If you’ve ever backpacked in anything other than warm summer temperatures, there is a strong likelihood you felt cold at one time or another. Feeling cold is a great equalizer as it not only makes you uncomfortable, but it pushes you towards feeling miserable. Fortunately, there are a handful of things you can do to give yourself the best odds of keeping warm even with lightweight gear.
The key to staying warm is to understand why you are cold. Backpackers lose body heat though evaporation (sweat), conduction (cold ground), convection (atmosphere), and respiration (breathing in cool air and/or changing the humidity of a closed area such as a tent). Most people erroneously think they need a heavier sleeping system, heavy clothing, heavier tent, or at the very least, to find some place where wind isn’t as prevalent. Although each of these would contribute to being warmer, it makes a lot of sense to try making smart choices before bringing your entire bedroom suite with you while backpacking.
For me, the first decision I make when trying to stay warm is to ensure I have a sleeping system which functions collectively with everything I’m carrying. This means the clothing in my pack needs to be worn while I sleep as otherwise it doesn’t make a lot of sense to carry it. Secondly, I want to ensure I’m not going to be laying in any moisture, so I make sure I bring a bivy with a waterproof bottom and breathable Pertex-type top (as to ensure the ground moisture doesn’t bother me and to allow my body to breath to avoid the build-up of cooling condensation). Next, I want to make sure I’ve got a healthy “R-value” sleeping pad. The higher the R-value, the warmer I know I’ll be. In the interests of staying lightweight, I usually carry two torso-sized closed-cell foam pads in winter and that’s sufficient to protect me against the cold ground. Closed-cell pads, in most cases, offer a higher R-value than their inflatable counterparts (and are cheaper and lighter too). I place my pad, stuff sacks, and anything else I can find under my legs, and my lower body is fine. Picking the right spot is also essential. Finding a place with a nice fluffy bed of pine needles, tall grass, or uncompressed duff is ideal. I also avoid staying next to bodies of water as it negatively affects the humidty. Camping within the tree line also helps avoid winds which blow across peaks and up valleys. If in the snow, camping under a big tree may give you some luck in finding softer ground with less snow, but be careful as things overhead may come down at any time. Going to bed shortly after getting off the trail is also a good idea as your metabolism continues to push along a little faster which aids your body's ability to stay warm. Your metabolism also works to your benefit if you eat shortly before going to bed. Adding “hot” spices to your dinner is also a smart choice as they too push your body’s warming system into the evening hours. I also personally find great comfort in having a set of merino wool/SmartWool top and bottom just for sleeping. They are clean, free of grease and sweat which would otherwise impair performance, and they are dry. Sleeping in anything wet is always a quick way to get cold. It is also a good idea to dry your bag/quilt out each day to help maintain its loft as any degradation of loft will lower the effectiveness of your bag/quilt. Don't put your head inside your sleeping bag/quilt either as your breath will cause water vapor internally which will hurt your ability to stay warm. Instead, give yourself a "mouth/breathing hole" and cover your head with your sleep system, or my preference, a nice warm fleece or down balaclava. Keeping hot water in your water container and placing it next to your body also makes a wonderful heat source as you try to sleep. Many will argue that a double-walled tent is a necessity, but for a few days on the trail, I’d stick with a single-wall if merely only for the ventilation. A tarp continues to function well even in snow, as long as it is pitched tight and at the right angles. If I choose to add ounces to keep me warm, it will be in the form of a warmer sleeping bag/quilt and clothing layers, not a tent. If you were wondering, a vapor barrier (VB) is really only needed in sub-zero conditions which I think is a rare situation for most backpackers to encounter unless they intentionally hike in environments where this will be the case.
Before putting yourself in a bad position, try these suggestions in your backyard when an escape into your warm home is an option in case your plans don’t work out. Once you’ve found a system that works, give winter camping a shot and find the beauty of solitude in the wilderness. With winter camping especially, there really isn't a "one size fits all" because extreme low temperatures affect each of us differently. However, with the right plan, gear, and smart choices, you can give yourself the best shot at being warmer than others who didn't think it through.