Hey Mike,
How about snow, for igloo walls?
Also sawdust was the vernacular insulating material of choice for ice houses for 100+ yrs around here. Most I've seen are dbl studded 12" thick walls. I've seen small backyard farmstead icehouses built on top of a deep pit into the ground as well as commercial icehouses 36' x 80' w 18'h sidewalls. When no longer used and empty they are legendary in local Fire Department memory if subjected to arson. With good roof maint and overhangs to keep walls dry there is no performance degradation. Sawdust fill could be considered precursor to various blown in and sticky cellulose forms.

What else? fur, leaves. feathers, dried horse manure?
Steve