Tents are a lot harder to warm than RVs and campers. That thin tent fabric just doesn't provide a lot of insulation. That being said, it is surprising just how much protection a simple tent can provide against cold weather. Temperatures once dropped below freezing during an overnight scout outing with one of my sons. We were sharing a 3-man dome tent. It was definitely cool inside when I got out my sleeping bag in the morning, but it was down right cold outside the tent! It was cool enough inside that I needed to get dressed quickly and was glad I had tucked my clothes into the bottom of my sleeping bag so they were already a little warmer than they would have been lying in the open. But it wasn't until I stepped outside that I needed a jacket! Don't know what the temperature was in the tent but it was around 24F outside! Just having a tent to keep the breeze off helps a lot. It also helps retain the heat released by your body instead of letting it simply blow away.
In the first place, tents have only a very thin fabric to provide insulation against heat loss. Even the walls of the heaviest canvas tents are only a fraction of an inch thick, compared to 4-6" walls in our residences and at least 2" thick walls in most RVs and campers. And it isn't really very feasible to add insulation to tent walls. However, tents with a rain fly do gain a little in the way of insulation from the air gap between the tent walls and the rain fly. This air gap helps keep tents cooler in summer and warmer in winter. For one thing, the rain fly protects the tent itself from direct wind. Some people add a skirt to the rain fly to ensure it reaches all the way to the ground to reduce heat loss. Putting a tarp over your tent might add little protection but unless there is some way to keep it from touching the tent fabric you will still lose heat through conduction between the tent fabric and the tarp. Rain flys are usually held a few inches away from the tent wall by the external framework of the tent. Using flexible fiberglass tent poles to hold a tarp a couple of inches above your tent fabric might be an option. Or just set up a dining fly over your tent.
One place you can add insulation to keep your tent warmer is the floor. Tents should always be placed on a sturdy ground cloth to protect the floor from being punctured or damaged by debris and to keep moisture out. A good ground cloth will also block a little bit of heat loss through the floor. Unless you are back packing, you can add foam anti-fatigue mats on the floor or your tent to protect the floor from damage, make it more comfortable to sleep on , and to insulate the interior from the cold ground. Of course you won't be carrying anti-fatigue mats anyplace you have to hike in very far, but having a good foam sleeping pad will help you sleep warmer and more comfortable.
Tent heaters are one way of warming up the inside of tents. The most commonly used tent heaters are catalytic heaters that use propane or white gasoline to generate heat. They must be used with adequate ventilation to prevent suffocation and need to always be kept a safe distance from all fabrics and other combustible materials. If you have access to 120 volt shore power or have a portable generator you could use an electric heater. While electric heaters will not consume oxygen you still need to ensure they are kept a safe distance from combustible materials and you have to keep the generator a safe distance from your tents to avoid breathing exhaust fumes.
The human body generates about as much heat as a 100 watt electric light bulb. That means that YOU and any companions will contribute to the heat inside your tent. I once had to open the windows in my house in Utah when outside temps were in the teens because it was getting too warm inside with 20 people in the living room -- and without any heat turned on inside the house! To keep warm use the smallest tent that is suitable for your needs. A big tent just means more air you need to warm to keep it comfortable. I typically recommend using a tent that is rated for one more person than will be sleeping in it to provide space for gear and room to get dressed etc. Using one rated for just the number of people sleeping in it might be a bit warmer, but you would sacrifice a lot of convenience and might end of having to leave some or your gear out in the weather. Using a lot bigger one gives you more room for gear or visitors, but will be harder to keep warm while you are resting.
Propane lanterns generate quite a bit of heat. But they can put out toxic fumes and definitely consume oxygen so you must provide adequate ventilation when using inside a tent. I know, it seems counterintuitive to open windows when you are trying to warm up your tent, but it is critical to provide adequate ventilation. Failing to do so could be fatal!
Sometimes you can make yourself comfortable inside your tent by simply dressing warmer: thermal underwear, sweaters, sweatshirts, jackets, etc. I don't recommend dressing a lot warmer at night than you are used to. Wearing warm, flannel "bunny" jammies instead of your regular PJs might be OK, but dressing in lots of layers will most likely be uncomfortable, confining, and you could get too warm and sweat which would then cause you to get even colder! Dressing differently than you normally do at night may interfere with sleeping. At night extra bedding may be enough to keep you warm enough to sleep comfortably, regardless of the temperature inside the tent. One trick I used when tent camping was to lay out an old sleeping bag opened up on the floor of the tent before rolling out the sleeping bags two or us would be using. Then putting another old sleeping bag opened up covering both sleeping bags. Outside temperatures were in the low 20s that night and we slept very comfortably.
Thermal mass is another way of adding heat to a tent. A thermal mass can be anything that can be safely warmed up and brought into the tent. I've seen people use hot water bottles, baked potatoes, and hot rocks. Just be sure nothing is hot enough to damage your tent fabric. Using leather gloves you may be able to carry hot rocks that are hot enough damage rather fragile tent fabrics so check them carefully or put them in or on fire resistant surfaces.
You can sometimes make it easier to keep a tent warm inside by setting it up where it is already protected from the wind. Solid wind breaks, like large rocks, walls, or buildings provide the best protection but you may get some benefit setting up next to vehicles or even trees and bushes. Vehicles may block a lot of the wind but usually allow quite a breeze underneath that can still blow on your tent and draw the heat away. You might also avoid shade to take advantage of any solar heating during the day. Tent fabric doesn't capture or retain a lot of heat but getting the tent and the air and gear inside as warm as possible during the day can help make it more comfortable after dark.
Keep warm!
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