Few topics are as frequently discussed in outdoor circles as the importance of a warm night’s sleep outdoors. In recent years the constant use of manufacturer temperature ratings and marketing fluff has made it difficult for newcomers to decide which kind of bag they can truly rely on to keep warm. This article aims to address a few common misconceptions to help you make sound decisions about sleeping warm outside.
Down vs Synthetic
When kept dry, no form of insulation offers a better warmth to weight ratio than good quality down. Even modern versions of synthetic insulation such as Primaloft Gold fail to match down’s performance in this respect. Down also lasts much longer than synthetic insulation when properly looked after, however down gear does have a higher initial cost.
If you’re confident that you can keep your sleeping bag from becoming saturated with water and you’re looking for the lightest, smallest packing bag at a given level of warmth, down is the natural choice. If you’re not confident that you can keep your sleeping bag dry, ask yourself why?
If you’re travelling to an extremely humid region such as the Amazon, synthetic fill is definitely recommended. For most camping in Europe and North America however, down is perfectly suitable. Your tent or bivvy bag will keep the rain at bay and a good outer shell can deal with the occasional droplet or case of condensation. I’ve spent well over 100 nights over the last few years sleeping in a down bag, usually inside a bivvy bag. I’ve never once got the bag so wet that I noticed a decline in insulation. A good compression drybag can help keep your bag dry in your pack and your tent/bivvy can do the same during the night.
With synthetic, the advice on fill is simple. Go for the best quality insulation you can find, such as Primaloft Gold. If you sleep warm, lighter is fine, with bags weighing around 800g-1kg being fine for spring and summer. For colder sleepers or those going to colder places, heavier is needed, something around the 2kg mark in terms of total weight (regular size).
A few words on “Fill Power” and “Fill Weight”
Down’s quality is usually measured in terms of “Fill power”. This figure is a measure of how much “loft” the down has. A higher fill power figure means the down will expand to a greater volume, trapping more air and keeping you warmer at a given weight. This, combined with the fill weight (how much down is in your bag) will determine how warm and how heavy your sleeping bag will be. Ignore any marketing lingo about hydrophobic coatings and the like, these two figures are what will determine how effective your down bag will be.
|Low quality down, primarily used for fashion clothing
|Good quality down. Good loft and warmth to weight ratio. Used by low cost manufacturers such as Alpkit
|High quality down
|Exceptional quality down
|Superior quality down. Very rare, packs small and has a high warmth to weight ratio. Very expensive. Used by PHD in their K series gear.
A higher fill power will mean a bag will pack down smaller and and weigh less for a given level of warmth than lower fill powers. A sleeping bag with 600 grams of 1000 fill power down will be very warm, keeping a fit, well fed man warm to temperatures as low as around -10 celsius. A 600 fill power bag with 600 grams of down however would be closer to -5 and will take up more space in your pack.
The best advice is to not get too wrapped up in calculating fill powers and just buy the best fill power you can afford.
The implications of “fill weight” are obvious. At any given fill power, the more down you have in the bag, the more insulation it will provide. Here’s a general overview of the commonly used fill weights for certain scenarios.
|Fill Weight (grams)
|Summer bag. Suitable for mountain marathon use, may be suitable for spring / autumn for fit individuals who sleep warm.
|A solid three season bag. Can be used in winter if paired up with an insulated jacket.
|Autumn / Winter use. Too warm for summer use for most individuals.
|Winter sleeping bag. For use in Scottish Highlands and other cold places. Very high insulation.
Maximising Your Bag’s Potential
Now that we’ve covered the different quality levels and fill weights of insulation available, I hope you have a rough idea about your needs. Remember that the above are guidelines, if you sleep cold you might need a little extra and vice versa.
All of the above does nothing more than determine the insulation of your bag. Sadly manufacturer temperature ratings have helped spread the illusion that sleeping bags warm you up. A sleeping bag merely traps the heat your body produces. You warm your sleeping bag, not the other way around!
Your body produces heat as an inevitable byproduct of its internal chemical reactions. These reactions are fuelled by the energy in the food you eat. Eat a substantial, energy dense meal before retiring to bed to maximise your heat production through the night. This will do more to keep you warm than any other measure. It is commonly held that fats provide more heat than sugars/carbohydrates due to inefficiencies of digestion. This is only a half truth.
Fats generally keep you warmer because they are more energy dense. Carbohydrates and Protein provide 4 calories per gram. Fat provides 9. As it is calories which drive your metabolism, a greater calorie to weight ratio makes for a more warming meal. Set aside the cookies and sweets and instead aim for a rich bolognese. I keep a small vial of Medium Chain Triglyceride (fat) oil in my pack for emergencies. It weighs very little and in the event that I’ve underestimated my food needs, drinking it will provide enough energy to ensure I get a warm night’s sleep, which can make all the difference to the hike back home in the morning. If you’re a little squeamish about drinking cold fat, a dollop of cream or butter in your evening coccoa can do the trick just as well.
We established earlier that body heat is a byproduct of your body’s natural chemical reactions. These chemical reactions require food energy (calories) in order to occur. They also require water. All chemical reactions inside our bodies take place in water. If you are dehydrated, your metabolic rate will decrease and you will feel colder. Drink some water or other non caffeinated liquid before bed and keep a water bottle somewhere where you can reach it during the night.
Warm the bag early
If you get into your sleeping bag cold, it’s going to take some time for you to warm the inside of the bag and start to feel warm. If your fleece is still dry, take it off when you’re already inside the bag to keep your warmth. Do a few situps in your bag, or a few star jumps before you get in to warm your body up.
When compressed, sleeping bags temporarily lose their insulation potential as the surface area for trapped air between the fibres / down plumes is reduced. Once it’s out of the compression bag, air is drawn in, the bag expands and insulation potential is restored.
Don’t lay inside your sleeping bag waiting for your bag to loft. As soon as you arrive at camp, set up your shelter and assemble your sleep system, then leave your bag to loft (expand) while you go build your fire and entertain yourself for the evening. At every camp the first thing I do is throw down my bivvy bag and insert my mat and sleeping bag, zip it shut to keep it dry and go off to leave it lofting. Coming back to a puffy, lofted sleeping bag at the end of the night is the smart choice.
Tighten Those Baffles
Sleeping bags come with drawcords for a reason, they allow you to draw the bag in tighter to your body around areas like the head and shoulder to prevent warm air from escaping. Once you get comfortable, tighten your hood and shoulder baffles to keep the warm air in. A warmer head temperature aids earlier sleep, so even if you are uncomfortable with using the hood, wear a hat or balaclava to keep your head warm during the night.
I hope this article goes some way towards demystifying the world of sleeping bags and helps you to choose the right bag for you. Make the most of whatever bag you choose by staying well fed and hydrated and most of all – enjoy the outdoors!