I was taking a look at the Handbuch der Architekture D L provided the address to earlier and thought I would quote the machine translation regarding the infill materials:

"As a filling agent use: hay, straw, woody parts, tan, straw, sawdust, wood wool, wood shavings, peat, soil, sand, ashes, charcoal, kieselguhr or diatomaceous earth, slag wool."

and the original text:
"Als Ausfüllungsmittel benutzt man: Heu, Stroh, Schäbe, Lohe, Häcksel, Sägespäne, Holzwolle, Hobelspäne, Torfstreu, Erde, Sand, Asche, Holzkohle, Kieselguhr oder Diatomeenerde, Schlackenwolle."

The book goes on to discuss the suitability of some of these materials. Great book!

Thanks;
Jim

p.s. I'll bet the Icelandic houses are cool and damp, but "way cool" in my opinion.