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Dense packed cellulose will not settle. It's installed at a density much greater than any recorded pressure. Vaopr barriers and fiber glass are a great way to cause moisture issues. That is unless your envelope is 100% air tight. Call Bill Hulstrunk at National Fiber. He can give you decades of information proving this. Originally cellulose was installed with the "two hole method" which involved drilling a second hole to allow pressure to escape allowing the bay to be filled. This would settle. Now we have extremely powerful blowers which just plain jamb it in there and there are no recorded cases of settling when it's properly installed. When you open up a wall that's dense packed you quite literally have to caw it out. It's mattress firm. One of the great perks of cellulose is it's hygroscopic value. It buffers moisture up to 30% of it's weight before failure. If you have that much moisture in your wall you have bigger things to worry about than insulation performance because you've got a giant frik'n leak from your roof. It removes moisture from the places where you don't want it, like in framing lumber and sheet rock where all other forms of insulation do the opposite. That is that drive moisture into framing lumber and sheetrock.