Carbon Footprints and Embodied Energy in Materials Choice
Some people have used straw*-bale walls for inexpensive building but I wonder about long-term strength, fire, rot, and infestation with straw.*
Some people choose straw* for a lower life-cycle impact or "footprint" on the environment. In practice, an efficiency goal often will overlap with low "embodied energy" (the resources needed to get your house ready for occupancy, including fuel costs to ship a roof shingle or bathroom tile from another state or country). However, embodied energy is not a deal-breaker for my project, except if it's reflected in my construction/maintenance cost effectiveness (bang for the buck).
*Corrected, do not confuse straw with hay (see comment).