Build walls with dirt
When in construction mode, consider using pneumatically impacted stabilized earth (PISE), which is a nearly perfect building material. The technique is a modern method of handling one of the oldest known building materials--earth. Similar to rammed-earth construction or adobe, the technique results in walls that are up to two feet thick, and never need to be painted. The walls absorb heat slowly during the day, and release it incrementally in the evening, which cuts down on energy consumption. More at [Dwell](http://www.dwell.com/green/].