Stuff sacks and compression sacks are a great way to keep your space usage down in your rucksack, but they should not be used for long term storage of your sleeping bag.
Long term compression of your sleeping bag can lower the effectiveness of the thermal fibres meaning that over time, the sleeping bag will become less effective against the cold. Sleeping bags can easily lose an entire season in rating, or up to 2 seasons over extended periods.
When "stuffing" the sleeping bag into the stuff sack, never roll it up neatly. Rolling up the sleeping bag neatly into the stuff sack aligns the fibres in the sleeping bag and means that the fibres are more likely to stay compressed when you remove it from the stuff sack. This reduces the thermal effectiveness of the sleeping bag over time. So, always "stuff" the sleeping bag into the stuff sack haphazardly from the bottom end to prevent trapped air, and remove from the stuff sack as soon as possible.
Sleeping bags should be stored loose in a rucksack or other large bag if not being used for long periods.
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