Despite what you may hear from clerks in outdoor stores, most backpacks will not keep water out. This is because most backpacks are made of polyurethane coated nylon in one form or another: the coating prevents water from making its way through the fabric (at least, it does when the coating is still relatively new).
The seams of the packs are not waterproof: the needle of a sewing machine punched holes with mechanical regularity a dozen times or so per inch along every join between two pieces of fabric. And the zippers – even the ‘waterproof’ zippers – are water resistant at best.
So – if you’re going to be backpacking anywhere where you can expect anything stronger than an occasional shower, one option is to use a pack cover to keep the pack and its contents dry. The other option (which can be combined with a pack cover) is to pack the contents of the backpack in dry sacks – you can read about that here: https://askbaz.wordpress.com/2012/11/28/modular-packing-in-your-backpack/
And – both ranges are a little more expensive than some other pack covers on the market.
In a market where price is becoming increasingly important, why would you choose a Sea to Summit pack cover?
In a word, quality.
Sea to Summit’s Ultra-Sil® Pack Covers are not just amazingly light and compact (a 70 Liter/4300 cu in size Medium weighs only 3.9 ounces), they’re also very strong. This is due to their construction: they are made of our Ultra-Sil® fabric, which uses Cordura® yarns for superior strength. Add to this the very high thread count (240 threads per inch) and the generous siliconization, and the result is both tough and light. The Ultra-Sil® Pack Covers are built from a single piece of fabric with an elasticated drawcord around the perimeter. This construction makes them both stronger and more waterproof.
If you’re looking for the lightest, smallest, best fitting pack cover, these are the ones to look at.
The Nylon Pack Cover is the model to choose if you will be bushwacking through undergrowth, or if you expect to encounter a lot of sharp abrasive surfaces (including rocky ground in which you may rest your pack). It is made of a 70 Denier nylon which has a higher abrasion resistance than the UltraSil Fabric. It also features fully-taped seams, and it has a drawcord which runs around the perimeter of the cover. Sea to Summit offers sizes in this model right down to the 20-30 Liter Cycling Pack Cover- this has a bright yellow color, a reflective print, and a Hypalon loop to attach your blinky light too.
The differences in quality between Sea to Summit pack covers and some other less expensive versions won’t be apparent if they remain in their stuff sacks in the store during purchase – so make sure you get the chance to open a cover and see how it’s made.
Because – where you will notice the differences is out in the field. A long way from home. When it’s pouring with rain…