By STEPHEN REGENOLD

Sea to Summit’s line of Ultra-Sil products, including shopping bags, a small daypack, and a duffle bag, use a siliconized 30 Denier CORDURA fabric that packs up super small for transport in a pocket. A new Ultra-Sil product, the Dry Daypack, adds a waterproof roll-top closure for protecting your goods inside.

Packed up in its tiny stuff sack, the Dry Daypack is smaller than a baseball and weighs just 3.5 ounces. Unfurl it and you get a thin, slippery shell of a backpack with adjustable shoulder straps and enough room inside for a day hike. It has about 20 liters (1,200 cubic inches) of capacity.

The bonus is the pack’s roll-top closure, which is similar to what is used for kayak dry bags. It rolls over itself on stiffened strands of Hypalon, and then a small clip seals it closed.

The closure and the CORDURA fabric are waterproof. Rain will not penetrate the system. But this pack was made for terrestrial use only, and it is not guaranteed to protect your goods if dropped in a river or otherwise submerged.

The pack is scheduled to be available this August for about $55. I tested a pre-release pack out briefly, and I liked what I saw.

To be sure, there are some big compromises with the minimal design: You can feel hard items pressing on your back through the fabric. The thin, unpadded shoulder straps are comfortable only up to a certain weight — maybe 20 pounds if you grit it. There is no hip belt or sternum strap.

But as a backup backpack, this is a neat innovation. The roll-top closure is a great upgrade to an already worthy and unique mini backpack product. Look for the Sea to Summit Ultra-Sil Dry Daypack in stores later this year.

—Stephen Regenold is founder and editor of www.gearjunkie.com.