A good sleeping bag makes all the difference for a comfortable night’s sleep. Whether you’re in a tent, bothy, bivvying or Alpine hut, the right sleeping bag can make or break a trip away. The question is, do you need that minus 15 sleeping bag? What is Fill Power? Down or synthetic?
Synthetic or down?
Down is a natural material produced by birds, which is found beneath their outer protective layer of feathers. Down has evolved to be an excellent insulator, with tiny fronds creating pockets of warm air. To allow the birds to fly, down is also tremendously light and so is an ideal material for filling sleeping bags.
When processed for use in clothing and sleeping bags, down is separated into different categories, called Fill Powers. Each Fill Power category is defined by how much volume a fixed weight of down can fill, and is expressed using a three digit number e.g. 600, 750, 875. The larger the volume the down can fill, the higher the Fill Power number. The higher the Fill Power, the more warm air the down can trap for the same weight. Put simply, the highest Fill Powers, are the ‘fluffiest’ down that have the best warmth to weight ratio. However, the price will increase with Fill Power.
The other disadvantage to down insulation is its ability to absorb water. It will easily absorb more than it’s own weight in moisture, thus removing most of its insulating properties. It will also take a long time to dry out. However once dried, down tents to ‘rebound’ very effectively and though a sleeping bags fabric may degrade with use, the down’s insulating properties will last a long time, with occasional cleaning where necessary.
Synthetic fibres try to mimic downs ability to trap warm air, but so far these man-made fibers have not managed to equal nature. Down (at least from about 600 Fill Power upwards) will still have a better warmth to weight ratio than even the best synthetic fabrics. However, synthetic insulation is cheaper than down and so allows for a less expensive bag to be manufactured. It is also far more resistant to water and will retain much of its insulating properties when wet. It will also dry out far quicker. However synthetic insulation has a lower lifespan than down if compressed for long periods of time (weeks or months) and will eventually break down. However, this may be offset by the lower cost of the bag.
Down sleeping bags are an ideal investment when weight needs to be as low as possible. However, you will have to take care not to get it wet. Synthetic is best for wetter environments, and where price is more of an issue. Ask yourself the question, what will I be using my bag for and what bag is best suited to my needs?
How warm is a sleeping bag?
Talking about the warmth of a sleeping bag in terms of ‘seasons’ is a common method, with a four-season bag being warmer than a three season. However, the difficulty is that there is no industry standard for what a ‘season’ means. Bags are designed and manufactured all over the world, quite possibly with no relevance to the seasons you will experience in your trips. The seasons in Cambridgeshire can be very different from the season in the Cairngorms, and a world away from the seasons in California or the Caucasus. Therefore quality sleeping bag brands have largely rallied around the EN13537 test. This attempts to apply real-world conditions to a variety of brands of sleeping bags, to give consistent results.
The test produces four results per bag; Max, Comfort, Limit and Extreme. The only relevant results are the Comfort and the Limit. These are the coldest temperature at which an ‘average’ adult female and an adult male respectively (clothed in a base layer) will experience a comfortable night’s sleep. This is of course extremely subjective, with a dramatic number of variables which will change your level of comfort in the real world.
The test is also expensive, so not every brand uses it, and has been shown to be unreliable in bags designed for use around -10 degrees C and below. However, it is the most consistent method we have of comparing warmth across rival brands and is far more reliable than the idea of a ‘season’.
Don’t forget you can add heat in lots of other ways too, sleeping in insulating layers and thermals creates a layering system to help regulate heat through the night. Sleeping bag liners are also great at trapping extra warmth and also keeping bags clean. A bivvy bag will protect your sleeping bag from moisture if you’re bivvying or sleeping without a tent.
A good sleeping mat is also very important when it comes to a comfortable night’s sleep. The EN test assumes you will be sleeping on a foam mat, which have largely been superseded by thicker, warmer inflatable sleeping mats. More detail is available here.
A sleeping bag is an investment that will last for years or even decades. Looking after it is therefore important. When not in use, ensure the insulation is not being crushed by keeping the bag in the large storage sack that is provided. If it gets wet, allow it to air dry, or use a tumble dryer on a low heat. Always check the manufacturer’s instructions before this. When packing the sleeping bag for a trip. Consider putting it inside a dry bag to ensure it doesn’t get wet being carried.
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