For a long time I have been in favor of the light clay brick, made of either straw, wood chips, or an experiment I have done using charcoal (it works rather well from an insulating standpoint, however the effort required to produce charcoal is likely too great). These can be easily mortared together with the same mud slip used to make the brick, and plastered over to make the wall more draft proof.

A rather clean and cozy design could be made using this in a half-timbered wall system, then cladding over the structure inside and out.