Originally Posted by
Mr. Regulator
Can you explain what you mean by that.
Cuben fiber is stronger than SilNylon and, for all practical purposes, is waterproof.
In essence, if you're a true UL backpacker, using a Cuben pack is a better choice than SilNylon and you can leave the cover at home since that indeed is redundant being that the pack itself is waterproof (assuming no gaping holes).
Cuben Fiber doesn't tear or puncture easily. A 1" wide strip of this material can suspend over 120 lbs of weight. No SilNylon can even come close. It is also highly UV resistant and will likely keep you and the contents cooler since the colors are generally white or very light. It is, however, somewhat noisy and it won't win any fashion awards. At present, Cuben Fiber is the lightest and strongest material on the market collectively. Obviously there are other lighter and stronger materials, but Cuben Fiber is the only one that offers both to the degree appropriate for this context.
If you're using it as only a pack cover, then it will be plenty sufficient and lighter than any SilNylon immitations (course, you could probably just line your pack with a trash bag and it would handle the water just fine). Keep in mind, many mainstream packs are now made of the Dyneema X fabric, which again, for all practical purposes is waterproof in the context of being used as a backpack. As long as you don't have any obvious open holes (a roll down top is a good idea) then a trash bag will likely be the lightest and least expensive option.