Sea To Summit Ultra Sil Compression Sack
Still indispensable, still made, and still one of the cheapest upgrades in a pack. Sea to Summit has kept the Ultra-Sil line going.
What it does. Four straps squeeze a sleeping bag or a pile of clothing to a fraction of its loose volume, in a sack that weighs almost nothing itself. On a full pack it is the difference between the lid closing and not.
Where it helps and where it does not. Compressing clothing is free space. Compressing a sleeping bag is free space on the trail and slow damage in storage, so use the sack for the trip and hang the bag at home. Compressing anything with structure, like a down jacket you will want lofted in an hour, is a false economy.
The version worth paying more for is the eVent one, whose base panel is a waterproof but air-permeable fabric. Air escapes as you compress; water does not get in. An ordinary compression sack traps air and fights you at the end, which is why people give up two straps early.
One caution the original did not give: silnylon this thin is not tough. It resists water and it does not resist abrasion, so it belongs inside a pack rather than strapped to the outside, and a snagged seam is the usual way one dies.
And the trick most people miss: stuff, do not roll. Rolling a sleeping bag creates the same creases every time and wears the baffles along those lines. Push it in randomly and it packs smaller and lasts longer.
Part of the guide to packing and organisation.