Staying Warmer in the Sleeping Bag you Already Have
In an ideal world we would all own 3 or 4 sleeping bags, and the weather would always stick to the forecasts so we would always be able to take the right bag on any trip. The way it really works - most of us get one, or maybe two, bags to use and we only have best-guess weather reports. The following tips can help to make that one bag that you have to work with more comfortable outside of its real-world temperature range.
Cold and warmth in a sleeping bag are pretty simple, though what you can do to adjust them are limited. The three things that make you colder in a sleeping bag are: 1) temperature, 2) airflow, and 3) water (yes, three simple things & nothing ingenious). There are two things that work together to keep you warm, 1) heat from your body, and 2) insulation that traps body heat. Based on the factors above every person will find a different real-world temperature range for just one model of bag. (When I write “real-world temperature range” I intend to mean the range of outside temperatures where a sleeping bag doesn’t leave you chilled or make you sweat.) The comfortable temperature range varies widely from the manufacturer’s rated temperature, plus it changes based on the conditions of weather, your body, and the sleeping bag itself.
Airflow:
In order to be warmer while sleeping we need to focus on just the things that can be controlled. Of the three things that make you cold, with the exception of flying to exotic locations, only “air” and “water” can be controlled. “Air” means both the air moving around outside, through the tent, and inside the sleeping bag. “Water” means both sweat from your body and any condensation or wet spots in your sleeping bag. Of the body heat and insulation that keep you warm, both are under your control.
Controlling outside air flow can be done by your choice of tent and where you place the tent. In areas prone to high winds and storms a 4-season or a 3/4-season “convertible” tent will make a big difference. The lack of mesh netting in a 4-season (or zip-over covers in a 3/4-season) tent stops wind from blowing through the tent and keeps the inside of the tent much warmer than it is outside. Unless you’re camping on a ledge that’s far from just about anything else flat - where you place your tent can help with airflow as much as the type of tent. Placing the tent behind man-made or natural wind breaks will reduce the air hitting and blowing through the tent. Wind breaks include stone and dirt walls in some developed areas (especially in high-altitude areas like on-mountain camps in Teton National Park), trees and heavy brush, boulders, hills, and even snow drifts in the winter. If you are using a wind break, pay attention to the winds in the evening; they’re usually different than morning and daytime winds.
Controlling airflow through and inside the sleeping bag starts as soon as you are picking your bag out from the gear store. Extra space inside the bag allows air to flow around your body and steals heat away by allowing evaporation to happen more quickly. Ideally a good sleeping bag would fit very closely to the body and the only air space would be inside the insulation. To get as close as possible, try to get a bag that is shaped and sized to fit your body. Check the “girth” measurement of bags and try to get the one with the smallest girth that you find comfortable. Also, “stretchable” bags like some from Sierra Designs and Mountain Hardware incorporate elastic into the diameter of the bag so that it will move with you. I’ve heard raves about these bags from most people who’ve used them, but the one complaint tends to be durability in the synthetic insulation versions. Since down isn’t a continuous material it doesn’t break down when the bag is stretched and twisted - stick with down for a “stretchable” bag. Even if you already have a bag that doesn’t fit - just add some filling. An extra pair of fleece pants and some long underwear will fill up extra space and keep extra air from flowing. I tend to move around a lot and have found that the extra clothes in the bag also reduce the chances of rolling onto my side and ending up on a cold zipper. Whatever you do, don’t wear ALL the extra clothes. Too much “stuffing”, whether you’re wearing it or just using it to take up space, inside the bag will compress the bag’s insulation and keep it from trapping as much heat.
Bag-length is important for warmth too. Extra length in the foot box means that your feet, usually the coldest and one of the sweatiest parts of the body will cool very easily. Too little length will compact the insulation under the soles of you feet and reduce the amount of heat that is trapped by that insulation. Too much length is a problem that’s easily solved by stuffing a fleece jacket or the next day’s clothes into the bottom of the bag. Especially in the winter, having some warm clothes and water that isn’t frozen is a must every morning. Personally, I try to buy every bag so it has 3-6 inches of extra length. In warm weather the extra space is nice because it allows some cooling air around your feet. Just remember to always fill that extra space with clothes any time the temperature drops.
Water: (this section has been revised with more detail: Surviving a Wet Sleeping Bag)
As anyone who has gotten a sleeping bag wet knows, water has the most dramatic effect on warmth of any one thing.
If you sleep in wet environments often (i.e.: the Pacific Northwest) then consider a synthetic fill bag a down bag with a DriClime or similar waterproof, breathable shell fabric. Since we’re discussing the bag you have already - do all you can to keep the bag dry. First, setup and check your tent before longer trips to make sure it doesn’t have seam leaks or a torn floor. Second, carry a sleeping pad on every trip. Putting insulation between you and the ground is the single most important thing to do in even moderate temperatures - especially if your sleeping bag is too wet to use at all. Finally, at every chance you get: drag the bag out of the tent, turn it inside out, and hang it up or lay it out somewhere dry. Even if you’re not sweating in the bag, skin does breathe and the insulation gets damp. Over the course of a week or more if that dampness doesn’t get flushed out the extra moisture can chill you in the bag at night and the bag’s insulation will not loft (”puff up”) enough to insulate well. Even if it’s raining, turn the bag inside out while you’re in the tent. Especially if it’s raining, make sure to keep the bag away from the tent walls and condensation drips. Easy enough? …when you’re cold and stuck inside the tent all day it’s even harder not to stay inside the bag all day long. The extra moisture in the bag if you stay in all day, can make for a cold night once the sun goes down.
Once you do get your bag wet or damp there are few things you can do to fix it. The most obvious method is to take a long break during lunch-time or during any breaks between rain showers. Pull your bag out and don’t just lay it out - shake it out and swing it around to get the air moving around it. Be careful to keep it from touching any puddles and stay near the tent if it still looks like rain may hit again. A half-hour break to “auto dry” your bag this way can dramatically help a synthetic bag and will even dry out down enough to make it useful after a good soaking. Without a chance to get a bag reasonably dry during the day, take a few minutes before sleeping to decide your best way to get through the night. A sleeping bag with synthetic insulation can be wrung out (gently, to not tear the insulation) enough to sleep in during moderate temperatures. Down bags are a big problem once the insulation starts to clump, only “auto drying” the bag will make it use as anything more than a blanket.
In near-freezing temperatures (20°-32° F) the water in even a synthetic bag is dangerous, in a down bag it’s sometimes best to abandon using the bag altogether. Unless it gets below freezing the water in either type of bag will still evaporate and can steal enough body heat to make you hypothermic. Hopefully, at those temperatures you’re carrying a good sleeping bag and decent top and bottom insulation layers. Put them all on before bed and turn in a little early. Do not get inside your sleeping bag. Either open it up and use it like a blanket or don’t use the bag at all. If you have a tent-mate with a dry bag, try to figure out how to share it with them. Assuming that you can find enough warmth to fall asleep the odds are that you’re going to wake up in the early morning when it gets really cold (the coldest hours are from 3am - sunrise). That is why it’s good idea to go to bed early, to get some sleep in before it’s really cold. In the worst case scenario stay awake all night and avoid hypothermia. Make sure to eat something overnight to keep core temperature up. Grab some sleep the next morning once exhaustion sets in, and once it’s warmed up slightly.
Below freezing (below 20° F) temperatures are a different story than the warmer temperature ranges. Water in the sleeping bag will freeze and frost-up, even when you are inside the bag. Frost in the bag is good and bad. It’s definitely not comfortable or particularly warm, but so long as the insulation can loft before the water inside freezes enough heat can be trapped to get through the night. It’s difficult to completely soak a bag when all the water is frozen, try not to spill the coffee. In the event something like that does happen - wring synthetic out immediately and then lay it out to loft; gently squeeze out down and then lay it out while gently shaking it. These steps will allow the bag’s insulation to puff somewhat before it freezes and the puffiness will trap heat. In cases where a bag isn’t completely soaked this temperature range actually gives some leeway for only a damp bag. In general a sleeping bag can be used as normally, just add a few layers of clothing to yourself to keep moisture away from the skin and even throw a layer of dry clothes over the bag to slow evaporation.
Body Heat:
Your body, the heat source that makes the whole sleeping system warm can be helped along several ways to save or make more heat inside the sleeping bag.
The most important time to think about body heat is before getting into the sleeping bag. Two simple things keep you warmer. The first is eating more, because metabolism turns up for a few hours after a big meal and higher metabolism means the body produces more heat. Make sure that dinner is your biggest meal of the day and don’t hesitate to have a hot drink before turning in. Alcoholic drinks don’t count, they are a negative for warmth because they move heat away from the body core and slow metabolism down. The second thing to do before turning in is a few jumping jacks or some mild running in place. The extra exercise helps to warm up the body and the extra body heat “fills” the insulation in the sleeping bag with heat. With the sleeping bag saturated with heat early in the night it takes much longer for the outside chill to set in.
A few last adjustments help keep heat under the covers if the sleeping bag can’t quite do it. Aside from the extra layer or two of clothing that was previously mentioned, a stocking cap helps trap a lot more heat than just the bag’s hood. The bag’s hood is the biggest vent for heat unless the draw cord is cinched completely shut. Since most of us don’t like to breathe through a pinhole the hat over the head traps even more heat than the bag’s hood. Personally I like to carry a balaclava or ski mask instead of just a stocking cap. In very cold weather breathing through the thin layer of fabric those provide seems to be warmer & easier than trying to get the hood closure just right. In a freak cold-spell when you don’t have a hat, try wrapping a t-shirt over your head. In addition to wearing a hat or balaclava, pull a t-shirt over the top of the bag (down to the shoulders, just like putting it on while already “wearing” the bag). The t-shirt over the top also helps to seal out the coldest air, but doesn’t restrict air enough to become uncomfortable to breathe through - I’m sure everyone remembers pulling the sheets over their head as a child and not having any trouble breathing.
Body heat is saved by the air that insulation traps; the least air is trapped wherever your body weight compacts insulation. Save more of that heat by adding a layer of insulation that will not be compacted between the sleeping bag and ground. Even the best inflatable sleeping pads compact slightly with a body on top of them. Add a layer of fleece in the small of the back (the kidney area can lose the most heat to the ground if you sleep on your back) or even empty your pack and slip it underneath. Try putting the pack under the sleeping pad if it’s too lumpy to sleep on otherwise. In the worst conditions and in areas with trees, or if you puncture your sleeping pad or use it as a broken-bone splint, stuff leaves under the sleeping bag to insulate yourself from the ground.
In summary, be creative when out with a sleeping bag that doesn’t quite cut it. Use everything at hand to trap more heat or even put off sleeping until it warms up enough outside that hypothermia will not be a problem.