David, you'll here a lot expert advice here fer sure, but you have to realise that the members of this group come from everywhere and their specific experience depends largely on where they had those experiences. All but one of my bags have goretex or Drylight shells. I simply have not experienced the dreaded "bag condensation" problems noted by others. I definitely think its worth the cost and weight water proof breathable sleeping bag shells. I think you are more apt to get the bag wet from soaking up moisture in your tent than from persperation. It can be pretty warm in South Alaska in August, and damp. You might consider a synthetic bag instead. I made the mistake of going to Alaska with too warm of down bag and sweated in it. (that was over 35 years ago and my equipment has changed a lot.)
Jim
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These are my own opinions based on wisdom earned through many wrong decisions. Your mileage may vary.