SPARC and TWARC are both similar in that they were the culminating projects after years of research and experimentation. The ARC was too big and experimental to be practical. So we built a Small Practical ARC or SPARC. SPARC was our first attempt at constructing a more standard single-family residence that can be both energy efficient and cost effective. 
 

Following up on thermal mass experiments in The ARC, this building sits on a “slab of concrete floating on a bed of sand in a highly-insulated box.” This time the box includes 10 inches of foam insulation on the sides and 10 inches on the bottom. The ARC had no air flow through the sand; Old Main did not have enough; the Mani Shop had more than was needed. We took lessons from all those buildings to develop the thermal mass under this building. Instead of the six layers of air flow tubes found in the Mani Shop, we used a single layer for this building, topped by 30 inches of sand.

The walls are made of Structural Insulated Panels (SIPS), offering R-80 insulation, and like the ARC, windows are a foot thick. Two double-pane windows separated by the width of the SIPS makes for a great space to display items or keep some heat-tolerant plants. In-floor heat is used as backup warmth for the tougher weeks in winter.