appeal ....

What's your tent, by the way?

Louise and I camp in Death Valley occasionally over Thanksgiving weekend; temps in the high teens, low 20s overnight. We use our sleeping bag plus a down comforter in the tent. We also have a space blanket and have needed it, too. Our bag is a 3-season one, so we need the extra insulation overnight.

The tent we use, is a 4-season tent from Mountain Research that's no longer made. I have an REI thermometer, and it shows about 10 degrees warmer in the tent than outside with only the heat our bodies provide.

Heating anything by flame may cause a problem unless you have special ducting. The hot air rises, of course, but this brings in cold air from outside as the hot air escapes out the top of your tent unless you have the flame ducted so that it brings the air into the combustion chamber, not the tent. There are some who say that having an unducted stove or fireplace actually makes a house colder than not burning a fire.

You may have a warm area at the top of your tent, and a puddle of cold air in the bottom brought in by the candle or other heater. Next time use a thermometer and check this out.

Tarp? What about the electrical options? I think a 12V powered blanket can do the trick. At 25 Watt (quite warm surface) your average car battery should take you through 2-3 nights on a single full charge (45Ah/10h/2A).

First, tents are pretty bad as insulation. Pretty much what you would expect from a thin piece of nylon. The physics of raising the temperature is that you have to gain more units of heat than your losing to raise the temperature.

A handy trick, if weight and bulk aren't too much of an issue, is to erect a tent inside a tent. Possibly both those inside another tent. The wall of a tent is pretty poor insulation but two or three tent walls with a space between them starts to add up as insulation. Siberians have been doing this tent within a tent thing for hundreds of years with hides. Hides are much more substantial than your typical tent material but then again they are fighting temperatures 40 below zero and fifty mile an hour winds. Three tents with one inside the other and a small fat lamp keeps temperatures in the forties.

Which brings up the other point; what is "warm"? If the outside is forty below keeping the inside a few degrees above freezing might be as good as it gets.

The principle of a shell within a shell is very useful if people are trapped at home in a blizzard. Shutting off most of the house, taking care to empty pipes so they don't freeze, and retreating to a small room can go a long way toward keeping warm. If you set up a tent in a small room you can gain even more.

Hint: the 'hook' portion of Velcro will grip most carpets and allow you to 'stake out' a tent and prevent it from wandering if bumped. A 4by4 piece of Velcro with a D-ring sewn in, prepared beforehand, will work on most commercial carpeting.

If you can isolate a small area and get good insulation between it and the outside temperatures your well on your way to being able to raise its temperature using a small heat source. A well built igloo, sometimes snow caves, might be raised to just above freezing by a candle or two even though the outside may be 20 below zero.

REI Hoodoo 3: http://www.rei.com/product/761889

It is a 3-season tent, I think. I use it for privacy only during good weather and just use a bivy sack if I am away from folks. In winter it is primarily rain and wind protection.

I was thinking about hanging a space blanket liner inside but am pretty sure condensation would make that a bad plan. So then I began thinking about hanging a wool blanket inside, but that is pretty heavy.

I have good sleeping pads, sleeping bag(s), and clothing so I survive just fine. But as I am looking at camping out again this winter, I was just wondering if someone had invented a better mousetrap.

Edit: I also have acquired a 12 x 12 silnylon tarp [with 4 adjustable height poles to make a summer sun shelter] that I might just lash down over the tent.

The tent keeps the wind out, the fly keeps the rain off, and the sleeping bag keeps you warm. A good down bag is a lot more effective than an electric blanket. My winter bag is a -20F or so Northface, and it's quite toasty at 5-10F. To insulate an entire tent to that level would require several feet of thickness of insulation - it would fill the back of a pickup truck. (If the square footage goes up 10x, the insulation thickness would need to do the same, for the same heat loss, so the volume of insulation would be 100x.)

If you are going to use a tarp set it up on sheer poles or guy lines over the tent but not touching it. It has to be far enough away not to let any water or frost freeze to your tent.
You want to avoid the plastic bag effect where condensation just drips back onto you.
You might find banking snow around the bottom of the tarp helps insulate you, but you still need to vent the tarp enough to breath and get rid of moisture.

I don't use down bags, especially in fall or winter.
The problem is that down gets wet and then it is cold and clumpy. I recommend a decent synthetic bag instead. Even when damp the better grade synthetics are still giving some insulation value and they do dry a lot faster than down does.

One trick is to buy a small bag and then a bigger bag and use one inside the other for winter.

Put plenty of stuff under you. It does not matter what it is just as long as it is insulation. You need at least twice as much under you as you put above you.
This is after the weight of your body packs it down. (Foam pads are good here, but use plenty)

If you are allowed a fire setting your tent and tarp to take advantage of external heat can work. I usually just ended up cold with holes melted in my tarp and tent from hot sparks when I tried that and was better off with an open tarp lean-to instead of a closed tent.

About tarps:
Silnylon is pricey. I would think about getting a cheap poly tarp for winter use and keep the silnylon for summer. (Not that there is anything wrong with the nylon, just that I tend to wreck tarps when camping in winter.)

I find winter camping is about bulk more than technology, and I seem to end up packing big bags of stuff that are more air than anything. I have been toying with the idea of bubble wrap ground sheets again.

I haven't done much winter tent camping, but what I did on the few occasions is gather lots of leaves and pine needles and make a pile, then put down a space blanket as a footprint and setup my 2-person backpacking tent over all this. The tent is 3-seasons so I setup the fly without the ridge pole to lay flat on top of the screen to cut down on airflow. Then I drape a sil-nylon poncho over the top and tie it down for a little better air tightness.

On one occasion my son and I spent two nights like that in temps that went down to the mid teens. Our sleeping bags were synthetic lightweight mummy bags rated for 20*, and we had regular thermarest pads. Filled our water bottles with boiling water and used them to pre-heat the foot of the sleeping bags, then wrapped them in our extra set of underwear and used them like a hot water bottle.

We had small, single-mantle gas lamp and backpacker's stove, but didn't use them in the tent at night for fear of setting ourselves on fire or getting CO2 poisoning. We did burn two candle lanterns, but I don't know if they really did much for heating.

It wasn't toasty inside the tent, but it felt at least 20*F higher than out in the wind. We slept in our clothes, thermal underwear, double layer of wool socks, gloves and hats. We also wore our rainpants and extra shirt, and draped our jackets over our legs and feet and used our second poncho on top to try and catch some of our body heat. We were actually pretty comfortable in our bags.

The worst part was getting-up in the middle of the night to pee. And waiting until we got the fire stoked the next morning.

We didn't have chemical heat packs then, I don't think they even existed. Today I'd bring a bunch and place them in my sleeping bag for warmth. Or better yet, just make sure the batteries are fully charged in the camper, turn on the furnace and plug-in the heated mattresses. Holiday Inn on wheels!

On one occasion wwe measured a temperature five degrees warmer underneath a tarp (basically a military poncho) compared to the outside, No significant wind.

The location of your tent matters. Avoid cold air drainages.