Most sleeping bags are filled with either hollow synthetic fibre or down (soft bird feathers, usually from geese or ducks). They both have their advantages and disadvantages, and it is worth looking in more detail at the differences between down and hollow fibre for sleeping bags to enable you to make a more informed choice when buying a sleeping bag.
Hollow Fibre Filling
Hollow synthetic fibres used in sleeping bag filling are slightly heavier than down sleeping bag filling. Hollow fibre filling cannot be compressed as much as down, and therefore the sleeping bag size cannot be reduced as much as one which is made using down filling. Therefore hollow fibre filled sleeping bags will take up more space than those filled with down. People who object to products which have come from animals may prefer to use hollow fibre filled sleeping bags.
On the plus side, hollow fibre does not absorb much water and this means that if a sleeping bag filled with hollow fibre gets wet it will still keep a lot of its insulating properties and will therefore still keep the occupant warm. This could be very important if you are camping in remote areas where your sleeping bag is likely to get wet.
Down Filling
Down filling is lighter than hollow fibre filling and compresses well which means that sleeping bags filled with down can pack smaller than those filled with hollow fibre. It also insulates very well when it is dry, its insulation properties being better than those of synthetic fibres. However, if down gets wet it loses all of its insulating properties and will therefore not keep the sleeping bag occupant warm at all. In the worst conditions in low temperatures this loss of insulation could lead to hypothermia. Many sleeping bags using down have been designed to keep the down dry even in wet conditions so that the bags maintain their insulating properties, so if you are concerned about this it is worth checking to see if a sleeping bag has this feature before you buy it. Down filled sleeping bags also tend to be a lot more expensive than hollow fibre filled bags.
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 New Goose down Sleeping Bag 14F Degree Camping XL Blue  US $49.99
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 New Goose down Sleeping Bag 14F degree Camping XL RED US $49.99
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 New Goose down Sleeping Bag 14F Camping XXL Gray US $59.99
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Not all down filling is of the same quality, and there are two main measurements for the quality of a down sleeping bag: fill power and down content.
Fill power is a measurement of the down's ability to stand up to being compressed and its ability to loft. The higher the fill power is, the better the down is and the warmer a bag will be. The fill power is given a numerical rating, but care must be taken when comparing US and European values as the system of measurement differs and therefore the same fill power will be given a different rating in the US and Europe.
Down content is another measurement of the quality of a down filling. This is a ratio which gives the percentage of down cluster to small feathers. For example 90:10 means that 90% of the filling is down cluster and 10% of the filling is small feathers. The higher the percentage of down cluster, the warmer the bag will be.
Which To Buy?
Which type of sleeping bag you choose depends on your personal preferences, and also in your budget. If your budget is not very large or if it is likely that your sleeping bag will get wet then you will probably want to opt for a hollow fibre filled sleeping bag. If you will be using your sleeping bag in dry conditions and have a big enough budget, then down filled bags would probably be the better choice.
The best down sleeping bag for you will depend on when and where you camp and also on how warm you like your sleeping bag to be. Don't buy something just because it was in the down sleeping bag sale. Decide whether you want a mummy-shaped or rectangular down sleeping bag, then get some advice from an experienced sales person in a reputable outdoor equipment shop. There is no substitute for the experience of somebody who has used the sleeping bags which they sell and can tell you which bags suit which conditions the best. If you have to buy online, then read a few reviews by people who have used the sleeping bag which you are thinking of buying before parting with any money.