Sea To Summit Ultra-Sil Compression SackDescription When you need to compress volume without significantly adding weight, the Ultra Sil Compression Sack is an excellent solution. Made from a water resistant, bluesign approved Ultra Sil 30D nylon fabric with Non PFC DWR this compression sack is far tougher than its weight would suggest. Ideal for use inside your pack or travel bag when you need to compress clothing or bulky soft items to allow space for other essentials or fun holiday items.