[Cob] plastic jugs in walls?

[Tys Sniffen]

So, as my walls get up over 7 feet high, I'm starting to remember how lower
down, when I had them laying around, I would toss in the occasional urbanite
chunk to take up some space. 

Now I'm starting to think about tossing in some one gallon plastic jugs
(vinegar jugs, that are slightly thicker plastic than the common 1 gal milk
jug) again, to take up space, but also I'm thinking having a void in the
wall that size could insulation in a way, and, of course, would make my tall
parts a bit lighter. 

Does that sound like a bad idea to anyone?

Tys