Honestly, I've never encountered rain with the Jensen. Knock on tree trunks! However, as a confirmed poncho user I don't worry about packs getting wet in the rain.
Rivendell makes a rain liner for the Jensen packs. I'm not so sure that trash bags or dry sacks wouldn't do as well or better. I bought a 15 liter dry sack to use my winter bag inside one of the vertical columns in the body of my Jensen. The bag is too big to fit in the standard size sleeping bag compartment. Yes, I'm still kicking myself for not buying the Giant when I had the chance. I did buy the external pockets. Glad I did! I love them. When I look at new packs without real outside pockets I just shake my head and walk away.
Bottom line: The Rivendell Jensen Packs are BULLETPROOF! The improvements made in the new versions address the few shortcomings in my original pack. I may have to buy a Giant Jensen for my winter sleeping bag and extra gear.
Caution: As you might guess, the back panel can be quite hot in the summer. I actually sweated through to the sleeping bag when I first used it. A trash bag takes care of that problem now. The new models have a vapor barrier built in.
I bought a 2 1/2 pound frame less backpack in 1974. Ultralight in the Dark Ages. Who knew that I was starting a trend?
I still have an L.L. Bean leather bottom book bag that my daughter used from 6th grade through graduating from Ole Miss. I can't kill it. Even if I did manage to kill it, Mr. Bean would fix or replace it. New stuff isn't always better. Just the opposite in many cases. Buy quality once!
Good luck on your hike!
Wayne