Talk to the folks at Prairiewoods, the Franciscan retreat in Hiawatha:
Prairiewoods: Franciscan Spirituality Center They have built two "hermitages" (small cottages) using straw bale construction.
My own preference is insulating concrete forms (ICF), leave-in-place foam forms that you put together like legos, inserting reinforcing rod as needed. You can do just the foundation this way or the whole structure. We only did the foundation this way, but use geothermal (closed loop!) heat with an in-floor loop and our heating bills, even this last horrible winter, are less for our 3500 square foot house than they are at my Mom's 800 square foot electric-baseboard heated place. We like to frame things, so it would have been hard to do the whole house with ICF, but there's a place I drive by regularly where that's what they did. Very efficient, and strong!