Sea to Summit Ultra-Sil Pack Cover

The Sea to Summit Ultra-Sil Pack Cover is a waterproof rain cover made from 30D Ultra-Sil siliconized nylon (silnylon) with taped seams, sold in sizes that fit packs from 15 to 95 litres. An adjustable keeper strap holds it in place over the pack in wind, and it stuffs into its own small sack when not in use. It protects the pack fabric and whatever is strapped to the outside from rain, but it does not seal the back panel against your body, so gear packed against your back can still pick up moisture in sustained downpours - pairing it with a pack liner or dry bags inside covers that gap. Useful from day hikes through multi-day treks in wet conditions, less relevant if you already carry a fully waterproof pack.