Getting good sleep is essential for any multi-day camp, whether in the mountains or the forest, and you can often tell how experienced someone is by how well they sleep outdoors. Having an effective sleep system will keep you warm and comfortable and make any trip more enjoyable. I define a sleep system as the kit or resources needed to sleep outdoors, consisting of a waterproof shelter to keep the weather at bay, insulation from the floor to preserve body heat, and finally an insulative layer. For this layer of insulation, the vast majority of people rely on a sleeping bag and in this post I plan to guide you through all of the technologies and jargon to help you choose the right sleeping bag for you.

How do sleeping bags work?

Understanding how and why sleeping bags work is the foundation for being able to make effective decisions when choosing one. Sleeping bags are in essence designed to preserve your body heat and they do that by trapping a layer of air around your body. The fill (stuffing) of a sleeping bag is designed to prevent that pocket of air warmed by your body heat from escaping. The more fill a sleeping bag has, the more air it can trap (or the greater its loft) and the warmer you will be. This is also why most of the time you need to pair a sleeping bag with a sleeping mat of some kind, because the weight of your body on the underneath of your sleeping bag compresses the fill and reduces the size of the air pocket to virtually nothing. Meaning you are missing out in insulation where you need it the most – in between you and the cold hard ground.

Which is best, down or synthetic insulation?

There are two main types of fill that sleeping bags use; down and synthetic and there are advantages to disadvantages to both. Which fill you choose will be down to your individual situation and based on your budget, activity types and environment.

Down Fill

Down is the smaller feathers that birds have underneath their main plumage (kind of like a waterproof and insulated jacket pairing), the delicate fluffy structure of these feathers makes them very good at expanding and knitting together to trap as much air as possible. Because it is so finely constructed, down is very lightweight for the amount of warmth it provides but it also absorbs water easily. When down gets wet it clumps together and loses most of its ability to trap warmth, so down sleeping bags in general need to be looked after carefully and can’t easily be washed.

Advantages of down

  • It’s lightweight and packable – Down is much lighter weight and packs down smaller than synthetic sleeping bags with similar temperature ratings. This is really important if you plan to carry your equipment for any distance; a smaller lighter backpack is much more comfortable to carry for long distance trips.

Disadvantages of down

  • It’s expensive – Down filled sleeping bags tend to be much more expensive than their synthetic counterparts. Make sure to avoid cheap down filled sleeping bags, if the price seems too good to be true, it usually is.
  • It’s harder to clean – In general you should avoid cleaning down filled products too often as every wash reduces their lifespan. When you do clean them they have to be cleaned carefully with down specific detergents and the down will need to be physically separated during the drying process to restore that loft. Tip: To reduce how often you need to clean any sleeping bag, use a sleeping bag liner and change into a clean set of camp clothes before turning in for the night.
  • It loses insulation when wet – While newer down fill sleeping bags often have a hydrophobic (water repellent) coating on the feathers, down sleeping bags still do not do well in damp conditions. However, with a care, a good quality dry bag and regular airing this is rarely a problem for most people (if your sleeping bag is getting wet, several other things have gone wrong before that point).

Synthetic Fill

Synthetic insulation is a man-made product designed to mimic the properties of down while being more cheap, robust and resistant to moisture. However, it is only recently that we have begun to produce fibres anywhere near as complex as those created by nature millennia ago. The warmth to weight gap between down and synthetic insulation is reducing rapidly, but for now, the majority of synthetic fill products are a lot heavier and bulkier than the equivalent down filled versions.

Advantages of synthetic

  • It’s cheap – Synthetic fill is in general pretty cheap to produce, meaning that there are some very moderately priced synthetic sleeping bags out there.
  • It keeps its warmth when wet – Most synthetic sleeping bags will retain a decent amount of loft and warmth when wet but in my experience it is still best to avoid being in that situation in the first place ‘An ounce of prevention is worth a pound of cure’.
  • It’s easy to clean – while you are still best using an equipment specific detergent, synthetic sleeping bags don’t need nearly as much work during the cleaning and drying process and will in general survive many washes.

Disadvantages of synthetic

  • It’s heavy and bulky – Particularly when you get into winter-weight sleeping bags, synthetic fill can quickly become prohibitively heavy and take up a lot of valuable space in your rucksack. The bigger your sleeping bag is the bigger and heavier your rucksack needs to be and weight can quickly spiral out of control

How to choose the right temperature rating

Once you’ve decided whether synthetic or down is better for you, it’s time to start narrow down the options and choose a temperature rating. Sleeping bags are typically classified into one season (hot), two season (mild), three season (cold) and four season (extreme cold), the UK being a temperate region – outside of unusually hot summers and the extremes of Scottish mountain winters – usually falls between two season in the middle of summer and three season in the middle of winter. For most people getting their first sleeping bag, I would recommend getting a good three season bag, as this is likely the temperature bracket most of the UK year falls into and it is easier to ventilate an overly warm sleeping bag than add warmth to a cold sleeping bag.

Narrowing it down further from the loose season categories, the majority of sleeping bags now come with three temperature ratings; extreme, limit and comfort. In most situations, the extreme rating can safely be ignored as it is an indication of which temperature the sleeping bag will keep you alive at with possible loss of extremities to frostbite (ouch), so consider it insurance against severe weather changes rather than a temperature to be outdoors in. The limit and comfort ratings are a bit more helpful, with the comfort rating being the temperature that the sleeping bag is designed to be used at and the limit being the temperature at which you should still be relatively comfortable at. While there are standards that set how these ratings are created, there are plenty of manufacturers out there that have clearly been less than honest in their rating system. So if you see a sleeping bag on Amazon with a -18*C comfort rating for £30, you may want to apply some due diligence to checking it out.

The issue with standardised ratings is that human beings rarely come in a standardised format, so your personal temperature rating may differ from other people’s, so here is how I (conservatively) decide on a temperature rating to look for:

  1. Get an average nighttime temperature of the area I am going to be travelling to or where I will camp most often.
  2. Subtract 1-2 degrees from the average if I’m in the countryside and 3-4 degrees if I’m going to be in the hills/mountains.
  3. As I am a cold sleeper I will then subtract another 2-3 degrees from that to arrive at the comfort temperature I am looking for, if you are a warmer sleeper then you may need to add a couple of degrees to get to your comfort temperature.
  4. Once you have your comfort temperature, aim to find a sleeping bag within a couple of degrees either way.

This process will hopefully give you a good all-round sleeping bag for your personal temperature range. You can then modify the temperature to suit a particular trip by adding or removing clothing layers or adding a thermal liner.

What is fill power?

Once you have decided on an ideal fill type and temperature rating you will likely be confronted by a variety of different sleeping bags at wildly different price points. This primarily has to do with the quality and quantity of the fill (in addition to the overall design, quality and construction of the bag itself). If you’ve ever bought or looked at a down jacket before, you will likely have noticed that the manufacturers list fill power as one of the specifications. Fill power is a measurement of how much a given weight of down expands (also known as loft), this is typically based on an ounce of down and the number of cubic inches it fills when fully expanded. So one ounce of 300 fill power down will fill a space of 300 cubic inches.

As we looked at earlier, the more space a fill lofts the warmer it will be. So 200g of 500 fill power down will be much warmer than 200g of 300 fill power down but it will take up the same amount of packed space and be the same weight. This is where things can get quite complex, for example; is 500g of 600 fill power down warmer than 200g of 700 fill power? I am not sure but it could be. However, we can distill it down to a basic principle that the higher the fill power, the lighter the sleeping bag will be and the greater the amount of down there is in the bag the warmer it will be.

While synthetic bags don’t have a similar scale, there is still going to be a spectrum of weights and quality of fibres that change the warmth, weight and packability. The big deciding factor at this point is likely going to be price, the better the quality of fill and construction of the bag, the more expensive it will be. Lighter weight and more packable will likely mean more expensive, but it is up to you to decide what is most important. My recommendation would be to get the best quality sleeping bag you can afford and treat it as an investment in your enjoyment of future adventures, a good quality sleeping bag can last you many years and one more expensive purchase may be cheaper than several smaller upgrades.

Final thoughts

There is so much more I could go into at this point including durability of construction, water resistance of fabrics, overall shapes, baffle construction, brands etc. but I hope that this has given you an in-depth understanding of the basic 3 criteria that I personally use and have guided many people through to find the right sleeping bag. If you would like to see part 2 on some of the more detailed criteria let me know. Until next time, stay warm!Bracken Van Ryssen
Bushcraft, Forest School and Outdoor Instruction